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  #1  
Old 11-24-2017, 01:15 PM
Teo2005 Teo2005 is offline
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Nakamichu BX-125 tampered with?

Hi all
Recently I bought a Nakamichi BX-125E in good condition. As the mechanical counter is working randomly, yesterday I opened it to check the counter belt.
As soon as I removed the bottom plate (the one that reveals the soldered part of the PCB), I saw some components soldered in a rather unprofessional way. Since I'm not an electronist, can someone with op-amps expertise to point what mod is that and what is the reason ? As far as I could investigate, it's related to the headphones amplification stage. I attached a close-up pic of the mod, also the Service Manual where the schematic diagram is. Please if someone can figure out what happened here or why one could do this modification, I'd be much obliged.

Thanks in advance

PS Mistyped the brand name
Attached Images
File Type: jpg IMG_0005.jpg (98.1 KB, 116 views)
Attached Files
File Type: pdf bx125.pdf (4.75 MB, 12 views)

Last edited by Teo2005; 11-24-2017 at 01:17 PM.
  #2  
Old 11-24-2017, 01:35 PM
mityooo mityooo is online now
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He has changed the gain (amplification).
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  #3  
Old 11-24-2017, 01:38 PM
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A.N.T. A.N.T. is offline
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Quote:
Originally Posted by Teo2005 View Post
Hi all
Recently I bought a Nakamichi BX-125E in good condition. As the mechanical counter is working randomly, yesterday I opened it to check the counter belt.
As soon as I removed the bottom plate (the one that reveals the soldered part of the PCB), I saw some components soldered in a rather unprofessional way. Since I'm not an electronist, can someone with op-amps expertise to point what mod is that and what is the reason ? As far as I could investigate, it's related to the headphones amplification stage. I attached a close-up pic of the mod, also the Service Manual where the schematic diagram is. Please if someone can figure out what happened here or why one could do this modification, I'd be much obliged.

Thanks in advance

PS Mistyped the brand name
Somebody wanted more output from the headphones socket, so the amplifier is configured with a gain about 5, which is too much IMHO for this deck, the amp will clip on loud signals.

Cheers

Alex
  #4  
Old 11-24-2017, 01:58 PM
bohelcho bohelcho is offline
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How is this to be restored?

So far so good.
However the OP wants how the deck can be brought back to original.
  #5  
Old 11-24-2017, 02:11 PM
Teo2005 Teo2005 is offline
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Thanks for fast replies. Yes I'd like to bring it back to original, guess all I have to do id to remove/desolder all components added and short 1-2 and 6-7 pins of the IC304 as they once were? If you can see on pics the cooper layer was damaged to isolate those pins...
  #6  
Old 11-24-2017, 02:17 PM
mityooo mityooo is online now
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Quote:
Originally Posted by Teo2005 View Post
Thanks for fast replies. Yes I'd like to bring it back to original, guess all I have to do id to remove/desolder all components added and short 1-2 and 6-7 pins of the IC304 as they once were? If you can see on pics the cooper layer was damaged to isolate those pins...
A drop of tin is enough!
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  #7  
Old 11-24-2017, 02:39 PM
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MagneticBlood MagneticBlood is offline
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Maybe you can check the sound first of making any changes. This way you can compare and try to understand why somebody did this.
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  #8  
Old 11-24-2017, 02:41 PM
Teo2005 Teo2005 is offline
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I'll do that. So normally won't be any other effect of de-modding, except some loss in headphones volume, isn't it? I don't use headphones on a regular basis, so I'm ok it... Now the hard part is to reach the linear rec level pots, hearing cracks/noise when operating them. Seems that most connectors on the PCB and daughter boards are sealed on the other side, so it will be a pain...
  #9  
Old 11-24-2017, 02:43 PM
Teo2005 Teo2005 is offline
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Quote:
Originally Posted by MagneticBlood View Post
Maybe you can check the sound first of making any changes. This way you can compare and try to understand why somebody did this.
Sound is ok, but I'd like it to be unmodified...
  #10  
Old 11-24-2017, 02:48 PM
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Try with IPA spray directly from front using special long pipe aplicator. I remember on my BX300 worked that way . IPA isopropyl alcohol. Will not leave traces neither grease.
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  #11  
Old 12-04-2017, 02:09 AM
Teo2005 Teo2005 is offline
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Ok, removed and restored the original headphones amplifier schematics, working good, enough level for regular headphones. After dismantling the transport for cleaning and oiling, I notice a couple of issues:
1 - in FFW mode, the left reel is working free (no friction or clutch) so when the stop button is pressed and the motor stops, the reel won't stop due to inertia and the extra tape loops inside the cassette. This happens too in REWIND mode if the belt going to the counter is removed. The brakes are clean and working, in STOP mode they engage the reels, maybe the timing is off... not sure if working as intended though
2 - In PLAY mode there is nothing to generate the required tension in tape. On other transports I saw felt clutches or soft brakes that creates the required tension in tape, but as I pointed above, the left reel is moving freely (apart from the STOP state when brakes are engaged and REW function when idler tyre spins the reel).
3 - When playing and STOP or PAUSE is pressed, the heads rig makes an extra move, causing the tape to advance a bit from the moment the STOP or PAUSE is pressed. No biggie, just lookin weird, might be normal as I have no experience in Sankyo transport.

