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Technics SA 202 with a dead left channel

I have finally had the time to sit down and test the pre amp stages, then remove the old STK.

After that I tested the transistor again, then DC voltage as instructed. Everything seemed to correspond with what you said, elnaldo, and the pre stages tested ok as far as I could tell (thanks for the advice, zebulon1). Again the schematics I have been able to find are blurry at best, so I just looked up values online based on what I could see on the parts. After this I did give the contact side of the new STK a light sanding with 220 grit, made sure the heat sink was properly cleaned, reapplied a layer of thermal paste, and installed the new STK.

I didn’t need to remove the heat sink fin or remove the entire board. The space to remove/install was very tight but going slowly and tight maneuvering was all it took.

And the result is a partial success! I say partial because the left channel is now functioning (I have left the unit on for 4 hours now. Both STK’s seem abt the same temp and nothing has blown up). However it is very distorted and full of static. I have tried different inputs (AM, FM, Phono, and AUX), switched speakers, and prior to replacing the STK I had hit all the pots with Deoxit, but I can tell I didn’t do that great a job (it was my first time) because the input and balance are still scratchy. So I will have to redo that.

Also on the positive note, the loud popping that occurred when turned on has been eliminated.

So if anyone can now advise me on how to fix this static and distorted channel I will be set!

Is it the STK? I feel that since it is the power pack mostly likely it is just amplifying a bad signal. Could it be just the pot needing cleaning or replacement? Could it be other components?

Any advice is appreciated, and thank you to those who have gotten me this far. It has been quite enjoyable attempting to fix this on my own.
 
I'd point to that transistor before the STK, if not the STK itself...

If you could build a signal tracer probe, like this one, you could connect it to the STK input and listen to the signal reaching the STK through a 2nd amplifier...

sonda_audio-2.jpg


P1090325.JPG
 
Dude, you put in a bad STK. classic symptom of a fake part - distorted sound. These things don't work, and I warned you about it.
 
Yup, you told me so. Lesson learned I guess. Well at least I hadn't put too much money into it.
I was fun playing around with it anyway.
 
So, it’s almost been 2 years since I asked you fine AK’ers for help on this project.
After the power pack I bought on eBay turned out to be junk, I pretty much shelved the unit and didn’t want to think about it. I did look into a new Chinese made STK and found one on Amazon for $15. I added it to a ‘wish list’ and didn’t buy it or think about it for months.

Then, during the Christmas season I was buying gifts for people and saw it was on sale for $8 so I thought ‘why not?’ And when it arrived I just put it in a box and sat it on top of the receiver.

I think after my last attempt I needed to build up my courage to try again.
But I started getting the itch for a new amp/receiver. Looking on eBay and Craigslist, dropping by the local thrift shops...
I decided before I spend any more cash I should try again to fix it, I mean I have everything I needed so I felt it was worth a shot.

The new STK works perfectly.
I’ve had the unit hooked up to my test speakers playing an FM station for the last 14 hours or so. Nothing’s blown up, no distortion. Good solid sound from both channels. All inputs are working.

Thanks to elnaldo for urging me to try the Chinese STK. It took me awhile but your advice paid off.
 
Good to know you solved it by now.

Some chinese chips work for a while, some for longer, some burn very fast. Some oscillate. But they are cheap. As you did, you can buy from one source and try another one later. I had to discard one that oscillated and no way to solve it. Replaced with a new one and it's working for some years now.
 
zebulon1 said:
Two years has also introduced these:
http://audiokarma.org/forums/index.php?threads/stk-0050-replacement-for-sx-780-and-others.721181/
Click to expand...

Yes, I have a local tech who builds discrete STK-0050 for more expensive units where quality is preferred over budget (I'm planning to install them in my own SX-680 (or 650, I don't remember exactly), given for free with blown outputs)

But the truth down here is that many of the burned STKs belong to lower line amplifiers, where 2 discrete STKs is more expensive than the whole unit, without taking labor costs into account.
 
Zebulon - Those look very cool, but also intimidating... I'm not sure I have the skill to attempt something like that. But it might be worth a try just to see if I could actually manage to build a few. I'll have to hunt down an broken unit and maybe give it a go. What's the worst that could happen?
 
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