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June 29, 2022, 12:06:24 AM

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1983 Fender Showman Channel Switching/Noise Issue

Started by DJ5D, February 11, 2021, 05:06:20 PM

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I have a 1983 Fender Showman, a solid-state amp from the Rivera era, and it has an interesting problem.

On the front panel, Channel 1 works great. However, I cannot switch to channel 2.

So I tried plugging in the footswitch to see if it was a bad switch on the front panel, and this is where it gets interesting. When I plug in the footswitch and hold down the Reverb button, I CAN switch channels, and play thru it with all channel 2 controls working, but only as long as I hold the Reverb footswitch down. As soon as I let it go, nothing.

So the switches themselves appear to work but somewhere there is a problem. The only thing i have done so far is to replace the input jack with a generic alternate- I'm not sure if that would affect channel switching or not.

On the schematic you can see where the front panel switches and the footswitch paths merge, so i have to assume that the issue is happening on the preamp board and not the logic board (but honestly i have no idea)

I'm attaching link to a PDF of the schematic- if anyone can help me trace the signal path and recommend some possible solutions that would be greatly appreciated!


So far I have tested IC's 1 (the schematic seems to indicate this is involved with channel switching) with a multimeter in diode mode. I don't know if this is useful info or not, i dont have an o-scope and a multimeter is all i have.

Here is IC 1 and the values I got:

Hopefully someone can give me an inspiration or two on this. Thanks in advance!


Google is telling me the schematic file you linked does not exist.

Found this thread: https://www.ssguitar.com/index.php?topic=3605.msg27527#msg27527


The first thing to do is to make sure all those grey colored ribbon cables are making connections from each end to the other end. I would literally check for continuity between each wire on all of those ribbon connectors. They tend to bend and wear out badly. What happens is that the connections become unreliable and introduce many problems between each board's connections. Only after making sure that these ribbon cables are making good connections on each wire can you expect to know they are not the main problem at the start. I attached a few pics of the ribbon cables that I am referring to on that amp.

Update: Make a note that Pic 2 and Pic 3 show a different ribbon cable. I took another cable I had and soldered the each leg to the corresponding pin. I left the ribbon connector on the logic board side and soldered the wires directly to the preamp board. This was the most important factor when fixing this amp in my opinion. There was no way to depend on that original ribbon cable after I ohm out each connection on each wire only to find the connections were failing. Also, note that the original ribbon cable in that spot was actually white.
"A person who never made a mistake never tried anything new." -Albert Einstein


I updated the link to the PDF.

To the best of my knowledge I have confirmed that all the connector cables are working (they have been replaced) but i have not confirmed that the pins they are attaching to are actually making a connection to the board. Some may have corrosion or wear.

That will be my next step!



I see that you soldered an orange wire directly to IC2.

Can you explain this?

EDIT: nevermind, I saw your other post, I wonder what they heck they were trying to do?


I think that whoever had opened up the amp before me was trying to figure out if the TL604 or TL607 chips were a problem. I got the amp that way and made for a confusing moment indeed.

Side Note: If you are looking for replacement TL604 or TL607 chips I found a good reputable source. I bought a few to have on hand after this repair. I made a test setup to make sure the analog switching was working and that they were actual genuine chips. The chips passed the test with flying colors.


"A person who never made a mistake never tried anything new." -Albert Einstein


And another alternative:   Peavey used a TON of TL604 in the 1980s, and when TI stopped making the chip, PV designed a small drop-in board to replace it.  They sell them through their parts department.


Thanks, this info about the replacement TL604s & 607s- immensely helpful.

I was planning on replacing all capacitors as well, I was asking someone at amprepairparts.com for a kit, and the told me that the 11000uF/60V capacitor was no longer being manufactured. I'll likely need to find that one as well if I want the job to be a complete restoration.

Hopefully within the week i'll be able to post a follow up with how things are going.


You can spend a fortune trying to match the original caps, especially if they are screw terminal type. 
There is no point in wasting that kind of money.  The modern common equivalent would be a 10,000uF 80V snap-in type.


Success! Well mostly. Channel switching now works.

Thanks for the recommendations on replacement parts!

When you replace cabling, you really have to replace ALL of it. The wiring, the connectors, the sockets, every bit of it. Also IC7 and IC8 had to be replaced. (CD4013BE).

So it's in a working state now, as far as channel switching goes, but there is one last issue...

There is a light distortion tail when playing clean, accompanied by intermittent crackling and pops. This happens on both channels- I'm hoping the last bit of cable/socket replacement will take care of it but I fear its in the caps.

I'm going to replace them all regardless, but does what i'm describing sound like a capacitor problem?


Here is an audio clip of the noise, if this helps anyone to ID an obvious problem.

The noise is happening regardless of whether anything is plugged in, knob position, i can even have the volume down all the way and it is still audible.


Anyone familiar with this and its cause?


Try plugging your guitar into the power amp input on the back,, that will (normally) disconnect the preamp circuit from the signal path.
So if that stops the noise then the main power supply and power amp power amp is likely ok and that tells you the issue is in the preamp.

Let us know what you find.

Could be a dying cap,, most likely in the preamp.


Thanks Phil.

Yep. Plugging into the power amp input, signal clean as a whistle. Nice way to quickly eliminate suspect components.

Next step replacing all electrolytic caps on the preamp board, and the logic board as well, for good measure.

16 in total:

1 16V 100uF
2 16V 10uF
7 25V 10uF
6 50V 1uF


well things did not go as planned.

When getting new caps i was given a different variety, they assured me that they would work fine despite having higher voltage ratings but the same uF.

So when putting them on, I had to guess as to the polarity of some because most of the original fender caps had no polarity markings on them. So i had to guess that the orientation was correct based on the few that were marked on the circuit board.

Nothing blew up, the channel switching and all buttons still work, i can hear the EQ change when i move the sliders, but alas NO SOUND from the input jack. The external input on the back still produces sound but channel 1 & 2 are not producing signal to the outs.

Any suggestions? How can I make sure all my cap polarities are correct?

EDIT: i have sound on channel 1 including EQ and Reverb. The switches are working but going to channel 2 no sound. right back where i started, almost.


No never remove Elcaps if you don't know the polarity. :duh
If they are part of supply they will certainly blow.

Just replacing parts without First establishing what has failed and WHY is just going to lead to tears.
You would need a signal injector to find what part of the circuit has failed but as the switching circuit is on the blink as well then it's likely that won't help much.
All I can say is
This is a rather complex circuit and maybe you should just take it to a repair service.