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

[ntehicks]

I was given this Stereo amplifier. I did some searching on Google and thought it was one of the two STK-0080 so I purchased one and installed it. I still get the same small static sound out of the left channel. I have worked on several flatscreen tv's but haven't done much in the area of classic audio. any help would be greatly appreciated thanks.

[budm]

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

On the back of the unit, does it have the Power amp input/Preamp output RCA jacks with the U jumpers?
These units are known to have bad/noisy transistors in the pre amp sections, the legs of the transistors are blackened due to oxidiation.

[ntehicks]

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

no it doesn't have the u shaped power out pre in. the static was from my input. I was using my cell phone and it was causing the noise. so I get very faint sound out of the left channel. I thought I would rule out the STK-0080 and put the one that is in question in the side that works. does anyone know where I can get a schematic or service manual.

[ntehicks]

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

Ok so I swapped the two STK-0080 and both chips are good. I found that the signal that feeds the amp one is much louder than the other. does anyone have a schematic that would show me the pre-amp?

[rhomanski]

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

The first thing I do is clean all the switches and pots. That's fixed a few problems. Use Caig Deoxit D5, spray it in the switch or pot and turn or flip it several times, probably about twenty times. Do this about three times, wait about a day between cleanings. Then follow up with some Caig Shield to replace the lubrication stripped out by the Deoxit.

[Radio Fox]

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

One good thing about stereo amps with one failed channel is that you always have the good channel to compare it with.

Sometimes, when you don't have a service manual available, you just have to do things the hard way & work out the route of the signal yourself, working your way through the circuit comparing the 2 channels until you find a difference.

[ntehicks]

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

I found a service manual it shows a af amp ic njm4558. does anyone know the best way to trouble shoot this chip.

[budm]

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

Do you have any signal generator? Did you already try cleaning switches and pots yet?

[redwire]

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

With the STK's, I find internal transistors go bad or the IC bonding wires get loose and tapping them with a screwdriver can flush that out.
Does this amp use a voltage amp (i.e. STK3042, STK3082)? It's usually one module that drives two channels, and it can fail. The venerable 4558 is usually in the protection circuit.
Here is the internal circuit of the STK0080 and STK0080II family:

[ntehicks]

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

I have tested by removing the stk-0080 and swaping the chips around. I get the same results. I have tested the audio that is fed to the input on the amp board one is significantly lower than the other that is why I think it has something to do with the pre amp. I have tested the VR Main volume pot and It test out perfect on both sides. The one thing I found that has me thinking I need to dig into the circuit that is just before the volume is that two wires on the volume board were touching each other.

[ntehicks]

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

The reason I think it is the 4558DX chip is in the service manual it shows a preamp between the main volume control and the STK0080 power amp. in the picture with the chip the green wire comes from the main volume control and on the other end of the board the white and silver wires go to the main amps or stk0080's
I have included the pdf file that is in the service manual to show the diagram.

[redwire]

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

One technique I use is to inject 1kHz sine wave into both channels (tape/aux) input, and use my multimeter on AC millivolts and compare the reading between both channels. I start at the balance potentiometer and volume control. This way is easier to find a few dB difference- which stage might be lazy (pre/pwr amp etc.)
Otherwise you could compare DC voltmeter readings too. The 4558 is just a generic dual op-amp you could swap out, if it's in a DIP8. I find any newer op-amp sounds a little better I.e. LM833

[ntehicks]

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

I replaced the OP amp but still have very low signal. I did put a low signal on each amp circuit and both STK0080 are working. so it looks like I will just have to trace back the channel starting with the RCA inputs.

[cadiman]

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

I normally pull the op amp after confirming outputs and signal trace (even with an old cheap scope) to check preamp in signals. All 80`s stuff is re-solder even on the output transisters. stk outputs were the good 80`s it got much worse from there.

For sure clean all input switches and contact switches as these were mostly the reason for preamp issues. no or low signal input.

[ntehicks]

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

so I started working on this project again. I went through and cleaned the switches and pots. I was testing the main volume control and did get some weird readings. on left channel the pot is normal 0 - 170k but on the right channel I get 0 - 56k. so I removed the pot from the circuit and tested it and it test good 170 on both. so I must have something that is pulling down the circuit. not sure where to start testing. I have included a picture of the pot. as well as the board that the pot wires goto.
when I test c03 and r03 in circuit I get 96k when I test c04 and r04 in circuit I get 98k not sure of that would be far enough off.

[Agent24]

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

Is there anything in parallel with the pot?

[ntehicks]

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

here are some drawings of the circuit.

[Agent24]

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

Doesn't really help though since it's for a different model...

Can you post a clear photo of bottom and top side of the board your volume pot connects to and show where it connects?

[ntehicks]

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

ok here is some top and bottom pictures.

[Agent24]

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

In regard to your schematic notes - What about the board you said goes between the pot board and the tone control board? If that is where the pot connects to then something there is more likely to be affecting the resistance reading.

Did you check back through the tone board and other board to the pot board to find out where the signal becomes lower?