Revox G36

[kevinkr]

My first post here. Last week I scored a fully functional Otari MX-55-T-M, luck seems to be holding for today a friend handed me a late production ReVox G36 (USA version, around mid 1966 based on date codes ). *Hi5* I have repaired units in the past (relatively distant past now) and although all of the selenium rectifiers appear to be good as do the electrolytics I know they are living on borrowed time. Back when I regularly worked on these the usual issues were wear and heated related.

The deck has good heads and relatively few hours, no signs of excessive heat from long hours of use. Just wondering what other people's experience with these decks is like and what I should plan on replacing first.

I could replace all 3 selenium rectifiers, the psu electrolytics, the motor run caps, and snubbers before ever attempting to use it or wait and see.. What besides the rectifiers and electrolytics usually need replacing at this juncture? There are a fair number of low voltage electrolytics in the audio path - I do plan to replace them all fairly quickly.

Thanks, Kevin

[Scorpion8]

I don't have any experience with that deck, but you're correct that at this age the elctrolytics start to dry out and fail, especially if it's sat unused for awhile. I just refurbished my Akai deck and it needed some new transistors that had failed. It's odd for the solid state stuff to fail, but these had and apparently it is common among Akai decks. Don't know if they sourced bad 2SC458 transistors, but I replaced those.

Keep us informed as you work along on this deck.

And welcome to TapeHeads by the way! You'll fit right in.

[kevinkr]

Hi Scorpion8,
Confirming what I already suspected. I'm thinking I should just get it over with and replace all of those components..

The unit incidentally is a MKIII and the original owner tells me it is equipped for 7.5ips and 15ips operation, but until I have it running it is hard to know. None of the documentation that came with the deck indicates that is the case so we shall see... *reelstop

It's pretty clean except that something used it to store little nuts, no damage that I can see..

Need a new felt pad for the play head.

I'm looking forward to learning a lot from all the reel people here... *reelspin*

[Scorpion8]

I take it you haven't played a tape on it yet? With aged components, all osrts of things can "drift" such as biasing, playback or record levels and so forth. But it should be a great deck if cared for properly.

[niklasthedolphin]

If you get that up running with orig. specs, you will have one of the best sounding R2R's ever made.

Congratulation.

"dolph"

[Scorpion8]

We need some nice good pics too! I love to watch a machine being restored.

[kevinkr]

I'll try to put some pix up in the next few days.. The machine mechanically is in excellent shape, while someone appears to have pulled it apart out of curiosity or for calibration absolutely nothing has been disturbed except perhaps to calibrate it for a specific tape formulation. I have worked on a number of these decks in the past, and find them pretty easy to repair.

I haven't even plugged it in directly, as the last time it was powered was at least 10 yrs ago, so I wanted to avoid the risk of inadvertent damage. Preliminary checks indicate that everything is probably ok in the short term, but 41 yr old seleniums and electrolytics probably cannot be trusted.

I first serviced one of these when I was around 17, around 1974 or so...
I think the last one I serviced was probably around 2000 or so.

[kevinkr]

Here's a site with a comprehensive English language service manual, and several articles on aligning heads and restoring one of these machines.

www.johnmcculloch.net/tapedecks.aspx

[kevinkr]

I was also given a dozen 7 1/2" reels of Scotch 201 in unopened, sealed boxes, opened one last night and it seemed just fine. How good or not is this tape?
I also have several reels of BASF that came with my Otari.. Will soon acquire hubs and 10.5" reels and tape as well.

Edit: It seems like this Scotch "Classic" 201 might be subject to the sticky tape syndrome discussed elsewhere, can anyone confirm this? The one reel I checked visually seemed ok, but now I am concerned that this tape could make a rather big mess if I actually run it through the Otari. This tape is on an acetate (not polyester) backing and is back coated.

[Scorpion8]

I haven't even plugged it in directly, as the last time it was powered was at least 10 yrs ago, so I wanted to avoid the risk of inadvertent damage. Preliminary checks indicate that everything is probably ok in the short term, but 41 yr old seleniums and electrolytics probably cannot be trusted.

"Put it on a Variac and bring it up slowly" is the usual recommended treatment for this. Or plug it in w/o turning on for a day or two, then turn on but don't play (just let it warm thru the circuits), before you try to play/test it.

[kevinkr]

Here are some pictures of the ReVox I got recently along with a gratuitous picture of my Otari..

I have sorted out all of the mechanical/electrical issues in the transport, and made modifications based on Charlie King's GA article. I found it easier to actually trace out the wiring and find the affected circuitry and make most of the changes there rather than add the additional resistors at the supply. Moving a single wire to the input cap and then changing some resistors is much neater.

The recording and meter amplifiers also appear to be functioning properly now. Not sure whether the deck records properly on both channels as the left playback amplifier is malfunctioning. Some investigation is still required. The working channel sounds pretty good actually.

The machine in case I did not mention it before is 3.75ips and 7.5ips, not as I had hoped the high speed version with 15ips.

The tape tension switch was bad, I fixed that.. Discovered that the tape tension resistor (used for 7" reels and below) was open. One clue was soldered wire across the tape tension switch which I thought initially was due to a bad switch, it wasn't - the 100 ohm resistor was dead.

The original electrolytic in the left channel metering amplifier shorted which resulted in a pegged meter, but no damage. I replaced the pair.

The rotary and pushbutton switches have been quite problematic, hopefully I will get them cleaned up enough to end the problems, and identify the other problems over the next few days.

You can see some of the mods in the third picture, there are a couple of new resistors. Voltages at the affected circuit nodes remain within 5% of what they were before I made the changes. I have removed the output tubes, output transformer, and speaker. The supply changes were made to correct for the higher resulting voltages. The amplifier generates quite a lot of heat and I will never use it - removing it should slightly enhance reliability.

