Fisher CA-880 Stereo Amp from 80's sound from only one channel

[ntehicks]

Re: Fisher CA-880 Stereo Amp from 80's sound from only one channel

I am amazed at how different the 800 model versus the 880 is. I started with the signal that enters in through the rca plugs. and followed that through the tuner selector switch. the switch is working good. I think it has to have something to do with the board right under the volume. I think that is the pre amp. I have looked to see if I could find the CA-880 schematic. not sure if one is available.

[Agent24]

Re: Fisher CA-880 Stereo Amp from 80's sound from only one channel

This is the preamp board you are talking about, right? The one you said you already tried replacing the op-amp on?

I assume the green wires are signal input, the white ones are output, and the red\blue are power?

I would check the power supply, what voltage you get on the red\blue wires? Is it the same on both pre-amp boards? Check the opamp supply pins 4 and 8 also, what voltage across those? is it the same on both boards?

Can you post a clear photo of the foil side of the preamp board? It would be easy to draw out a schematic.

[ntehicks]

Re: Fisher CA-880 Stereo Amp from 80's sound from only one channel

I am having a hard time tracking down what is pulling down the resistance. I have started to test OP amps I have one that on pin1 and pin7 are both suppose to be output but I only get output on pin 1. I tested one on the tone board and pin 1 is over driven and distorted and pin 7 in very weak and quiet.

[Agent24]

Re: Fisher CA-880 Stereo Amp from 80's sound from only one channel

First of all you should verify that the opamp is getting the right supply voltages. If it is not getting the right voltages it can start clipping the outputs and causing distortion.

Compare the supply on the working preamp board with the non-working one, check that both op-amps have the same voltage on their supply pins (pins 4 and 8)

[ntehicks]

Re: Fisher CA-880 Stereo Amp from 80's sound from only one channel

Red and Blue wires are 20.5 -20.5 volts and the op amp is getting 20.1 and -20.1
The green wires come from the volume VR and there is a Brown wire the goes from the volume VR to the Tone Board. The input in the back of this unit goes to the selector board and then there is a wire that goes to this toner board.

[Agent24]

Re: Fisher CA-880 Stereo Amp from 80's sound from only one channel

The voltage may be too high - datsheet says maximum +/-18v supply for that IC. Is it the same on both boards? Check your voltmeter doesn't have a flat battery?

I attached what I think is the schematic of the preamp and the opamp datasheet

[ntehicks]

Re: Fisher CA-880 Stereo Amp from 80's sound from only one channel

So I removed the Op amp to see if I could get some different readings. still getting one channel really low. sometimes the one channel that works will stop and if I pulling the mic mixing knob the channel restores. so I did some testing with wire coming from the mic plug. the red wire which is the side that has the short once removed from the toner board. the short goes away. but when you play now no sound at all will play. I have one connector on my toner board it is labeled 10 and it has no wire going to it. not sure if it is suppose to have a wire or not. it is the only connecter on the tone board that has no wire. I did find one cap that looked bad but it tested out perfect. I replaced it anyway.

[stj]

Re: Fisher CA-880 Stereo Amp from 80's sound from only one channel

so this has 2 pre-amps right?

swap the outputs going into the driver stage and see if the fault jumps to the other channel.
if it does, put the wires back,
and swap the pre-amp inputs over.
you should be able to find if the fault is before, on, or after the pre-amps that way in a few min's.

[ntehicks]

Re: Fisher CA-880 Stereo Amp from 80's sound from only one channel

ok so I swapped sides on the volume control and the other side works. I cleaned the mic pull and volume and the balance knobs and now both channels work. thanks for your help.

[budm]

Re: Fisher CA-880 Stereo Amp from 80's sound from only one channel

So it was just dirty pots and switches?