Intermittent Rhythm/Solo on Crate GLX1200H

[kms0521]

I've had a problem for a few years now where the amp switches intermittently between rhythm and lead, very sporadically. I took it to AmpWerks in Long Beach, CA, but they couldn't find the problem. I have a 3-switch foot switch that controls the switching, as well as turns the effects on an off, and there is also a rhythm/solo button on the amp head. Even when I disconnect the foot switch, I still have the problem, and I notice that without the pedal connected, I cannot switch between rhythm and solo with the button on the head (it stays in solo). Seems like this button might be the issue, but that is so obvious that I would think AmpWerks, who has high scores, would have found that right away.

Very frustrating and distracting when this happens and obviously can't go on stage with this.

Any thoughts from anyone. Should I get the schematics?

KMS

[Enzo]

Should I get the schematics?

Can you think of any reason NOT to get the schematic?

[garytoosweet]

Please post the schematic.

[nosaj]

Schematics in this thread
https://music-electronics-forum.com/...ad.php?t=37081
nosaj

[TomCarlos]

Schematic and Board Layout Attached.

As I mentioned in the other thread (non abandoned), take a look at page 3 of the schematic. It shows you points A, B, and C. Tell us if you get these voltages and whether or not they change when you press/depress the channel switch button.

[g1]

The schematics are missing the footswitch jack board. Would be real nice to have for this particular fault.
I would think the footswitch jack is at least the reason front panel switch doesn't work without footswitch.

Enzo, you mentioned those other drawings before, do you still have them?

Here:

There is also a sheet for teh FX card, and another sheet for just the FS jacks. I don;t think we need them, but ask if we do.

[TomCarlos]

I believe this amp uses the CFS3 (3 button) footswitch. It needs a 9v battery. Not sure if the battery does anything beyond powering the LEDs.

KMS: If G1's theory is correct, how about we start with checking the cables to make sure they are good. Make sure the tips are not loose, broken etc. Check for continuity. Next, open the CFS3 and give it a look over. All the wires connected? Jacks clean and look ok? The battery is good?

I looked for a footswtich schematic. There are some photos of DIY footswitches on the net- cannot be sure they are correct.

[kms0521]

Thank you gentlemen,

I checked all the cables and jacks (all good) and cleaned up all the switches, dials and receptacles with some electronics cleaner. Also, there is no 9V battery in the footswitch. If the intermittency is happening without the 3-button footswitch connected, wouldn't that rule out the foot switch as the problem? Unfortunately I am not an electronics technician, although I am a mechanical engineer, and not sure where to apply the probes from my simple radio shack multimeter. Was hoping there might be a simple fix. Might be best to find another repair technician and give it another shot (unless you can convince me otherwise . I do recall the prior technician said all the relevant voltages checked out fine, but worth a look again. Any recommendations in the Los Angeles or Long Beach area?

Cheers,

Kurt

[Jon Snell]

These intermittent faults can be a trial.
Do the lights reflect the issue when the floor switchbox is unplugged?

[kms0521]

Yes, the lights do reflect the issue with the footswitch plugged or unplugged. I figured out how to take the measurements. This is what I got. Wasn't sure which side of the resistor to measure, so I did both. Also I used the footswitch to toggle between 1 (solo), 2 (rhythm) and clean. Also did 1 + 2 together. Looks like an issue with B??

A B C
Resistor R71 R110 R19
Side of resistor measured left right left right lower upper
Channel A Gain 1 -0.01 -0.03 0 2.3 14.7 14.7
Channel A Gain 2 -7.7 -15.2 0 2.3 14.3 0
Channel A Gain 1 & 2 -7.7 -15.2 0 2.3 14.7 14.7
Channel B Clean 0 0 0 0.3 14.7 0

[kms0521]

I have the measurements in excel but can't seem to paste it as a table or a picture.

K

[kms0521]

Basically, channel B measured 0 to the left of R110 and 2.3V to the right for 1, 2, and 1+2 and it was 0 to the left and 0.3 to the right for Clean. Seems like this might be an issue.

Kurt

[Jon Snell]

We are close to it. Discounted foot pedal and the rest of the pre amps etc.
Consider just one part of the schematic.
I would check D25, R74 may run hot and Q22 could cause issues when warm.

If you look at the truth table and check A, B1 and D1 for errors or noise.

[kms0521]

Good point on warming. Seems like the intermittency doesn’t start for a few minutes so it appears warming is a contributor.

[nosaj]

Good point on warming. Seems like the intermittency doesn’t start for a few minutes so it appears warming is a contributor.

Warming could be a cold joint opening up then closing when cool.

nosaj