The case of the disappearing reverb

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workshed
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The case of the disappearing reverb

Post by workshed » Sat Sep 23, 2006 11:23 am

So I have this old Rosac solid state guitar amp, which I actually love. I love it because the spring reverb gets this perfect surf instrumental sproing, and the tremolo has this thick presence that is almost cutting in nature. The only problem I've had with it is that the reverb tends to stop working at random intervals. I finally took the thing apart the other night, and while I had a guitar plugged in, tried to check the wires running through the reverb circuits for loose connections. What I ultimately found was an old IC that, when gently moved will create a sharp popping sound and then the reverb will either cut out or come back. I finally found a position where the reverb seems to be working, but am wondering if there is a component in this old IC that could be on its way out. Sometimes simply touching the insulation around the IC will cause the popping. The solder joints actually look and feel pretty solid all through that circuit. And of course, I really don't know a damn thing about electronics. I don't even play an amp tech on TV. Here is a photo of the IC in question. The gray wires run to and from the reverb tank, back to the reverb footswitch jack and level knob:


Image

The reverb tank is in pristine shape, connections to and from it are solid (it looks to have been made by a company that was owned by hammond).

If anyone has any thoughts, I'm all ears.

-Bret

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Re: The case of the disappearing reverb

Post by radiationroom » Sat Sep 23, 2006 2:51 pm

workshed wrote:What I ultimately found was an old IC
Image

That looks to me more like a hybrid module than a true IC. The good news about hybrid modules is that **IF** you can get a schemo of it including part numbers/values you can reconstruct the circuit on a hunk of vector board with new parts.

If you can't locate the schemo for your amp, take a good look at the rest of the circuit and draw everything out on paper as what connects to what. Trace out where the B+, B-, and ground connect to along with the voltages at various points in the circuit. THEN PUT YOUR ENGINEERING SKILLS TO THE TEST AND TAKE A GUESS AS TO WHAT THE CIRCUIT INSIDE THE MODULE IS.

Do not destroy the original circuit or remove the board, but construct a new version of the entire circuit on a separate piece of vectorboard, including your "guess" as what is inside the hybrid module. When you have verified that your new circuit passes a signal, "piggyback it" into your amp by carefully de-soldering the original circuit and hooking up the one that you just built, attaching it with a plastic strap or something else that is non-conductive and can easily be removed if need be. If your replacement circuit doesn't meet your expectations, "guess again", build another replacement circuit and repeat the process. If worse comes to worse, by leaving the original circuit board mounted in your amp, you have the option of restoring your equipment to it's original form if your alternate replacements don't work out.

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Post by winky dinglehoffer » Sat Sep 23, 2006 9:49 pm

If you're reasonably handy with a soldering iron, you might as well touch up all the solder joints where this module connect to the circuit board and/or other components. It may not solve the problem, but it's a cheap and simple first step.

Tom

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workshed
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Post by workshed » Sun Sep 24, 2006 8:59 pm

Thanks guys for the ideas... I'll see if I can scare up some free time to explore the circuit further.

-Bret

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