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IC Power Amp and Voltage Amp Question- Sansui R-99Z

W

W.T. Holt

Active Member
I'm currently working on a Sansui R-99Z that has some real issues. As soon as it powers on a faint, crackly signal comes out the left speaker but quickly (5 seconds or so) fizzles and goes completely quiet. This is a later model Sansui, and the power amp is completely contained in an STK2139 IC, which I suspect is my culprit. There is, however, another IC labeled "Voltage Amplifier" which has given me pause... possibly this one has gone bad instead or as well? I don't work on this type of gear with integrated circuits often, but from my research the IC's seem untestable with a MM and I would rather not buy $20-30 in parts on a hunch, so I'm hoping for a bit of advice on where my problems may be stemming from. Are any of these parts notorious for going bad? Is there in fact a way to test an IC? Or is there possibly something else painfully obvious that I'm missing?

Thanks!
 
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you need find the manual or the data sheets to see what voltages are expected at each pin . then measure all voltages and see which are wrong
 
What Pete said.

Also, while I wouldn't ignore IC's as ever being a problem I tend to look at mechanical, rather than electrical, issues first. Can you get it to "fizzle" by pressing on circuit boards with an insulated object? If so, possibly loose solder joints or other internal connectors?

Must admit I'm not familiar with this model. But, are there any switches and/or switched jacks? Maybe with dirty contacts causing the noise?
 
This STK "voltage amps" are notorious for failing. A good indication is after power up, spray cold-spray on the voltage amp (alu back side) and many times it works for a few seconds.
Also, some of these voltage amps are interchangable so once in a while people go for higer voltage units (see forums info on marantz PM80 on the internet, it also uses these voltage amps).

Note, all voltage amps you can buy nowadays are fake, so this is the only way you can go. Put a heatsink on it if no heatsink is there.
 
If you have a audio tracer or can make one, I think I have seen a post on here where someone talked about using a tape deck as a signal tracer but just about any small amp will work, you could check the input and output pins of the IC's to see where it gets distorted or is missing. That may narrow it down to the stage with the problem.
 
Thank you everybody for your input. Upon further testing and inspection, a few results:

1: No crackling, fizzing or really and noise at all from tapping on circuit boards.
2. Switches were cleaned first thing, no positive result.
3. Tested 2 small 1 ohm resistors soldered onto the pin outs of the voltage amp IC. They had a black sootlike material on the leads. Still read 1 ohm on my multimeter.

At this point im still wondering about the ICs. Im assuming they will need to be removed from the board to test the voltage yes?
 
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