Covert Pre-Amp Control not Working

[evitative]

Hey everyone,
I just finished putting together a build of the Covert but I've run into a problem. The Pre-Amp/Gain control doesn't seem to do anything at all. I can run the pot through a full sweep while playing and here no affect on the sound whatsoever. I've already taken the build apart and replaced the A1M pot, just in case it was faulty. That had no impact at all. Everything else about the pedal seems to function as you'd expect, with the exception of the Pre-Amp control. Any suggestions would be appreciated.

Edit: I wanted to add that I ~think~ the pedal is basically stuck playing maximum gain, regardless of the Pre-amp setting, but I'm not sure since I really haven't been able to adjust the gain one way or the other.

 

[Chuck D. Bones]

Make sure R7 is 47K and is properly soldered. Make sure C7 is 47pF and not 47nF. If those check out ok, here's a quick in-circuit check you can do:
1. Pedal power off.
2. Set PREAMP to 0 (full ccw).
3. Measure resistance from pin 2 of PREAMP to ground. Should be 47K.
4. Measure resistance from pin 2 to pin 3 of PREAMP. Should be 1M.

I strongly advise against removing and replacing soldered parts unless you are sure they are bad because even though these board are well made, they can take only so much soldering and unsoldering before the pads come off. There are usually some easy in-circuit tests that will prove a part good or bad.

 

[evitative]

Make sure R7 is 47K and is properly soldered. Make sure C7 is 47pF and not 47nF. If those check out ok, here's a quick in-circuit check you can do:
1. Pedal power off.
2. Set PREAMP to 0 (full ccw).
3. Measure resistance from pin 2 of PREAMP to ground. Should be 47K.
4. Measure resistance from pin 2 to pin 3 of PREAMP. Should be 1M.

I strongly advise against removing and replacing soldered parts unless you are sure they are bad because even though these board are well made, they can take only so much soldering and unsoldering before the pads come off. There are usually some easy in-circuit tests that will prove a part good or bad.

You are a gentleman and a scholar, sir! Many thanks! It actually wound up being an incorrect value used for C7. I wish I could say that I used 47nF instead of 47pF, but let's just say the value of the one I used was "Samsonite". Would you mind shedding a little light as to what led you to think that might have been the problem? Since the problem was the Pre-amp control I figured it had something to do with the topology around Q3, but I didn't get any further with it than that.

 

[Chuck D. Bones]

The PREAMP control is a variable voltage divider. It lets some or all of the signal get from the 1st stage to the 2nd stage. When you said "The Pre-Amp/Gain control doesn't seem to do anything at all," that told me that the PREAMP pot was either not completely connected or it was being bypassed. Not too many ways for that to happen. The resistance measurements would tell us if the bottom leg (pin 3) was connected to ground via R7 or if there was a short between pins 1 & 2. The only other way to bypass the PREAMP pot was with C7. C7 is a "treble bleed" that adds brightness at low to moderate volume (gain) settings. C7 is supposed to only let the high freq stuff sneak around the PREAMP pot. But if C7 is too big, it lets everything sneak around the PREAMP pot. Had you posted a pic of the board, one of us would for sure have spotted C7. I've said it before and I'll say it again: the most powerful troubleshooting tool is a thorough visual inspection. You can't solve every troubleshooting problem with inspection, but it's a good place to start. I'm an EE. Me and my colleagues have literally spent weeks troubleshooting a board only to find something that could have been discovered by a few hours of visual inspection. These pedal boards are simple by comparison. On most of them, you can inspect every component and solder joint in under an hour. A $5 magnifier head strap from Harbor Freight is all you need.

Glad you got it fixed, the Covert is a great foundation pedal. Makes a little amp sound like a big amp at any volume. I have one and I'm planning to build another one with a few simple mods pertaining to the 4PDT switch and the presence trimmer. When it's done, I'll post a description in the Modifications forum.