So far it is going relatively well. A little probing with the scope and Amber has helped.

[braxus]

Always thought it would be nice to own one of these Revox tube decks. This would be the one I'd get if I had the chance.

Nice work. Wish I could do my own repairs, but its over my head.

[kevinkr]

I think these days if you're going to own one of these decks you almost have to be able to work on it. Finding someone to service it when something goes wrong would be problematic, and I've worked on enough of these to determine that 40yr old + tape decks aren't the most reliable devices out there. *headache*

This one was supposedly in good running order, well it really wasn't.. None of the problems are that complex to solve, but do require both good mechanical skill and electronics troubleshooting ability. In my real life I'm an EE and am expected to be fairly good at this sort of thing & I actually enjoy it to a point.

Once I get it sorted out, provided that it stays working I will probably miss the challenge.

Anyone know where I might be able to get a new belt for the tape counter? The existing belt has gotten rather slack due to age and flops around quite noisily during wind/rewind operations. *reelspin*

[kevinkr]

I recorded on one of the blank, NOS 201 reels I was given recently using the Otari, no SSS going on with this tape, but it sure doesn't sound great either. (Even at 15ips) I think the specifications may give some clue as to why... In any event the fact that the output is 6dB below reference level on playback even with the bias adjusted kind of says something..

[Jay Pemberton]

Scotch 201....introduced 1962. Not a back coated tape. (The first of those were 206 and its 1 mil opposite number 207, introduced in 1969.) 201 was one of the original 'Dynarange' series, and was touted as giving fidelity at 7 1/2 previously only possible at 15, etc. It USED to sound good. It was widely used as mastering tape in many top studios back in the 1960s. A lot of well-known, classic music of that time was recorded and mixed onto it.

Where the penny dropped: The binder became quite brittle with age. As a result, many reels of 201 exhibit severe flaking of oxide. I first heard of this phenomenon in 1986, in reading an interview with Bill Inglot and Dan Hersch, then of Rhino Records, in THE ABSOLUTE SOUND magazine. They had just completed a compilation CD of many of Dionne Warwick's Scepter-era material, which was recorded at A&R in New York for the most part, with Phil Ramone engineering. And they said that you could see light through bits of some of the tapes, meaning, at those points, all the oxide had left the backing!

So they had to either find safety copies or remix from the multitrack tapes. The tell-tale sound of 201 falling to bits can be heard on a number of CDs of 1960s music, including 'Christmas time is here (vocal version)' on the original Vince Guaraldi A CHARLIE BROWN CHRISTMAS CD on Fantasy: it's that gargling, crumbling, staticky noise in the background you can especially hear when this track goes into the instrumental passage in the middle.

You can also detect it on several cuts, including 'A taste of honey', 'Tijuana taxi' and image shifts on 'Whipped cream' on the 2001 cd HERB ALPERT--DEFINITIVE HITS. Same tape type, same problem. (Note to Beatles fans: Not to worry, they used EMI tape at Abbey Road, and nothing else, in their day, so nothing of theirs of UK origin was recorded on 201!)

Good luck on that G36 though!

[kevinkr]

Hey Jay, thanks for the interesting post on the 201 tape. I knew the tape was introduced back in 1962, but that is about it. It was really interesting to read a little more about the tape and where/how it was used. The comments on cd rereleases of of these old masters has me thinking.. These reels appear to have traveled through a time tunnel or the like as they are in absolutely as new condition. I was amazed, not sure why they survived in such great shape, but they are neither shedding nor brittle.. They were completely sealed, never opened and stored in a cool NH basement if that is any sort of explanation, but they were purchased about 42 yrs ago. I wonder how long they will last. (My basement is quite warm due to the mandatory heavy duty dehumidification required to keep things very dry.) I have a couple of reels of BASF Audio Professional ex studio tape which sounds much better, but the Otari is probably calibrated relatively closely to the requirements of this tape. Even on the low bias setting the 201 sounds just a bit duller and less dynamic than the BASF - it's not terrible though, and it is quiet. It sounds ok at 15ips, but the BASF tape just creams it.

[kevinkr]

Emailed JM Technical Arts in Old Hickory, TN last night and heard back that they still have parts for the G36 in stock. I ordered a replacement counter belt. An extensive Google search turned up nothing useful.. Pinch rollers and tapeheads in abundance, other than that.. nothing

[kevinkr]

Tonight I spent some time investigating the serious playback imbalance that exists between the left and right channel. I estimate an average difference of -10 to -12dB on the left channel relative to the right channel. I verified that this difference exists right at the input to the playback amplifiers so the problem is with the head itself.

I measured the dcr (I figure this head is a goner and if it isn't I can always degauss it) and it was approximately 400 ohms on both channels.

I grabbed a very bright light and took a much closer look at the heads and I now can see a significant amount of head wear on the record and playback heads - not sure what the story is because otherwise there is little evidence of wear on this machine.

I'm debating whether I want to lap the heads or not, I do have a friend who is more skilled with these machines than I am, and perhaps he can do something to improve the situation. Replacement heads are about $600 and frankly I'm not up for that expenditure - I have one great Otari already and can get another for far less than a set of G36 heads, and I need 15ips capability as well... I think I have already invested enough time in this machine - too bad because other than the heads the machine is in pretty good running order at this point.*bash*

[Flyquail56]

Kevin,

Sorry to hear that things are turning out the way they are. I owned a G36 for a short time a few years ago, when I thought I would get back into R to R. Fantastic machine; it is the one piece of audio equipment that I regret having sold.

Best regards,
Mike

[kevinkr]

I'm still trying to decide what to do.. I've been told I might be able to get my hands on an old Nortronics head (NOS) and that is one of several routes I will look into.

I'm also thinking about pulling the heads and having them professionally lapped. (Edit: Actually I will probably do this myself.)

