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RA-970BX - worth repair?

A

AndrewSan

New Member
Hey folks, my RA-970BX began exhibiting distortion in the left channel a few weeks ago. I hoped it was a dirty selector switch but a good de-oxit didn't improve the condition even a bit. It sounds to me like a legit issue with a transistor or something, hard to explain but it just sounds like an overdriven amplifier sound not a dirty switch sound.

Anyways, how hard might something like this be to track down? And if it can be done, are the components even available for repair? I.e., the transistors used when this thing was built?

I have moderate electronics expertise and have a lot of equipment (4 channel PSU, soldering station, o-scope, basic signal generator). But I don't want to spend a lifetime messing with it. It's for "my wife's" TV and is used as a real workhorse. I'm on the fence about whether to just find another affordable ebay amp, like a 15 year old NAD integrated, and just pass this one on cheap on cragislist.

Any tips or advice is much appreciate.
 
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Assume you have tried other inputs ie, CD/AUX/TUNER to rule out dirty switches.

Apart from opamps in the preamp the RA-970BX is fully discrete, no STK packs etc...
Modern day equivalent parts should be available

For distortion in one channel, and with the tools you have, the approach would be to feed a low level 1khz (or whatever)
tone/sine wave into the AUX inputs and then trace with your oscilloscope as the tone passes through the various stages.
You might get lucky, the first test may indicate dirty switches... I would rate this task as routine.

First download the service manual from hifiengine, need to register, it's safe.
Feed 1Khz sine wave into AUX and check the waveform at the left speaker post, so you know what the distortion looks like.
Next test point should be half way, eg, at the pre/power stage connection, eg, R681, check for distortion, signal will be
smaller so change volume and oscilloscope scale.... If distortion at R681 then it's a preamp/switches issue, maybe an opamp?
If no distortion at R681 then it's a power amp issue. Note if it's the top or bottom half of the sine wave that's distorted that
would point to the top transistors (eg Q619, 623, 627, 629...) or bottom (Q621, 625, 631, 633...)

Fault finding/the fix could be easy or a real PITA... If this sound like too much work then get the NAD or whatever
 
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