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Pioneer A-331

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info@toniolo

New Member
Hi Guys,

I'm new to these forums. I'm a qualified tech who's getting back into audio repair after a few years of absence. I have a Pioneer A-331 with no sound on all input circuits except the CD and only when the CD Direct button is turned on. The sound is less amplified, the Balance fader doesn't work and the left/right circuits seem to be overlapping. After reviewing the service manual, the CD Direct bypasses all tone controls, which might be why these are not working. Speaker system B doesn't work and bass/treble/loudness don't work either.

Do you have any clues as to where I should start looking? It sounds like a power supply issue. It's a 1987 model. Should it be recapped? Are there any other known issues or points of failure for this amp?

Thanks

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Check your power supplies rails (+/-15V) for the tone control circuitry. They are dropped and zener derived from the main HV rails.

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The balance control may need a squirt with CRC/Dexoit and worked back and forth.

Speakers A/B may be wired as one or the other, but not both in such a basic amplifier.
 
Thanks for your reply. I checked the power lines and I'm getting +/-40V on the main power rail and +/-18V on the tone board. I finally found my way to the problem being the balance slider and the CD Direct switch. After some tough cleaning the sound came back. But the CD Direct switch seems mechanically faulty and I decided to bypass the switch. I have hard soldered the switch in the "OFF" position as indicated in the schematics as follows:

1736784141783.png1736784229589.png

Now I am getting sound from all inputs, but it's highly distorted like the sound from an older amplifier with no protection relay after powering it off until the capacitors discharge. Could faulty capacitors be the cause of this? The amp smells like bad caps. Does it make sense to recap it all?

Is the hard wiring of the above switch correct to maintain the CD-Direct function off?

Thanks
 
There's a set of op-amps for a makeshift pre-amp. The distortion might be from those?
My experience with that generation is seeing a few failed op IC's.
Look with a scope using a 1kHz tone, if you can. Or a poor mans signal tracer?
There is a few 2.2uf coupling caps in the signal path that could cause a similar issue.
 
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