One other thing I have yet to look at is the head alignment, while I don't believe they have ever been messed with I will make a homebrew alignment tape on the Otari and see if careful adjustment mitigates the issue.

The head dcr is consistent on both channels so I believe the coils are ok. I believe it is a head contact issue, lapping may still be required.

Edit: Based on my not so thorough inspection of the heads I believe the zenith may have been misadjusted as it appears that both the record and replay heads are more worn on the bottom than on the top. This could be an illusion, but as the right channel sounds pretty good I have to conclude that they have not worn similarly. I don't see any evidence of tampering with this part of the recorder so I am wondering whether it came this way from the factory..

I'm just going to continue the slow, methodical approach to this and see where it gets me.. The machine otherwise is in great shape, a shame really. If I am lucky a judicious adjustment could do the trick. (or not...)

I downloaded the old Nortronics article on lapping heads, and as I have lapped both tape heads and fiber optic connectors in the past I think I can do this successfully.

[kevinkr]

The mystery continues as I mistakenly perhaps believed that the lower part of the head was the left channel, track diagrams I have seen seem to indicate that this is in fact the right channel. I will need to confirm which track is which.

I will make a left, right, left + right tape with tones on the Otari in the next few days and see whether or not I can confirm this.

[Jay Pemberton]

Top half is left channel, bottom half is right.

[kevinkr]

Thanks Jay for confirming that, means that the section of the head I initially thought might be the culprit actually isn't.. I'll have to take a closer look at the alignment and the head surface.

I'm probably going to send the heads to JRF to be relapped, but I need to make sure there is no other source for the problem. Electrically the head checks out fine so it is a big puzzle..

[kevinkr]

The tale goes on... *reelspin*
I just ordered a replacement set of NOS Nortronic heads for a very reasonable sum of money from The Magnetic Head Company. http://www.magneticheadcompany.com/

Given the very small difference in cost getting new heads rather than just lapping the existing heads seems like a no brainer.

[kevinkr]

I picked up the heads this morning at the Post Office and hope to be able to install them by week's end and get them lined up..

I'll say Magnetic Heads Company is very prompt about shipping out replacement heads - I did not expect it to be so fast, and they followed my shipping instructions to the letter.

Everything required to do the work including very good instructions is included.

I can't wait to give them a try..

[kevinkr]

Once I get the new heads installed and the other remaining issues sorted out I am going to start thinking about a new playback amplifier design - this is where the fun begins.. *Hi5*

I'm starting to think about a high speed capstan for this deck.. Anyone got leads? I plan to contact JM Technical Arts, but just in case I'll ask here too..

[kevinkr]

I had a few free hours tonight and decided to install my new heads. I had some sort of deja vu about having installed heads on one of these machines before - I've definitely done it in the past, and didn't remember doing it.

I removed the old heads and shields and installed the NOS Nortronic heads without the original shields as they are already well shielded and it is way too tight with them in place. (The only way to make it all fit is to bend the clips at right angles and even then it will be too tight. This is something I "remembered." You cannot solder to Nortronic heads as the coils may go open in some instances.)

I also measured the inductance of the old heads and the playback head is BAD.. The left channel measured 415mH whilst the right measured 465mH. I did not remeasure the resistances which were in spec when checked.

The record head was 33mH on both channels to a couple of nH..

The heads are not that worn, but unfortunately ReVox head QC left a lot to be desired, and those shield cans hid some real sins.

The epoxy on both heads is full of voids, and there is a huge one on the left channel pb head.. There is also some evidence of a metallurgical problem in the head steel on this channel as well - there are deep rusty pits visible through an 8X loop just about .100" to the right of the gap.

I apologize for the fuzzy quality of these pix, my digicam is 10yrs old and is not really up to this sort of photography on the best of days.. ;);)

Installing the heads is not too difficult but requires some patience and an ability to work with wire cursed with low temperature insulation which is also placed in a very tight spot.

The Nortronics head finish and QC is orders of magnitude better looking than the original heads, and if the internal construction matches I expect a significant improvement in performance over what could be achieved with good NOS ReVox heads.

I have left the Nortronics head covers in place for protection until I am ready to start the alignment and calibration.

[kevinkr]

Anyone got a 1/2 ReVox erase head knocking around? I will contact JM Technical Arts to see whether they have stock. I recently determined that the original heads were in fact 1/4 track heads which would not have changed my plans to install new half track heads on this machine.

The new heads are installed and working well. The old PB head is definitely bad so no regrets. Had I realized I might have hunted for an erase head as well.

Lined up the playback head using a tape made on the Otari, and lined up the record head today. I have not calibrated to a new tape, and apparently the machine was set up for 201 as it records just fine on that tape.. I just can't erase it.. *quiet**flame*

ReVox PN is 1.017.240..

[kevinkr]

New erase head is on the way from JM Technical Arts and it was very reasonably priced. (Including shipping less than $80.) Check with them before buying any heads on eBay as they will likely be much less expensive.

So soon the erase issue will be resolved, next I just need to get the 15ips capstan assembly or build a sleeve for that purpose myself and then to make the changes to the EQ.

Later I will start to work on a replay amp design using D3A as inputs and 5842 as output stage. I plan to use IXYs ccs chips as loads and since I dumped the speaker amplifier I have both the sockets and B+ current required for something interesting.

[DaveInVA]

I have three G36 machines. Two are quarter track low speed and one is half track high speed. I have two of the machines working good now and the third one still needs help and a new counter as someone removed it from this one in the past.
I also had one that both channels sounded different. On mine it ended up being the coupling caps had all drifted to all different values. After replacing them all it now sounds great. On the other working machine one record channel was dead and that turned out to be a coupling cap also. I guess these things runs hot enough that it really cooks the electrolytics (that and the fact they are almost a half century old).

