Randall RG100es Frequency Oscillation Problem Howling Noise Troubleshoot Help

[aczfenderstrats]

Hello all,

I have a vintage Randall RG100es that has some issues. The main problem is that the amp will start to produce a loud frequency oscillation on both channels when you start to turn up the volume past 1:00. It happens sooner on the red channel. The frequency will actually sweep up in pitch as you turn up the volume knob. It is not a guitar feedback issue obviously. This will happen with nothing plugged into the input.

I would guess that this is a problem in the circuit somewhere after the volume knob, but I don't have a ton of evidence to back that up. Have already cleaned it out and DeOxit-ed the pots and jacks. I am an audio engineer with good knowledge of circuits and repair. Can anyone point me in a direction on this specific issue?

Will new output transformers get rid of some of the ridiculous hiss this amp emits?


Thank you all!

[The Dude]

schematic here:

http://music-electronics-forum.com/a...525-image.jpeg

[52 Bill]

Welcome to the place.

Which controls do this, the gain controls or the master volumes? The amp is oscillating at higher volume settings because of too much gain, bad lead dress, who knows why. You will need to start by trying to find out what section of the circuitry is causing the problem and they fixing it.

As for the hiss, try plugging a guitar into the FX return jack. Now turn the guitar's volume control all the way down. How loud is the hiss? The hiss that you hear now is being generated in the power amp circuit. Now remove the plug from the return jack and compare the level of the hiss. Any increase in hiss is caused by the preamp circuit.

[aczfenderstrats]

Welcome to the place.

Which controls do this, the gain controls or the master volumes? The amp is oscillating at higher volume settings because of too much gain, bad lead dress, who knows why. You will need to start by trying to find out what section of the circuitry is causing the problem and they fixing it.

As for the hiss, try plugging a guitar into the FX return jack. Now turn the guitar's volume control all the way down. How loud is the hiss? The hiss that you hear now is being generated in the power amp circuit. Now remove the plug from the return jack and compare the level of the hiss. Any increase in hiss is caused by the preamp circuit.

Thank you, Bill!

The master volumes are the ones which do this. Although I believe they will begin to oscillate sooner if the gain is higher.

I will do that trick with the FX loop when I get my hands on the amp again. It's in storage. Also, I meant transistors not transformers.

[vidmachine]

Have the same issue on my RG100ES XP. One of the Ceramic Capacitors on the Circuit board near the reverb connectors is microphonic I bet. I changed mine because it thumped when tapped with a tweaker tool. (yellow 472Z 1KV (.0047uf 1KV) schematic says ".005uf") The replacements I tried also thumped. Bummer right? Well I guess that part of the circuit is for the reverb and meant to amplify the signal from the tank. I am not an expert but I think that is causing the feedback.

Let us know if you make any progress with figuring it out.