Help with my G&K G210 amplifier

[Sleet]

So I've been working through my inventory of amps and have focused attention on my Gallien & Kruger 210G amplifier. It is misbehaving in a variety of ways. When I power up the amp it begins to squeal and feedback without a guitar plugged in.

I decided to scope the output and used my signal generator to input a sine wave signal into the inputs. I've attached some images of the scope which are, at times quite different. The overall volume of the amp seems a bit low, but I can play my guitar through it. It sounds a bit fizzy and distorted when it should sound clean.

This could be something simple that I'm overlooking. I did clean the output jack for the "speaker" (internal), that seemed to help a little. It's pretty unstable in general.

[Sleet]

Here are some shots from my scope with a sine wave input.

[Enzo]

When I power up the amp it begins to squeal and feedback without a guitar plugged in.

There, that, fix that before worrying about passing signal.

[Sleet]

You're right, I'm getting a little ahead of myself. The amp is unmodified and seemingly original. I did, some time ago replace a filter capacitor, but nothing else. Here are a few pictures of the amp internals.

[glebert]

Since you have a nice scope and seem to know how to use it presumably you have some electronics skills.

The Input A jack is supposed to ground the input with no guitar plugged in, I would start by making sure that is happening. The preamp out jack is TRS and also acts as a power amp in. You can short out both tip and ring to the shield and see if that stops the squeal and feedback. That will help determine if it is a preamp or power amp problem. Then I would start looking at the preamp out signal, measuring voltage rails, etc.

With any amp it is also a good idea to try bypassing the effects loop switchjack(s) to make sure a bad contact there isn't causing problems. For this amp you would do that by shorting the tip to ring of the preamp out.

[Sleet]

glebert

Thanks for the suggestions. I will take a look at the input jack and output jack for the preamp. The amp is crusty, so that could certainly be causing the problems.

[g1]

If the amp is crusty, check for corrosion at surfaces where pots and jacks bolt up to the chassis. Like the star washers if it uses them.

[Sleet]

Update. I've taken the front panel preamp section out and checked for corrosion or loose jacks and pots. Everything seemed okay. I dripped some deoxit 5 down the shafts of all the pots and cleaned the inputs jacks. Reassembled with no real improvement.

I did notice that when turning the power limiter control to above 5 caused an audible noise in (I think) in the power amplifier section. When lowered to below 5 the noise would diminish. At the time I had the speakers disconnected, but the external speaker output jack was connected to an 8 ohm dummy load. Should I look into the power amp board?

[Sleet]

I've decided to look at the power amp board and do some basic measurements. The schematic has a note to adjust the power transistor bias (not sure about this description) so that there is a 15 mv reading across R18 and R20 using the the pots at R10 and R13.

My measurements across the resistors was high to start with around 60 mv for each resistor. The best I could get by adjusting was 26 mv.

Next, I checked the supply voltages for the power amp. They seemed correct:

0 (ground)

-32 V

+66 V

+32 V

The cable connector from the preamp board were not correct:

0 (ground)

CL (not sure what this does?)

66 V

-32 V

Last pin is intermittent goes from no reading to about 8 V

I noticed when applying pressure on this end pin of the preamp connector that the voltage reading oscillates some. There is also some static sounding noise heard from within the power amp board at the same time. The other curious thing is that the power limiter control when turned to 9 or 10 causes a noise within the power amp section, perhaps overloading the power transistors?

I have yet to remove the power amp board and check the pc board for bad traces or cracks. FWIW, there are no signs of burning or overheating of the board.

[g1]

I think the line called 'C.L.' on the supply drawing is the called 'limit' on the preamp drawing and comes off the power limit pot. You should be able to get a stable DC reading there (with no signal applied). You should also be able to get a lower bias idle current with no signal applied, but that could be some kind of oscillation issue, try to get the power limit control working right first.

[Sleet]

So, I'm back again with this amp. As g1 mentioned I think there may be a problem in the power limiter section of the preamp. I checked the resistance on the power limiter potentiometer without power supplied. I resistance doesn't seem to change as I rotate the knob. It reads 109 K through out as I rotate it.