Dave

[kevinkr]

I have three G36 machines. Two are quarter track low speed and one is half track high speed. I have two of the machines working good now and the third one still needs help and a new counter as someone removed it from this one in the past.
I also had one that both channels sounded different. On mine it ended up being the coupling caps had all drifted to all different values. After replacing them all it now sounds great. On the other working machine one record channel was dead and that turned out to be a coupling cap also. I guess these things runs hot enough that it really cooks the electrolytics (that and the fact they are almost a half century old).

Dave

Try JM Technical Arts in Old Hickory TN, they might have the counter you are looking for and unlike most dealers in ReVox spares charge a very fair price for their spare parts.

They are scorchers for sure, and mine has so few hours that you can really tell the difference between this deck and most of the ones I've run across.

I've almost finished the conversion to half track, new erase head is here, just have to install it and line it up..

[DaveInVA]

Thanks, I bought a couple of pinch rollers from them a while back I believe. I'll see if they have a counter, maybe it won't break the bank :) I was also going to convert the currently non working quarter track machine to half track and already have the new play/record heads. Where did you end up finding a half track erase head?

Dave

[kevinkr]

Thanks, I bought a couple of pinch rollers from them a while back I believe. I'll see if they have a counter, maybe it won't break the bank :) I was also going to convert the currently non working quarter track machine to half track and already have the new play/record heads. Where did you end up finding a half track erase head?

Dave

Got erase head from JM Technical Arts/ReVox.. Highly recommended.. Half track record and replay heads from Magnetic Head Company at: http://www.magneticheadcompany.com/ also highly recommended.

[kevinkr]

Well I finally installed the erase head and did a full alignment on the new heads. The playback head was first and I used a 10kHz stereo reference tape I made on the Otari to align it, then I aligned the record head to that, and finally did an "optical" alignment of the erase head which worked out well despite lacking some obvious aids.

I spent some time checking components and concluded that the cracked carbon cathode resistors in the input stage of the replay amplifier should be replaced. This version of the deck uses 3.9K 10% resistors which I replaced with 3.92K 1% metal films.

There are small imbalances between the channels on both record and replay, but cumulatively they add up to about a 1dB mismatch or less - and seem to be caused mainly by capacitor tolerances and the heads which are actually very close overall.

The record and replay heads came from the Magnetic Head Company and the erase head from JM Technical Arts/ReVox. (All 2 track)

I made all of the changes in the record and replay electronics required for the 2 track heads except for the level padding for R>L and L>R "sound on sound" record options which I will never use.

I was going to bypass the "diode" amplifier on the input and go directly in, but the amplifier seems to tolerate high input levels from the dac without difficulty so I did not bypass it, the ability to use it with microphones may be an asset in the future.

The machine sounds really good, far better than I remember these machines sounding, and I suspect that is a commentary on just how good those replacement heads actually are. It is also true that I have made a lot of mods and replaced suspect parts.. All electrolytics actually in the signal path are now Black Gates, and I replaced a few mustard caps with Fdynes, and resistors as required for the new heads..

Incidentally the tape I was using on the machine is Scotch 201 which, well sounds ok.. My normal tape is BASF Audio Pro which the deck is now calibrated for and which outperforms the old Scotch by a wide margin.. (Much better HF response and more headroom before the tape saturates.)

I included a few pictures of my listening room, as well..

Here are some nice pix. (I recently picked up a Nikon Coolpix 5700 on eBay, photography on a budget - another of my passions.. *quiet*)

[kevinkr]

One thing I haven't done and now desperately need to do is bypass the input record amplifier circuitry ahead of the record level controls. I decided on Friday night to dub a cd (recorded at quite high levels) onto tape and discovered that I was clipping that record amplifier rather frequently - this was so that I could demo the deck the next day during an audio get together I was hosting, needless to say listening to the ReVox was not part of the day's activities..

The clipping sounds rather bad as you can imagine, and I decided not to use the "diode" inputs which have attenuator pots because of the additional gain employed and the likely reduction in sound quality that would result from the input pots..

The existing input amplifier in on the "Radio" input has about 10dB of gain and really performs best with input signal levels no greater than the old IHF line level of around 700mVrms..(I think 200mVrms is more appropriate..) My diy dac at 0dBFS puts out about 3Vrms..

[TheReeler]

Good work!!!. One thing of my never ending list of "Things I don't know how to do" is the head aligment. Hope some of you someday want to film a video doing a complete head aligment in a R2R to learn more about it.

[kevinkr]

This evening I bypassed the input stages of the record electronics ahead of the record level controls - with modern source amplitudes this appears to be absolute necessary, although you could pad the inputs as well. I choose not to made the changes required. I am very pleased with the results and recommend this easy change to anyone recording modern digital sources or anything else that has very high output levels relative to what ReVox designed the deck to handle.

The Nortronics/Magnetic Head Company replacement heads are excellent, and I prefer the sound of this deck at 7.5ips to the Otari MX-55T-M at this speed.

Now just to find some "reel" *reelspin* ReVox NAB hubs because the aftermarket ones I bought that work perfectly with the Otari are working much less than perfectly on the G36. I normally run the deck in the vertical orientation but with 10 1/2" reels and these adaptors the reels actually move about the center of the hub rocking from flange key to flange key - this does not happen when horizontal or with a lightly loaded reel, but I figured it was not good for the tape (tape tension varying) or the deck - things banging around at high speeds.

Least you think one of these decks does not have the torque to handle large reels think again, in fact it is quite frightening just how fast it winds, if a hub ever failed I think the ensuing events would be more than exciting if not downright dangerous.. (Mine rewinds 2400' in way under 1 minute.)

[kevinkr]

Good work!!!. One thing of my never ending list of "Things I don't know how to do" is the head aligment. Hope some of you someday want to film a video doing a complete head aligment in a R2R to learn more about it.

