no volume on Crate G120C

[merk]

Hi all-I started this on a different thread on Schematic Requests and moved it here-hope that's OK. I did get the schematic from Crate. I started out with no volume at all and found an open switching jack at FX In (thanks to the forum sticky). I fixed that and now the amp plays fine for about 10 seconds then goes to no volume unless I turn the gain and vol all the way up. Then the note will sound with a hard pick attack then dies out immediately (and sounds bad like it's clipping). Plug the guitar into the Line In and it works fine. So I assume the problem is in the preamp. (The power supply is OK as the filter capacitors after the rectifier diodes measure +/- 29.4 vdc and the spec is 29.5.)
I read some of the archives and thought I'd try to inject a signal to follow. Haven't been able to do it yet. I get 0 ac volts across the speaker leads as well as the input jack with the DMM while strumming the guitar even during the 10 seconds or so when the amp is working. The schematic says it should be 25v p-p @4 ohmns load at the speaker. I hooked the output of an ipod to the input of the amp and also got 0 volts at the jack while the music was playing through the speakers for the 10 seconds. ??? Help. I'm sure you guys could fix this in 10 minutes but hey at least I'm learning. Thanks

[g1]

Can you post the schematic?
Do the line in & out jacks switch the signal the same way the FX loop jacks do? If you put a patch cord from line out to line in does it work?

[merk]

schematic

The patch cord did the same thing-worked for about 10 seconds then the volume faded out. I plugged an ipod into the input jack on the amp and followed the signal. The results:
IC 1-pin 1 had the signal on both channels A & B, pin 7 had a signal on Ch. A
IC 2-pin 1 had a loud signal on Ch A & B, pin 7 had signal on Ch B
IC 3-pin 1 had loud pop on Ch A & B, nothing on pin7
IC 4 & 5-nothing on pin 1
Also took some dcv readings-J5 had all 0v except for pin 12 had +28.5 and 13 had -28.2
IC 2A-pin 1 3mv Ch A
IC 1B-pin 1 8 mv and feedback Ch A
IC 3-pin 1 +15.07 Ch A & B
at A (by R24) Ch A=-15.3 and Ch B= .14

G120C & CXL Schematic.pdf
Hope the schematic is readable.

[merk]

Can any tech look at my posted schematic and give some advice?

regarding interrupted signal per the previous posts on this thread? Would it be in between IC 2 and IC 3? Any specific components in that area I should check? I don't know enuff to decipher the schematic so I'll just keep testing components to see if I can tell exactly where the signal ends. Don't really want to start randomly replacing stuff. I sure could use some direction from the old hands here. This forum is pretty cool.

[merk]

Update-after the signal drops off if I then touch the probe (from the ipad) to R31 (in between the treble tone control and IC 3 the signal comes back and I can play the guitar for 30 seconds before it quits again. Touch R31 again and I'm good for another 30 secs. I replaced R31 and it didn't help. (I know, I wasn't going to throw parts in there in my ignorance but I couldn't help myself.) Am I getting close? I have a capacitor checker but I'm assuming I would have to remove a capacitor to test it?

[g1]

Are the IC's getting proper supply voltage? (+ & - 16V at pins 4 and 8)
If they are ok then I'm a little suspicious of Q5 (lower left of IC3), I would try removing it and seeing if that makes a difference.
Sorry I can't offer more, but I'm having problems dealing with this schematic.

[Enzo]

Touching a probe to R31 wakes it up a while? SOunds to me like that input is allowed to charge up to something that pinches off the signal. Your meter impedance completes an open circuit to ground that drains off that charge.

The only DC path to ground from there is through the tone stack. SO is the MID or BASS control open? Or their solder cracked?


Power off, measure resistance to ground from R31. DO you get resistance that seems like what those controls add up to? Or is it open to ground?

In other words, the schematic says fully CCW is the zero ohms setting for BASS and MID controls, so from the wiper of the treble, with treble at max, to ground you should get the value of the treble control, which looks like 50k from here.

[merk]

g-one both IC 2 & 3 have approx. +/- 15.5 vdc on pins 4 & 8

enzo-with power off, R31 to ground is open
TREBLE from wiper at max rotation to ground is open
max rotation to end of winding is 0 ohms
min rotation to beg. of winding is 48k ohms
MID wiper and end of winding are grounded
wiper to beg. of winding tab is open (1.8 M ohms) fm min to max rotation-no change

solder looks OK (for what that's worth). All pots show typical increasing resistance when turned except for MID-can I remove it to make the amp work (until I can replace it later)? The 3 tone controls don't really seem to do much (altho I haven't heard the amp when it's working properly). I can still touch R31 repeatedly and get the signal to start back up...

Thanks very much for the response guys, I'm feeling hopeful again about this thing being fixable.

[g1]

All pots show typical increasing resistance when turned except for MID-can I remove it to make the amp work (until I can replace it later)?

You don't even have to remove it, just solder a jumper wire across the outside legs and see what happens.

[Enzo]

As I suggested, you have an open DC path to ground. From the treble control, the path to ground is through the MID and BASS controls. If those two controls THEMSELVES are OK, then a connection between them is open. Either a good looking solder joint is still defective, or you have a cracked/broken pc board trace connecting them together.

The controls will of course have little effect - their circuit to ground is open.


Sit on a kithcen chair. Set your amp in front of you. Now reach out and lift the amp by the handle a little. Now try it without putting your feet on the ground (or anywhere else.) Chances are as you try to lift, it will just want to tip you forwards off your chair. Without a solid ground connection to push against, your body can;t do the job. Just like this circuit.


Removing a bad control won;t make it work, an open control is the same thing as a removed one.

I agree a jumper wire across the control would tell you if the rest of the amp works.

[merk]

A BIG THANK YOU to Enzo and g-one for diagnosing my amp problem. You nailed it Enzo the MID pot was open. I put a jumper across it like you guys suggested and that fixed it. The amp is back in business and sounds good. I'm totally impressed and happy to find this forum with all the great info and people. Funny thing, you guys figured out my problem over the internet pretty much immediately while our bassplayer's spare tube amp has been at the local tech's place for over 6 months and he keeps saying he's gonna get to it ...soon. Anyway, we have practice tomorrow so the guitar player/owner will be loving it! Thanks again guys.

[Enzo]

Oh well, of course. Online, we do things immediately. But if you bring it to me, I am obligated by tech secret society rules to wait six months before looking at it.