Crate GTD 120/212 Problem - Treble Changes Volume

[Hicks86]

Im having an problem where the EQ High knob is adjusting volume of the entire channel on my Crate GTD 120/2x12 amp. The crunch channel and the high gain channel share the same 3 band EQ, and now when I turn up the high / treble knob it turn the volume of the whole channel down. I can’t find any information on the web on how to fix this. Anyone out there have some insight on this issue? This is a 3 channel, solid state amp. Clean w/eq, crunch w/eq, and a high gain channel. The clean channel in unaffected. Anything helps! I dont want to trash this thing. I like it. Thanks!

[DrGonz78]

Could be something wrong with the treble pot. To me it sounds like the wiper loses contact at some point of rotating the control. All of the signal flows through that wiper connection. So it just might be dirty or it is truly faulty and needs to be replaced. It also could be C56 either has bad solder joints or is bad itself. Here is the schematic.
GTD120-212-1.pdf

[Hicks86]

Awesome, thank you for the info. I've taken the pot out and tested it and it seems to be okay. I can't get the same one anymore anyhow since Crate is no more. I'll clean it up and put it back in then check out the C56. Thanks for giving me something to run with!

[glebert]

It also could be C56 either has bad solder joints or is bad itself. Here is the schematic.
[ATTACH]n912574[/ATTACH]

Clean channel is unaffected, so C56 seems like it wouldn't be the culprit.

Do the low and mid frequencies seem OK (hard to tell sometimes on a distortion channel)? When you move the knobs on those does the tone change?

[Helmholtz]

Confused. Aren't we speaking of P5? Then why C56?

[Hicks86]

I can't remember if the high and mid channels are working in relation to tone (I think they are, been apart for a while), however they do not affect the volume as the high knob is.

[g1]

Confused. Aren't we speaking of P5? Then why C56?

Yes, I think DrGonz78 got the channels mixed up. C30 should be the suspect.

[DrGonz78]

Sorry guys thanks for the catch. Yeah I was rushing off to work and read the schematic way too quick.

[Hicks86]

Thanks everyone! I'm going to put the P5 back in and then pull out C30 and check it out!

[vintagekiki]

Confused. Aren't we speaking of P5? Then why C56?

P5 and C30 (.0047) treble in distortion channel
P10 and C56 (.0015) treble in clean channel

[Hicks86]

Okay, I took out C30 and tested it with my digital multi meter. Its not an electronics multi meter, but an electricians meter. The value adjustment is automatic. If I check the two sides of C30 the resistance is gradually dropping. It should be going up right? It started at about 1.6, and then slowly decreased. I am no electronics expert but this means that the capacitor is bad right? If that is the case, are there any suggestions on the best place to order a good replacement? I am fully aware of my naivety, so don't worry about correcting me if I am off base.

[Enzo]

Reverse your probes and see if it goes the other way.

A cap will slowly charge up from the tiny current your meter puts to it. That makes a resistance reading go higher and higher. But this leaves a small charge voltage in the cap. If you reverse the leads, now that voltage fights the meter voltage, which slowly discharges the cap, and once it gets to zero, it will start up the other direction. Also any residual voltage from the circuit can confuse a meter.

[The Dude]

I'll add: On high resistance or auto ranging scales, your fingers can have an effect on readings. Don't hold the probes on the cap leads with your fingers touching the leads.

[Hicks86]

Very interesting. Testing after discharging, without letting my fingers touch the leads, I get no reading at all in either direction. Should I go ahead and try replacing the cap? Would this one work? https://www.allelectronics.com/item/...pacitor/1.html

[DrGonz78]

You can’t say “no reading” on the meter. Does it say OL or something?