The most important thing seems to be lots of patience and a good understanding of how the tape is supposed to cross the heads. Frankly if you can afford to pay someone else to do it I would. Considering what I already have in this deck $ wise and what it is worth should I need to sell it I did not think spending several hundred $ to have the heads aligned was a good use of my limited funds. (Despite what I said above it is very unlikely I will ever sell it.)

Head alignments are only necessary if you are lapping the heads or outright replacing them. Short of some excessively inquisitive fingers there is no reason any R2R should need its heads aligned.

Doing an alignment is easier on half track heads than on quarter track heads.

There are several things you do first before you do the final alignments, first if replacing heads you must set their penetration into the tape path so that it is comparable to the originals. Next you set the zenith so that the head is parallel to the tape surface. (Usually special inks and transparent tape are used to check head contact.) Once this is done you move on to the playback head azimuth alignment which is done with an alignment tape usually provided by the deck manufacturer. The most simple procedure is just to maximize the output at 10kHz on one channel and then check the other to make sure it is ok. (rock the head azimuth a bit to check) Even better is to use a scope and look at both channels simultaneously or sum them and adjust azimuth for maximum output of the combined channels. There are several other ways to do it as well.

Next you do the record head and in this case just record on either or both channels at 10kHz (check at 1kHz too!) and adjust record head azimuth for maximum output from the playback head and amplifier.

Aligning the erase head is more amusing because other than checking that it only erases the tracks you want there is not much you can do beyond doing a very good optical alignment. I checked the erase head alignment by taking a previously recorded tape and checking for adequate erasure during record.

Mine is a half track machine and the active head widths are twice that of quarter track and there is only 1 stereo track pair to worry about. Aligning quarter track heads is more difficult for both of the above reasons. The service manual in question should provide a good workable alignment procedure and you should follow that as closely as possible - both my Otari and ReVox manuals do, and I followed the procedure very closely and improvised in some instances.

I can't imagine being video taped whilst doing this stressful and tedious work. For the casual hobbyist it can take days to do this to his/her complete satisfaction. I revisited the alignments more than once and tweaked things (which really didn't need to be) repeatedly. I think I took at least 6 hours over 4 different days to install, test and align the new heads - and it was worth it. (The heads are great, and I suspect perform far better than the stock early replacements available on eBay for about 3X what I paid for the Nortronics heads.)

[kevinkr]

No way to delete duplicate posts?

[TheReeler]

Well, your method is pretty similar to the one I use. I never try to align heads if they are OK. The problem is when you buy a deck and you hear the other side backwards by a channel or simething similar.

I've aligned maybe 3 different decks, all 4 tracks 2 channels stereo and the result is always good, by I spend a lot of time doing it.

Usually, to check the final result, I thread the tape thru TWO or THREE different decks and switch the sound to hear if it sounds well or not, and most times, using headphones.

The last time was this past weekend with a Pioneer RT-707 that I bought in France and was with the heads COMPLETLY out of align, it didn't sound in any track correctly, so I had to do all completly from the beginning. And this ocurred 2 years back, but this weekend I adjusted the capstan motor mechanically and had to open the fly wheel because the tape position were different when it was playing in forward or reverse.

After adjusting the capstan motor, the tape was OK positioned in both directions, but then the playback forward head needed to be realigned again.

Lot of time, but finally done. BUT I'm sure this can be done of a perfect way without spending so much time.

And of course I use too transparent tape to watch thru it and see the head.

PS You talk about two screws in the heads... but they have three :(

[kevinkr]

Well, your method is pretty similar to the one I use. I never try to align heads if they are OK. The problem is when you buy a deck and you hear the other side backwards by a channel or simething similar.

I've aligned maybe 3 different decks, all 4 tracks 2 channels stereo and the result is always good, by I spend a lot of time doing it.

Usually, to check the final result, I thread the tape thru TWO or THREE different decks and switch the sound to hear if it sounds well or not, and most times, using headphones.

The last time was this past weekend with a Pioneer RT-707 that I bought in France and was with the heads COMPLETLY out of align, it didn't sound in any track correctly, so I had to do all completly from the beginning. And this ocurred 2 years back, but this weekend I adjusted the capstan motor mechanically and had to open the fly wheel because the tape position were different when it was playing in forward or reverse.

After adjusting the capstan motor, the tape was OK positioned in both directions, but then the playback forward head needed to be realigned again.

Lot of time, but finally done. BUT I'm sure this can be done of a perfect way without spending so much time.

And of course I use too transparent tape to watch thru it and see the head.

PS You talk about two screws in the heads... but they have three :(

Actually ReVox heads have 5, but I was not trying to go into that level of detail. Head height is set using 4 of the 5 screws on the individual head mounting assemblies and can be done optically. Zenith is next and that is optical. Azimuth is done via electrical measurements after this. All of this presupposes that the head penetration into the tape path was also set properly.

[ecir40]

Hey Kevin, thanks for letting us see your rebuild, nice job. BTW can you tell us about or have any links to those speakers you have there.

BR

[kevinkr]

I designed and built the speakers about 2 yrs ago, they are based on the old Jensen Ultraflex more commonly known these days as Onken. The bottom end is handled by Iconic 165-8G which are basically the same as the Altec 515-8G currently sold by Great Plains Audio.. The mids and highs are handled by vintage JBL pro horns, respectively the 4333 and the 2402 annular ring radiator. The X-O is my second attempt and is basically butterworth 2nd order, the first one worked, but had some design errors that resulted in response errors in the cross-over regions. Cross-over points are 800Hz and 8kHz which work well with the chosen drivers. The cabinets were built to my specifications by my father in law, (and extensively stiffened by me) and the box calculator I used is the Onken calculator by Cyr-Marc Debien based on the work by Koizumi, Mahul and Hiraga.

