CR-7A needs new transformer

[scan80269]

A CR-7A that arrived today was immediately diagnosed with a bad power transformer with open circuit on the primary side. *headache*

Any ideas on how a replacement transformer can be acquired?

[Nakdoc]

Check first with ESL labs for a replacement. I may be able to help if no new part is available. All transformers contain thermal fuses. Yours has opened.

[LesX55]

Hi,
You can also try Bowers and Wilkins UK, though they may only have UK spec..

[scan80269]

Thanks for the pointers.

I can't see the thermal fuse inside the transformer, so I may have to tear into it to some degree. Is there any way to replace the blown thermal fuse, or should the whole transformer be replaced?

This CR-7A also has other suspicious areas. Two out of three fuses mounted on the power supply PCB are blown, but all three fuses have been bypassed with wires on the PCB solder side! There are also signs of caps and transistors in the regulator circuitry having been soldered (replaced?), so it looks like I picked up a unit that has been badly traumatized.

I almost hate to find out what else may be wrong after the transformer is fixed...*headache*

[Pentium100]

This CR-7A also has other suspicious areas. Two out of three fuses mounted on the power supply PCB are blown, but all three fuses have been bypassed with wires on the PCB solder side! There are also signs of caps and transistors in the regulator circuitry having been soldered (replaced?), so it looks like I picked up a unit that has been badly traumatized.

Fixing this will be fun :) My first impression is that some component somewhere developed a short and blew out the fuses. Since the owner did not have replacement fuses/was too lazy to go buy them/ found out that replacements blew out too he decided to bypass them. This burned out the regulator which was replaced probably with a more powerful one. This was followed by the transformer overheating and blowing out its thermal fuse.

[gamve]

Try Luis,
He is offering torroid conversions. He might have the voltage you need

[speakerman1]

I like toroid transformers. I can get some; but I need the particulars. I think they do improve the sound. If you get me the specs I'll see if I can get one.
Larry

[Nakdoc]

This CR 7 was 'hot wired" and has more problems than the transformer and fuses. Many thermal fuses are impossible to bypass. Please note: bypassing the thermal fuse creates a fire risk should the regular fuses be defeated. Always add a new fuse in the primary if none exists in the original design. I do not recommend this procedure unlees there is no other option. Luis's torroids are a great idea!

[scan80269]

Even before I deal with the transformer, I'm going to check the power supply circuits and the downstream analog circuitry for any shorts. I want to know the root cause of the PCB fuses and the transformer thermal fuse getting blown and fix the issue prior to connecting another transformer to this deck. I don't want the new transformer to get taken out the same way as the original transformer.

The electronic copy of CR-7A service manual that I have is of such poor resolution that I can't read any component values from the schematic pages. I'll need a better copy before I can do much more work on this deck.

As a Nak newbie I have become very fond of the better Nak decks, so I think my CR-7A is worth fixing despite the effort required.

BTW, I also picked up a Nak T-100 audio analyzer with a broken display (big crack in the glass) *headache*

[scan80269]

I just found one culprit in this CR-7A deck: a shorted 2200uF electrolytic cap at C411 on the power supply PCB.

My outlook on being able to repair this deck has just improved!

[scan80269]

Hmmm, ESL does not have a replacement transformer for CR-7A.

If there is a way to replace the transformer fuse it would be great.

Alternatively, if I want to go for a toroid transformer, how do I contact Luis? Is he a Tapeheads member?

[Pentium100]

Hmmm, ESL does not have a replacement transformer for CR-7A.

Does the transformer have a single or multiple secondaries? If single, just find out the voltage and power rating (should be on the transformer) and buy such a transformer from an electronics store. It won't be described as "for CR-7A" but will work. If the transformer has multiple secondaries then the odds of finding it in a store decrease considerably.

[gamve]

Hmmm, ESL does not have a replacement transformer for CR-7A.

If there is a way to replace the transformer fuse it would be great.