I'd be grateful to any opinion on these issues, I'm mostly concerned in the first one as the tape loop inside the cassete can put strain in tape on further operations, causing it to snap eventually.
  #12  
Old 12-04-2017, 03:38 AM
bohelcho bohelcho is offline
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Sankyo transport is easy to work on (or disturb!)

I would suggest that you carefully read the first pages of the service manual.
The transport has obviously been tinkered with and major adjustments have been changed. This is the reason for the disturbed functions you have observed.
The BX-125 utilizes a tape transport that is easy to work on.
Change all rubber, check the motors for proper operation (no dead spots!) and hopefully the deck will be fine. Just follow the service manual.

Regards
Nick
  #13  
Old 12-04-2017, 03:57 AM
Teo2005 Teo2005 is offline
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Thanks for reply, I read the service manual several times but there's nowhere how the brakes are adjusted or anything related; upon disassembly I made several photos of the mechanism in order to help me assembly correctly, changed and/or cleaned thoroughly all parts... Anyway I'll go back to SM
  #14  
Old 12-04-2017, 05:39 AM
bohelcho bohelcho is offline
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I checked the SM, in particular the Exploded View of the tape transport.
Your described malfunctions 1) and 2) are probably due to the lack of back tension.
Check SM p. 11 showing the exploded view of the transport.
Check your transport to verify the presence of part #24 Brake Spring and part #40 Back Tension Spring (x 2), the latter as parts of the Reel Motor Holder (#41).
These are either “stuck” (insufficient lubrication) or missing!

Section 3 “Mechanical Adjustments”, Item 3.9 Lubrication: Washer L8 (x2) (between the Reel Hub Ass’y(s) and the Back Tension Spring(s) should be provided with some lubricant.

Maybe this will help solving the problem.

Regards
Nick

Last edited by bohelcho; 12-04-2017 at 06:34 AM.
  #15  
Old 12-04-2017, 07:05 AM
Teo2005 Teo2005 is offline
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Hey Bohelcho thanks for interest
I checked the SM and I suspected that too, but:
- #40 Back Tension Spring (in fact 3 of them, 2 under the Reel Hubs and 1 on the capstan) are all there along with washers, cleaned and oiled
- #24 Brake Spring. Saw it too as it jumped in my eyes from the table following the exploded view. It is missing, but... checked the pin where it should be and no marks of being something there, so I checked the internet for videos and/or pictures of BX-125 transport disassembled to figure out what this spring should do (Ex. https://www.youtube.com/watch?v=RWZNe7dxGdg at 3:30 time), and is nothing there!!! So I assumed that SM is for a variant of the transport mechanism that is using this spring, others don't or use smth else instead...
  #16  
Old 12-04-2017, 07:06 AM
calijente calijente is offline
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(2) I wouldn’t worry about the back tension. Most (if not all) single capstan two head Sankyo transports don’t have any back tension adjustment.

Most double capstan machines (at least all of mine) do have a back tension belt and a back tension pulley with adjustable friction. When this belt is broken, the tape can skew and get eaten. Not the case of single capstan transports, though. I imagine Nakamichi engineers relied on the head tape guide to prevent the tape from skewing?
  #17  
Old 12-04-2017, 07:27 AM
Teo2005 Teo2005 is offline
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Quote:
Originally Posted by calijente View Post
(2) I wouldn’t worry about the back tension. Most (if not all) single capstan two head Sankyo transports don’t have any back tension adjustment.

Most double capstan machines (at least all of mine) do have a back tension belt and a back tension pulley with adjustable friction. When this belt is broken, the tape can skew and get eaten. Not the case of single capstan transports, though. I imagine Nakamichi engineers relied on the head tape guide to prevent the tape from skewing?
Might be true i don't argue with that as my expertise in Naks is nowhere near most members of this forum, but tbh if that spring (as I imagine) would provide some kind of gentle pressure or friction on the left Reel Hub, it would solve at least the #1 issue, meaning that stopping from FFWD operation won't make excess tape looped inside the cassette. Otherwise the tape path is ok, checked with a DIY mirror cassette i made, no skew or tape bending on guides.
  #18  
Old 12-04-2017, 02:56 PM
Teo2005 Teo2005 is offline
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Is it possible for someone who is servicing Nakamichi to post a picture with #24 Brake Spring (according to SM Exploded View) or at least to tell me what this spring is doing? And if it's presence is a must? Apart from the point of mounting, the SM isn't clear enough although it seems to put some pressure on the left Reel Hub...
  #19  
Old 12-04-2017, 03:24 PM
mityooo mityooo is online now
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Error :(
Pleace delete!
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Last edited by mityooo; 12-04-2017 at 04:11 PM.
  #20  
Old 12-04-2017, 03:46 PM
calijente calijente is offline
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Quote:
Originally Posted by Teo2005 View Post
Is it possible for someone who is servicing Nakamichi to post a picture with #24 Brake Spring (according to SM Exploded View) or at least to tell me what this spring is doing? And if it's presence is a must? Apart from the point of mounting, the SM isn't clear enough although it seems to put some pressure on the left Reel Hub...
Here is a picture of a Sankyo transport I happen to have on my bench:



#24 is actually essential for braking, it pushes the brakes against the reels. It looks like you nailed it
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