Sony Stereo repair - suspected dead STK

Hi all,

I'm trying to repair my Sony CMT-M373NT bookshelf stereo which has rendered excellent service as a USB sound device with my PC for nearly 20 years. 25W + 25W output, with 6R speakers

Symptoms:

  • System starts as normal, relay clicks, sound is produced, but:
  • Right channel has a buzzing/hissing sound and none of the actual signal. Constant volume, regardless of volume setting.
  • Left channel is 100% normal
  • Result is the same when switching speakers around
  • Same can be heard through headphones
  • With right speaker disconnected, the system can be used Left channel only with no issues
  • With right speaker connected, the system drops to protection mode within a few seconds

Looking at the circuit diagram in the service manual and knowing that these components are well known for going bad, I’m strongly suspecting the STK403-030 amp module. Anyone think I might be barking up the wrong tree?

1693383573409.png


However I can’t probe it in situ because of the way the system is built; the STK and all of the supporting components are masked by other closely stacked and rigidly interconnected boards. However with the board out and checked on the bench there are no dead shorts on any of the transistors or capacitors and no open resistors.

IMG_20230829_172840.jpg


I was planning to solder fly leads to the pins of the STK in situ and then reassemble the unit to allow me to probe voltage the STK pins with the whole thing powered up. Assuming that’s confirmed bad then a repair would be pretty straightforward if it weren’t for the lack of availability of these STK modules. They are listed on ebay and from China but it seems that more likely than not they will be fake, although they do look quite convincing, with the same moulded top cover.

1693385551481.png


Are ALL STKs subject to fakery? This system was bought in 2004ish so we’re not talking about parts that haven't been produced in 40+ years as with some models of STK. Does the top cap come off these easily? If so I could probably buy an ebay one, pop the lid and compare the internals with the original one, a fake will be easy to spot I assume?

Then as I’ve started reading I’ve realised that it’s probably fairly simple to directly replace the dead STK with a couple of either LM3886 or TDA7294. Could anyone offer some tips/pointers here?

As a hobbyist electronics tinkerer I’ve designed, built and coded many Arduino-based devices to do with supporting functions in car engine conversions etc so I’m no stranger to the skills required and I have a decent working understanding of the electronic principles at play, but the specifics of audio/amplifier design are beyond me at present.

Cheers!
 
Last edited:
Administrator
Joined 2007
Paid Member
It does sound like the STK is at fault. Never come across that particular variant but generally yes, its always the STK.

Although either of the chips you mention could be used I'm going to say a couple of genuine LM1875's might be an easier option and essentially all you need do is tweak some values and decouple the chip (I would do this) directly across its supply pins with say a 10uF 63v cap.

No guarantees but I bet it could be an easy conversion.

Screenshot 2023-08-30 111353.png
 
I managed to get a resolution on this…

As much as the idea of a little project to convert this to a pair of different amp chips appeals to me, with a 3 year old and 3 month old, it just wasn’t going to happen. So I figured I’d just buy the replacement STK from ebay and if it didn’t work or whatever I can get my money back easily. Then I’d buy another stereo, shove this one in the loft and come back to it in a few years.

Replacement turned up and except for the different printing of the part number on the front and the lack of etching on the back, it visually is indistinguishable from the original. Weight within a fraction of a gram the same.

Original on the left, ebay special on the right.
IMG_20230908_141822.jpg


IMG_20230908_141831.jpg

Soldered it in and used some CPU paste between it and the heatsink and… success. Is it a fake? Who knows? Maybe, maybe not. Either way it works and to my ear it sounds the same as the original although this isn’t high end kit and I’m not an audiophile. Perfectly fine for its purpose though.

I am intrigued by the counterfeit STK phenomenon though. I find it hard to believe that anyone would bother counterfeiting a module that doesn’t seem to have a demand. Cannot find a datasheet anywhere, cannot find any mention of it in relation to any piece of equipment it was used in. As far I can tell my system is the only device it was ever used in. All really odd.
 
Last edited:
  • Like
Reactions: 1 user
Administrator
Joined 2007
Paid Member
Well that's great news (y) The more oddball (and particularly physically) a part is and the harder it is to fake so a very good chance it is what it is supposed to be. Things like the 3 square corners and rounded 4th corner of the metal tab also tally between the two.

So a good result :)
 
  • Like
Reactions: 1 user
I managed to get a resolution on this…

As much as the idea of a little project to convert this to a pair of different amp chips appeals to me, with a 3 year old and 3 month old, it just wasn’t going to happen. So I figured I’d just buy the replacement STK from ebay and if it didn’t work or whatever I can get my money back easily. Then I’d buy another stereo, shove this one in the loft and come back to it in a few years.

Replacement turned up and except for the different printing of the part number on the front and the lack of etching on the back, it visually is indistinguishable from the original. Weight within a fraction of a gram the same.

Original on the left, ebay special on the right.
View attachment 1211930

View attachment 1211931
Soldered it in and used some CPU paste between it and the heatsink and… success. Is it a fake? Who knows? Maybe, maybe not. Either way it works and to my ear it sounds the same as the original although this isn’t high end kit and I’m not an audiophile. Perfectly fine for its purpose though.

I am intrigued by the counterfeit STK phenomenon though. I find it hard to believe that anyone would bother counterfeiting a module that doesn’t seem to have a demand. Cannot find a datasheet anywhere, cannot find any mention of it in relation to any piece of equipment it was used in. As far I can tell my system is the only device it was ever used in. All really odd.
it seems not original Sanyo STK, as long as not turn the volume loud, should be good . STK module was common before Sanyo Discontinued it. at least you have extra years before "not original" stk would pop-up :).

if I were you, I just bought small/cheap class D amp and transplant it.
 
I Do have :-D. taking care my kids and elderly parents. :).
simple way, buying cheap class D chi-fi :-D,

but it is all up to you on how to manage smartly,
I do have "good" tools that speed me up to fix/replace...

let me know the long term result of "3rd" party STK, I have bad experience with it.
* Bought 2 , first was not good with distorted output. and 2 was ok with non-even balance but not distorted at least .
tested with audio generator and cheap oscilloscope.
 
  • Like
Reactions: 1 user
exactly,
totally forgot my experience with new "NoS", bought one initially. it is ok but with oscilloscope and audio generator, seeing the distortion starting from 10Khz and when volume up, the clipping happening with other channel (original not replaced) is perfectly working.

and the only solution is, get hybrid module replacement if available, or replace with LJM amp board (been posted many times in DA).

my other approach for Yamah CR-840, I bought another one with broken one channel, and salvage all good STK :). how about the another one? remove the amp board components that related to the power amp, and add LJM L20SE board. and call it done :p.

apologize for forking the discussion and I should stop.