Alternatively, if I want to go for a toroid transformer, how do I contact Luis? Is he a Tapeheads member?

Luis's site is here

http://naks.es/start.html

[Nakdoc]

luis is in Spain, but he has reasonable shipping. His torroids are shown in a movie on his site. The idea was to make a lower noise transformer and a universal voltage transformer available.

[scan80269]

After having detached a shield from the transformer, I got a peep of the blown thermal fuse. It is deep inside the transformer. Instead of further disassembling the transformer to get at the fuse, I decided to replace it with a 1.1A resettable fuse that I stole from an old PC motherboard. The new fuse is in parallel with the blown thermal fuse. I know it is not as good as the original fuse, but at least there is some overcurrent protection on the primary side now.

I also replaced three blown glass fuses on the power supply PCB with resettable fuses of ratings close to the originals, and replaced the shorted 2200uF cap.

With these changes, my CR-7A actually powered up! The display lit up, and the capstans are spinning. If all goes well, a replacement transformer should not be needed, However, transport function buttons don't work, and Q657 started spewing smoke about 10 seconds into the power-on. *eyepop*

I'll need to do more detective work, but I'm beginning to suspect one of the motors may be shorted.

[scan80269]

I have just restored my CR-7A to working order! *Hi5*

Here are the things I did to fix it:
- repaired the transformer with a blown thermal fuse
- replaced a shorted 2200uF capacitor (C411) on the power supply board
- removed 3 blown glass fuses soldered on the power supply board, installed fuse sockets and new fuses
- replaced two transistors:
- Q410 in +5.6V regulator circuit. This transistor was likely taken out when C411 shorted
- Q657 in the control motor drive circuit. This transistor overheated. I saw some smoke *eyepop* when Q410 was bad and the +5.6V rail was at 3.2V when I initially restored power to the deck. I suspect both forward and reverse motor drive paths were enabled simultaneously, which shorted Q657 output to ground. A replacement NPN darlington transistor was a bit tricky to find.

All deck functions now appear normal, though I have not tried recording yet. The azimuth adjustment knob works fine, and so does the remote control! The sound is great and is literally music to my ears.

I'll be checking the wow & flutter soon to determine whether the capstan belt should be changed (it feels somewhat loose). This deck has a gear-type idler, thus no idler tire to replace.

This CR-7A will be my third Nak and a great addition to my collection! *thumbsup*

[perry]

Well done Scan!! Isn't it a rush when you fix something THAT good and it started out as a possible write off? I love that feeling of accomplishment. That's how I got started down that slippery slope of "Well, now I need a better iron, and a solder vacuum station, and a better set of meters, well now I need better calibration tapes, and may as well try to get the alignment jigs.....", 2 years and a few thousand $$ later and this once bargain hobby has morphed into an audio money pit...but it's worth it! (as long as my SO doesn't find out what I've spent)

Again, congratulations, very nice job!

[Skywavebe]

Very Good Scan,
You did what I would have done except the resettable fuses are a little on the dangerous side- not to a fire but because a fuse is suppose to blow to let you know the current draw is outside of parameters. You then fix the problem and then install a new fuse. With resettable fuses, they keep on trying to turn on and the bad part down the way will eventually go because of the several retry attempts- just my opinion.
The previous owner was a typical knumbskull. At least you came out ahead.
I have repaired many a thermal fuses in many types of equipment AND put a fuse in the primary.

[scan80269]

Thanks for the good advice, Sam! I'll insert a normal fuse into the primary side of the transformer to get some peace of mind. What would be a good fuse current rating to use (CR-7A is rated at 55W), fast acting or slow blow?

At first, I thought the previous owner had soldered down the three glass fuses on the CR-7A logic PCB (as well as bypassed the fuses with wires *eyepop*), but yesterday I opened up my Dragon, and saw three glass fuses soldered onto the fuse PCB! It appears that Nakamichi had assembled these decks with the protection fuses soldered down! That pretty much guarantees a service trip for any deck that blows the fuses. In my case, the shorted electrolytic cap in the CR-7A power supply section was likely the root cause of the fuses blowing, among other collateral damages.

