Pioneer SA-420 amplifier

[112stereo]

I have just bought one of these for myself as I am a fan of Pioneer products.
I was told it was faulty on one channel so I decided to take a chance.
It's been standing for a number of years, but the previous owner had plugged it in.
The right channel is ok, but the left is very low !
Playing a tablet through the aux sockets at low volume so I can turn the volume up to full the sound comes through both channels equally?
If I turn up the input level so I can turn the volume control to below half way the volume level on the left channel falls right away !
This is the part I can't explain, touching a screwdriver on each input socket the hum level is the same on both channels and fully adjustable on the volume control.from it being off to a high level.
Someone before me has changed the volume control and put in a 100k potinstead of a 250k pot. Whilst I don't think this is the problem as the right channel is ok.
Does anyone have any ideas, I don't have a scope.

[paulsherwin]

Sounds like dry joints, or a faulty speaker protection relay (if present).

[112stereo]

Thank you for your thoughts.
No relays present, the only thing I need to look at are the switches, although they all seem pretty positive on the surface.
I keep going back to the volume control area, something doesn't seem right, but I could be barking up the wrong tree.

[112stereo]

Had another session with it, yes it was the volume control!
Found an old 250k in my misc box fitted it and behold it works a treat.
The only issue I have with it now is when there is no input the right hand channel motor boats, it's probably been like it all along but just noticed it.
I presume it might be a cap somewhere which might take some finding again as no scope it might be a bit of trial and error.
Any thoughts ?

[112stereo]

This amp motor boats on the right channel except when the tape adaptor button is pressed. The plot thickends?
Why just one channel?

[Radio Wrangler]

Then I would suspect decoupling capacitors along the path feeding power to the stages that come before the tape monitor switch.

David

[112stereo]

Found it .,
When I was an apprentice I did a year on quality control and it's stood me in good stead doing repairs on tv's, video recorders and satilite receivers.
This amps components looked tatty so I decided to tidy it up by straightening up generally, touching a cap it came away from its pins and left them in the board. Replacing it like for like it now works perfectly. I had actually bridged this cap with another with no difference . Anyhow, job done now to concentrate on the Rotel.
Thank you everyone for your help and advice, I often feel just talking to someone helps resolve issues no doubt I will need assistance again and it's good to know there are good people willing to help.

[Herald1360]

Come on then- which cap was it and what was it supposed to be doing?


Schematic clip with cap shown in its circuit surroundings would be nice.

[112stereo]

C19 68pf was the damaged cap. I don't know how it still worked all I know is it did !

[Herald1360]

Interesting- it's not a decoupling cap but along with its series resistor it would tailor the open loop response of the amplifier. Without it the amp's negative feedback probably wasn't negative at some frequency and oscillation resulted.


Thanks for "closing the loop"

[Radio Wrangler]

I would have expected that to have induced oscillation above the range of hearing, but then you can get oscillations that demand a lot of current and stop themselves only to restart as thew supply recovers. This can mean a high freq instability creates an audible effect... Squegging!

David

[qualityten]

Originally Posted by 112stereo

... I often feel just talking to someone helps resolve issues no doubt I will need assistance again and it's good to know there are good people willing to help.

I agree. Thanks for the update.