Low plate 124-style, signal bleed in OD normal?

[jph118]

Hey guys, been doing my own take on the 124 circuit with a few tweaks, and while I’m incredibly happy with the tone both clean and overdriven, I get just a touch of signal passing thru with OD Level and Ratio controls set to zero... seems to be coming from as far back as the clean Volume, goes away if I lower the OD trimmer to zero or if I switch back to clean. Turning up OD Level increases it. Pots are all CTS. I’ve checked and rechecked my lead dress, using shielded coax for all pot runs, moved wires while observing signal on scope, and no solution. Any general ideas?

[pompeiisneaks]

Since it's your own take, probably would be best to show a schematic so we can see what may be causing it circuit wise?

Photos help too.

~Phil

[jph118]

I don’t have a full schematic handy, and will snap a few pics later, but from input thru OD stages I’m going by the book... after that, I have a clean Master 1MA pot that only switches in when OD is bypassed (removing this had no effect on signal bleed). If I ground the signal at either OD pot input, the bleed vanishes... could this be a grounding issue, even if all ground points show solid continuity on the DMM?

[pompeiisneaks]

It could be that the pots '0' setting isn't truly 0 and thus when you create the ground that is a dead short it works. I'd check the pot resistance between wiper and ground with them all the way to where it's barely bleeding back. If so you might want to replace the pots or something similar? Maybe just clean them a bit?

~Phil

[alkuz1961]

Try this experiment - take a small metal plate, solder a long wire to it, and then isolate this plate well from a possible electrical contact. I use a wide masking tape. Connect the other end of the wire to the ground. Move the plate between the components and listen for the effect of this on the signal transmission level. Sometimes closely spaced components can affect each other at high gain stages.

[Charlie Wilson]

If the clean / OD relay is wired correctly, the OD should be grounded out when in clean mode.
CW

[jph118]

If the clean / OD relay is wired correctly, the OD should be grounded out when in clean mode.
CW

Yes, everything is fine in clean mode. This minor problem only occurs in OD mode with pots at zero. I’ll be trying the ideas above tonight or tomorrow.

[jph118]

Try this experiment - take a small metal plate, solder a long wire to it, and then isolate this plate well from a possible electrical contact. I use a wide masking tape. Connect the other end of the wire to the ground. Move the plate between the components and listen for the effect of this on the signal transmission level. Sometimes closely spaced components can affect each other at high gain stages.

This did the trick! I didn’t have metal, but took a small piece of cardboard, wrapped it in shielding tape, wrapped that in electrical tape (left a corner exposed for alligator clip), and found the culprit... alongside the final OD stage was a voltage divider, and the weak side of it was picking up the OD signal. Placing this grounded shield in between reduced the bleeding signal from 240mV to 38mV on my scope. Thanks so much!!

Now, to determine a permanent solution... I suppose I could fashion a more permanent shield out of wrapped aluminum, or just alter my layout.

[erwin_ve]

Try this experiment - take a small metal plate, solder a long wire to it, and then isolate this plate well from a possible electrical contact. I use a wide masking tape. Connect the other end of the wire to the ground. Move the plate between the components and listen for the effect of this on the signal transmission level. Sometimes closely spaced components can affect each other at high gain stages.

This did the trick! I didn’t have metal, but took a small piece of cardboard, wrapped it in shielding tape, wrapped that in electrical tape (left a corner exposed for alligator clip), and found the culprit... alongside the final OD stage was a voltage divider, and the weak side of it was picking up the OD signal. Placing this grounded shield in between reduced the bleeding signal from 240mV to 38mV on my scope. Thanks so much!!

Now, to determine a permanent solution... I suppose I could fashion a more permanent shield out of wrapped aluminum, or just alter my layout.

Could you make a pic of the area you found the unwanted coupling?