I'm now thinking of removing the soldered fuses in my Dragons so that sockets can be put in. The PCBs carrying the fuses in both Dragon and CR-7A have additional pads and holes to accomodate fuse sockets for standard 5x20mm glass fuses. I was just a bit surprised that Nakamichi didn't install sockets for the fuses in the first place. They probably assumed that for most of their customers, cassette decks are in the "no user serviceable parts inside" category, but then I'm not exactly a typical customer...

[perry]

I've soldered in a a few fuses. though a pita, no big deal really, though, of course I prefer fuse clips as well.

[scan80269]

Well done Scan!! Isn't it a rush when you fix something THAT good and it started out as a possible write off? I love that feeling of accomplishment. That's how I got started down that slippery slope of "Well, now I need a better iron, and a solder vacuum station, and a better set of meters, well now I need better calibration tapes, and may as well try to get the alignment jigs.....", 2 years and a few thousand $$ later and this once bargain hobby has morphed into an audio money pit...but it's worth it! (as long as my SO doesn't find out what I've spent)

Again, congratulations, very nice job!

Thanks, Perry. Yeah, I was quite thrilled when my CR-7A started working again. You are also dead on about these decks becoming an expensive hobby. A second CR-7A arrived a couple of days ago, and the only thing wrong about it was a slipping idler. Now I have 4 Naks: two Dragons and two CR-7As, and the sound from these are so good they blow away just about all the other decks I have.

I know exactly what you mean. Within the past month, I have acquired a wow & flutter meter, a Metcal soldering station, and built up a collection of about two dozen test/alignment tapes plus one torque meter. I also have a Fluke DMM and a ESR meter kit on their way to me. I would love to get my hands on a set of Nak gauges and alignment tapes, but understand they are unobtanium. There is also a T-100 analyzer with a broken display that I love to get working one day. My to-do list just keeps getting bigger!

[perry]

Yes, it's scary how fast it can get out of hand. I bid up to about $150 on that T-100, but the no display scared me away from going any higher. Finding a unit with a good display and bad electronics is gonna be tough. I paid almost $400 for my working T-100, and it needed some caps and transistors replaced on the display PCB to fix an issue, but I was sweating it when I thought it was the display. I'm up to 3 Dragons (one missing the REW button), 4 RX-505s (one sold, another for sale), a ZX-7 I got today, (needs all new rubber EXCEPT the original capstan belt!!), a 680ZX (that needs all new rubber and a stuck pressure roller arm freed, that looks like it was made yesterday), a 660ZX (bias orange caps, both roller arms stuck, all new rubber, looks like it was made the day before yesterday), 2 BX-300s (one to sell after I re-rubber and cal/align it), an RX-202 (needs new rubber), and a Cassette Deck One. 14. I have belts on order, except the original capstan belts, and some new caps to replace the orange caps. Unfortunately my honeydo list and work are getting in the way of having fun working on these decks.

Until you menioned it, I never noticed that all the fuse PCBs have slots for fuse clips. Did you just get them at Ratshack, or did you order them? I should stock up on a few. It would be faster to solder in the clips , than solder in new fuses, if one blows.

[Nakdoc]

Perry,
Soldered fuses sound better than clip in. No oxide!

[scan80269]

Until you menioned it, I never noticed that all the fuse PCBs have slots for fuse clips. Did you just get them at Ratshack, or did you order them? I should stock up on a few. It would be faster to solder in the clips , than solder in new fuses, if one blows.

Perry, I picked up the fuse clips and the fuses from a local electronics store (Frys). They just happen to have what I needed.

[perry]

The fuses only supply power. I understand that the fuse ratings can be artificially increased by the clips, but how can they affect the sound? Each fuse handles a voltage or a circuit and all the power for that circuit goes through that tiny high resistance wire. How can oxide make any difference?