I designed them to work well with moderately low powered DHT SE amplifiers with 0 fdbk. The bass alignment is optimum with a source impedance of about 2.5 ohms which makes them compatible with most SE amplifiers out there with sufficient power. (>6W per channel)

They are quick, extremely detailed, image very nicely, go down to the low 30s and are efficient, just a bit below 100dBspl/W/M. Yeah they go very loud... *Hi5* They aren't perfect but handily outperform the Magnepan QR1.6 I used to own as well as the JBL C-37 Rhodes they replaced. High risk project that paid off better than expected.

There is a lot of information on these particular speakers and the project on diyaudio, my userid is the same as here.

[TheReeler]

Five screws?... Three in the mounting plate and two more for the shell?

[kevinkr]

Five screws?... Three in the mounting plate and two more for the shell?

Four in the head block and one between the head and mounting plate. #24 and its cousin retain the mounting plate to the head block. Newer ReVox models which I have not worked on are probably a bit different. Between them the 4 screws determine the head height, zenith and azimuth. I have included an illustration from the manual that should give you an idea of what I mean.

The old Revox heads have a fixed relationship relative to penetration into the tape path, and are not adjustable in this regard, but the replacements are adjustable and have to be set. (They slide front to back within their mounting adaptors.)

[TheReeler]

Thanks for the pic... wow, really scary to adjust *hypnot*

[ecir40]

Thanks for the description on your speakers. Found one thread at diyaudio http://www.diyaudio.com/forums/showthread.php?postid=930925#post930925

Sorry about going ot, keep up the great work. You got me now wanting one of these decks, have to keep an eye out.

BR

[kevinkr]

Thanks for the description on your speakers. Found one thread at diyaudio http://www.diyaudio.com/forums/showthread.php?postid=930925#post930925

Sorry about going ot, keep up the great work. You got me now wanting one of these decks, have to keep an eye out.

BR

If you don't mind the relatively (very?) dated design these decks can be really nice performers - not as kind to tape as the Otari, but not terrible either. I really wanted one when I was in my teens, and had quite forgotten when this one "fell" into my lap.. *reelspin**reelspin* It was a lot of work getting it into good working order, and the tab was rather a lot too. Try to get one that has been serviced recently and does not have a lot of hours or has had new heads fitted. The original heads are not as good as I expected and as you can tell from the pix I previously posted. Mine now sounds very good particularly with the vintage BASF LPR tape I am currently using. I'm quite pleased with its performance now that it is mostly "done."

BTW it is virtually mandatory to bypass the record input amplifier as it does not have the headroom to deal with modern digital sources - you can use the adjustable gain input at the expense of greater noise and poorer resolution, I don't recommend this at all.

[kevinkr]

Just a quick update, having made the mod to bypass the first two stages of the record electronics (prior to the record level pot) I would have to say this is just about mandatory if you record from any sources that output more than a few hundred mVrms - noticeably cleaner sound.

The 1/2 track Nortronics heads I got from the Magnetic Head Company are absolutely what is needed to make these decks perform comparably to much newer machines in terms of snr, and bandwidth.

Note that all of the inventory of Magnetic Head Company has been acquired by JRF and consequently I would assume some price increases are likely - even so these heads are likely to cost less than the remaining OEM heads and I believe based on my experience that they will outperform them in terms of longevity and electrical performance.

The machine sounds great... *reelspin**reelspin* *yip*

[TheMooN]

Greetings Folks *wave*,
Pleased to make your acquaintance , particularly Kevin the OIP of this excellent thread .

I have recently acquired a ReVox G36 with recent professional service
as well as an F36 from the originator of this You Tube showing the exact same deck http://www.youtube.com/watch?v=4pd4XLnw-yM

Kevin , I am quite interested in your input bypass mod , Would it be possible to post your method and any components required on this thread please ?

Roger ~ London , UK

[disco]

...
Later I will start to work on a replay amp design using D3A as inputs and 5842 as output stage. I plan to use IXYs ccs chips as loads and since I dumped the speaker amplifier I have both the sockets and B+ current required for something interesting.

Hi Kevin

I came across this old post and wondered if you started building on this playback amp.

Regards, Jaap

[kevinkr]

So far haven't been highly motivated to design and build another playback amp, the stock one with a few refreshed parts sounds pretty good to me! My G36 seems to be working quite well and has elicited some jealousy surprisingly in one case from someone who thought this machine was a waste of time. It obviously isn't... :)

I've got 3 other projects going - diy TT, GM70 SE amp, and a pair of Fostex full range drivers which might just end up in teeny Onkens. (The sims look good)

My contract consulting gig keeps me very busy and hence my long absences from this and other boards where I generally hang out.

[kevinkr]

Greetings Folks *wave*,
Pleased to make your acquaintance , particularly Kevin the OIP of this excellent thread .

<snip>

Kevin , I am quite interested in your input bypass mod , Would it be possible to post your method and any components required on this thread please ?

Roger ~ London , UK

It is a very simple mod that requires you to identify one set of input jacks and their connections to the record source selector which you will then unsolder from the switch. Then just disconnect the two large "mustard" caps that feed the record level pots (I'd just remove them) and solder the connections coming from the input jacks right to the pots where those caps were previously connected. You will need to check that you do not swap left and right channels.

[Anna_Logg]

I chanced on this thread and was fascinated to read it.

I used to work as a service engineer in the 1970's in the ReVox service department of the UK Studer Revox agents, F. W. O. Bauch. Most of the machines that came in were A77's and B77's, and solid state ReVox amplifiers and FM tuners.
However, a fair number of G36's turned up too (with the occasional E36 and F36 deck), and I repaired about 50% of these 'antiques', bringing them back to original factory specification. Only myself and one other engineer would touch them, the younger engineers looked at them in horror.

I cannot give much specific advice, as it was a long time ago, but here are some recollections....

