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JVC JA S31 looking for repair advice

Daan*

Daan*

Member
So i stumbled onto this nice looking JVC amp for 10 euro´s and have been busy trying to get it back into working order. This is my first time trying to repair an amplifier and I could use your advice!

So here is the situation:

When i got the amp it wouldn´t turn on. After replacing the fuse the amp turned on but two resistors (r640 & 638) started smoking on the right side. I figured the STK0040 was defective so i replaced it, i also replaced the two restitors (MPC71). After that the amp turned on!

But not perfectly. When using speakers the sound from the right channel is really distorted.
Sound from headphones sounds normal, but when you connect speakers the sounds distorts both in the speakers and in the headphones. I think that means the problem is not in the pre-amp? I don know if itś important but when using the black connection from the bad channel and red connection from the good channel sound is normal. The other way around sound is distorted.

After that i did a full recap but that didn´t improve the distortion. I then thought the STK (being an ebay replica) might be the cause and got a new one but the problem is exactly the same.

I´m stuck! I have a cheap multimeter, but don really know what to look for. Maybe a transistor? Any advice is appreciated! I really like the sound from the good channel.

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Check the supply voltages on the STK.

Could be a power supply issue.

Also check R637, 638, 639. and 640.

Rob
 
Thanks Rob, i switched R639 and R637 from the good channel for R638 and R640 in the bad channel but problem remains the same (distortion in the right channel). I will do some googling to see how i can check supply voltages on the STK.

BTW i also measured DC offset in both channels. The good one reads around 10 and the bad one around 220
 
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Where did you get the STK? They are not made anymore and many Ebay suppliers sell junk. Liberty Electronics has a good source for theirs.
 
It's not the same problem. Originally there was a short and resistors were burning up. Now you have distortion. Check the transistors, and the .47 Ohm 5 Watt resistors but if they are ok, your STK is probably counterfeit.
 
Your right but after that i replaced the first ebay STK again with a new one because i also thought the it might be defective. So i now changed STK´s twice.
 
Daan* said:
Your right but after that i replaced the first ebay STK again with a new one because i also thought the it might be defective. So i now changed STK´s twice.
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If they are both from the same source they could both be bad. What some of these Chinese knock off makers do is put tiny transistors in them that won't work under a speaker load.
 
Thanks for the advice everyone. I found the cause. It's my lousy soldering skills. Two of the pcb pads came loose when i installed the new STK. I just noticed it. I bridged those two points to the nearest point in the circuitboard and now sound is normal! I still have to test it some more but it's way better then before.
 
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