The fault we all dreaded with the 36 series was a faulty switch bank. If you pressed a piano key button and the machine didn't do what it was meant to, chances are one of the metal contacts in the switch bank had broken. Replacing a contact is not the easiest of tasks...

The wow and flutter on these machines is usually very good, but I found on some capstan motors the grease in the bottom bearing had solidified. Often disassembling, cleaning and correctly lubricating the capstan motor gave a great improvement. We used Studer capstan motor oil (synthetic I believe) on the top bearing.

If the pinch roller is not badly worn, but has hardened with age, you may be able to get away with cleaning away the hardened surface with a cotton bud and acetone - but don't breathe in too much.

KevinKr says 'Head alignments are only necessary if you are lapping the heads or outright replacing them. Short of some excessively inquisitive fingers there is no reason any R2R should need its heads aligned.' - I would agree with this except to say that the head azimuth should be adjusted fairly frequently, as it does drift on all R2R and cassette machines. It is easy to do if you have a reference tape, and misalignment affects sound quality a lot. Our practice (and the practice in recording studios, where I later worked) was to adjust the azimuth every time the machine was realigned (a daily event in recording studios).

When doing a full head alignment, we used to paint the surface of the heads with a felt tip pen, then play some transparent leader tape for a few seconds. The tape rubs away the ink, showing the head contact, and any misalignments are very apparent.

Biasing and e.q. adjustments on the G36 are fairly straightforward, but you may have to compromise between good noise figures and low distortion. The exact best bias point seemed to be different on each machine.
The G36 could be set up to record well on 'modern' (in the 1970's and 80's) tape, eg Ampex 456, giving better noise and distortion figures than the original specs.

Occasionally we had difficulty finding replacement parts, even though we were dealers. Our company also had the UK Studer agency and we found that many Studer tape machine parts were identical to the Revox G36 ones, despite having different part numbers. (I believe the internal factory part numbers are the same.) So if you get stuck for something, try looking at any Studer spares available too.

I loved the sound you could get from these machines. While probably not as accurate as a B77, to my ears they sounded much more musical, and have that elusive 'valve' sound. I even sold my high speed B77 and bought a G36 instead, which I refurbished (alas long gone).

I am pleased people are keeping these beasts alive, and wish you luck! Your machine looks magic, KevinKr!

[kevinkr]

Hi Anna_Logg,
I'm still using and enjoying the G36, with the better heads I probably ought to look for a modern tape and rebias for that. No question it is a good sounding machine as long as I don't saturate the tape during recording, and it is quiet enough.

I was lucky as my machine was in pretty good condition cosmetically and apparently did not have a lot of hours on it. The bank switch is in pretty good shape and many of the components are still original. The heads were not OK however and I took the opportunity to upgrade to half tracks along with the changes required to use them.

So far so good, supply electrolytics are original as are the selenium rectifiers which I ought to replace although they work fine at the moment. Don't want to let out the toxic magic smoke these things contain. *flame*

Future work will include replacing the couplings in the capstan motor and servicing the capstan motor bearings. This machine has low enough hours that I have not had any mechanical problems.

Rewind achieves pretty impressive speeds which freak out the unwary.. *reelspin* I warn them not to get too close..

Thanks for all of the insight and the comments on my particular deck.

I'd love one of the rare high speed half track versions.

[kevinkr]

Looks like I will be posting some more information on tinkering with the G36 shortly as I noted a rather ominous ticking noise from the capstan motor which means an overhaul and some replacement parts as the internal coupling has clearly failed. It's not something possible to overlook.. *fit**wink*

The investigation will start shortly as the unit as it stands is unusable..

Since I last posted regularly I have gotten a much better camera and promise to share what I find..

[kevinkr]

Looks like I will be posting some more information on tinkering with the G36 shortly as I noted a rather ominous ticking noise from the capstan motor which means an overhaul and some replacement parts as the internal coupling has clearly failed. It's not something possible to overlook.. *fit**wink*

The investigation will start shortly as the unit as it stands is unusable..

Since I last posted regularly I have gotten a much better camera and promise to share what I find..


Starting on the tear down today.. Not really looking forward to it. *reelstop Will post pix when I have opened it up.. Far too noisy to use.

[kevinkr]

Tear down and service done. Not as bad as I expected and the motor really needed a thorough cleaning and lube.

I will post a few pictures sequentially as I think this makes things a bit clearer. Unfortunately despite the great camera, my focus was on the repair and less on the pix so there are some gaps and a few that just didn't come out well enough to be useful.


Notes:

Unit is a North American specification MKIII however a lot of this should be applicable to any model.

The front and rear chassis sections do not need to be fully separated to do this work. No wires need be unsoldered. Power may need to be applied to verify motor work is successful as we go.



Start by removing the head covers and the tensioning control.
Next remove all 4 grey knobs, the felt washers, and the clear knobs.
Remove all six screws..
Carefully lift off the plastic cover and set it aside.
Place chassis as shown in picture, electronics side down.
Remove the 4 screws that hold the front and rear chassis together - these can be found at the left and right bottom corners and centered between the hub motors.
On the MKII carefully remove the two screws and spacers that hold the tape path sensor electronics in place and set the assembly on some insulating material. (MKI and MKII owners refer to service manual.)
There are four nuts located in and around the head block area. Using a spin tight and your left hand placed under the motor remove these 4 nuts. The motor will drop an inch or so..
Carefully lift the top chassis away from its mounting brackets, angle rear slight higher and angle motor until the capstan clears the cutout and pull motor out. Note orientation of wiring for reinstallation.

[kevinkr]

This section deals with actually fixing the motor.

Disassembly:


Mark top cover and motor for proper reassembly orientation.
Looking from the top down orient the flywheel such that you can easily remove the two screws that couple the spider to the motor. Do this first!
Next using a deep 7mm socket remove the 4 threaded standoffs from the top plate. (Or improvise as long as the standoffs are not damaged in the process.)
Lift flywheel and capstan assembly carefully off of the motor.
If you run the motor now you should hear just about the same noise level as when it was in the deck provided that the spider and capstan bearing are in good shape.
Unplug!

Service:


Referring to the bottom of the motor you will see a large C clip that is the first item that must be removed. I made a clip extractor by modifying an old pair of needle nose pliers. You must have a suitable tool to remove this and other clips in the motor.
The bearing cover, thrust plate and ball come out next. Most likely there will be a fully disintegrated cork gasket glued to the cap.
Once you clean out the mess you will see there is another clip to be removed, and then a phenolic washer and finally a metal washer.
Having removed all clips and washers pull out the rotor. (You can gently pull out the rotor instead and then remove the two washers as they fit rather tightly.)
The bearing pedestal has two metal sleeve bearings at the ends and a nylon sleeve in between. Clean this carefully with lint free wipes, swabs and alcohol..
Clean the motor shaft (really bearing surfaces) with alcohol and a lint free wipe.
Lubricate the upper and lower bearings, and the top and side of the top of the bearing housing. I used light teflon grease. It remains to be seen how good this is long term, but whatever was there had almost completely disappeared.
Reinsert the rotor in the motor housing having lubricated all moving parts. Now place the motor on its side, and run it. The rotor will center and should run quietly. Unplug the recorder! Repeat until you have eliminated all noise.

Motor Reassembly:


Re-assemble the motor in the reverse order of disassembly.
Reinstall the metal washer, and then the phenolic washer, then the smaller C clip.
Place a dab of grease on the bottom of the shaft to retain the ball.
Place some grease on the thrust plate and then push the bearing cover into place. If the thrust plate is dimpled it would probably be a good idea to turn it over first. I didn't and probably should have..
Install the C clip.
Run again to assure it is still silent. Unplug!
Before installation lubricate the capstan bearing. (20wt light machine oil/turbine oil [zoom spout] recommended)
Observing the mark you made earlier reinstall the flywheel/capstan/top plate assembly on the motor. Install the 4 standoffs next and torgue in a cross pattern. Finally install the two screws coupling the spider to the motor.
Run it again and make sure it is relatively quiet. Unplug!

Note that originally there was a small quantity of oil in the bottom bearing of the motor housing, but mine was almost but not quite dry. Given the lack of seals after service I recommend the grease instead.

it appears that the noise was coming from the exterior of top bearing assembly which runs inside a cup that is part of the rotor. The original lubricant had disappeared and the wear marks made it clear that there was direct metal to metal contact sans lubricant.

This motor runs quite slowly so I believe that this grease will do the job nicely - if it doesn't I'll know in a while I guess.

[kevinkr]

Reinstalling the motor is the reverse of removal.


Angle top plate to permit installation of motor. Make sure to not snag any nearby wires.
Support the bottom the motor and install the 4 nuts retaining it to the top plate of the chassis.
Reinstall the 4 screws holding the chassis together.
On MKIII reinstall the tape path sensor assembly.
Reinstall the shafts and knobs and check that the motor runs quietly before proceeding. Unplug!
Install the chassis in the cabinet.
Install front panel. Be patient and make sure the tape speed and source control buttons on the right also line up properly.

Having completed the whole thing it definitely runs much quieter than it ever did previously. I think it sounds significantly better, all of that vibration was probably modulating the tape. I should have done this long ago..

The work required took less than 90 minutes to perform from start to finish, and the machine just works so much better now. I hope this is useful to at least one other person here.

[Naknut]

Good stuff kevinkr. Great that you kept at it and thanks for following through with the project and allowing us to see the results. Should be a wonderful resource for others.

[kevinkr]

Naknut, thanks for the response, it's much appreciated.

My biggest concern is my choice of lubricants, we'll just have to see how good long term the teflon grease I choose is.. Can't be any worse than no lube at all since most of these motors must be running dry at this point in time. My other concern was that there is a long nylon or some other plastic sleeve that supports the bearings and I was not certain how it would react to oil or other commonly used lubricants. I very carefully applied the grease to just the metal bearing surfaces - hopefully this proves ok in the long run.

The good news in all of this is that it turned out not to be nearly as hard as I expected.

I listened to some tape last night and it definitely sounds better, and that odd ticking, scraping noise which is now gone was far more distracting than I thought.

*reelspin* *reelspin*

[kevinkr]

I'm back to thinking about designing an alternative playback amp, nothing very imminent, but I will post what I come up with when it happens.

I'll probably be refreshing the power supplies in the nearer term - they're living on borrowed time at this point. The fact that this deck had low hours of use has probably helped.

I'd love to get my hands on a 15ips version of this deck too. Maybe someday..

[kevinkr]

I bypassed the input switching that feeds the recording amplifiers the other day and there seems to have been an audible improvement in cross-talk and in overall resolution.

This is pretty easy to do by running a wire directly from the wiper of the appropriate pot to one of the 100K input resistors of the appropriate channel - the other should be grounded.

I haven't gone any further than that.

I long ago bypassed all of the input switching circuitry and that high gain line/mic amp which saturates on most high level line sources unless you use the diode inputs.

Note that I only record in stereo and don't use any of the other features like sound on sound.

[petronne]

Good morning, from Paris France, *reelspin**reelspin*

In first i give you my apologies for my poor english,

I save from garbage and old REVOX G36 SN:52250, all valves have been tested and they are good, my problem is that i have a little, little, sound. One of my friend take to me to change the selenium rectifier, what do you think about this answer?

Do you know the number of these, and the modell i have to buy?

Could you explain to me how to open the chassis and locate these part's?

Picture of this old reel to reel:

http://www.audiovintage.fr/leforum/viewtopic.php?f=70&t=10798&hilit=LUNDI+HEUREUX

Thank's

Philippe

[petronne]

Nobody to help me?