Tonepad Rat huge output drop when stacked after other Rat pedal … grounding..?

[nF]

Hi, yet another Rat topic...
Just built a Rat according to Tonepad schematics/pcb.
It seems to work pretty good except for slight different behavior of the tone control (likely just the pot raper..?).
Except ... when stacked back to back after a ProCo The Rat from '91 for comparison purposes when both engaged volume drops to 'almost' zero. the same does not happen with either of them im bypass or with The Rat second in the chain...

would it be a ... ground issue..?

[GibsonGM]

Hi and welcome, nF! 

What happens if you turn the volume way down on the Rat feeding the suspect one - does anything change?

Can you post the actual schematic you built from?

[nF]

hi, and thank you for having me.
the schematic is the one from tonepad's site, I attach it here:


pedals need to be both engaged so, in a configuration old -> new rat, choking the volume on the old one effectively  cuts all signal.
but more importantly, at a second inspection at decent hearing volumes, my build has a very faint high pitch when engaged even if nothing plays through it. the high pitch disappears when unplugging the input jack (some... stray DC in the signal path..?)

EDIT:
while building it I happened to find here and there ~1.3V in places I thought there shouldn't be any. But that  behavior was inconsistent, at times would disappear, hence hard to trace.
Anyhow, I imagine the DC readings on the IC should be, normally:
1- 4.5V
2- 4.5V
3- 4.5V
4- 0
5- 0
6- 4.5V
7- 9V
8- 4.5V

and on the Transistor
G- 0V
S- 9V
D- ~4V

[antonis]

Welcome, also..

pedals need to be both engaged so, in a configuration old -> new rat, choking the volume on the old one effectively cuts all signal.

Nothing strange here..

at a second inspection at decent hearing volumes, my build has a very faint high pitch when engaged even if nothing plays through it.

I'd try making 100pF cap (the one across Distortion pot) 150pF to 220pF..

[nF]

hi, you mean the one in the feedback loop?
ok, may I ask why?
thank you!

[antonis]

To prevent high pitch oscillation..

https://www.allaboutcircuits.com/technical-articles/miller-frequency-compensation/

[nF]

fantastic, thank you for the article.
at any rate, that's likely issue n.1 ... I guess either a cold solder joint of a dead cap altogether... it seems to me that at the moment the one in place behaves as being out of circuit...
wouldn't know whteher that affects the volume drop with pedals in chain yet ... first things first

ps
this verification here under is killing me ... 

[MrStab]

What voltages do you read at the input and output of each pedal? On the tip connections? Should be close to 0V.

[nF]

ok, interesting.
the readings all all pretty much 0V except for the input tip of the pedal upstream in the chain (the old rat).
there I read a variable 100mV but only if the new pedal downstream is engaged.

[GibsonGM]

So to be clear, you read 100mV at the input of the new build?  Is it steady?  Leaky input cap.  But that doesn't seem to be what would cause your 'problem'.

I'd audio probe the new Rat while inputting a signal thru the old one into it, and see where this volume drop occurs, and at what volume (of the old Rat) it does.

If you WAY overdrive the input of a circuit at the  voltages we commonly use, you will run out of headroom and the signal won't get any louder; it will be maximum clipped, maximum fuzz.   But that doesn't sound like what you report.

[nF]

thanks again.
ok, for clarity I'll call the old rat just as such (rat) and my pedal just 'clone'.
the chain order is rat -> clone.
both pedals are run with distortion gain at a minimum (5-10%) and volume output at 80-85%.

I did read 100mV at the input tip of the rat, it was variable but behavior is erratic and now seems to be gone.

I swapped the 100pf in the opamp feedback loop for a 220pF one but the high pitch resonance is still there.
there is no resonance when no input jack is plugged in the clone.
EDIT: hooking the clone to a separate power supply removes the resonance, it seems (I use a Mosky ISO 10 at the moment).

more importantly, though, I found out that the volume drop with both pedals engaged only hapens when the volume of the upstream pedal (rat) is less than, say 95% in output. when the volume pot of the rat is anything less than cranked the volume drop appears again (bad volume pot in the rat..?)

[MrStab]

100mV definitely quite low, in so far as causing issues like this - I was wondering if it was being bypassed altogether!

Does the 95% volume thing happen if the old Rat is going straight into the amp?

[nF]

no, it doesn't. but that pot is indeed kinda 'touchy'.
so it might be that it is going/it is gone bad and letting weird amounts of ... uh ... current through when not fully cranked..?

as for the resonance, I was thinking maybe something in the filtering of the power supply in the clone is not doing its thing? leaky cap..?

[MrStab]

no, it doesn't. but that pot is indeed kinda 'touchy'.
so it might be that it is going/it is gone bad and letting weird amounts of ... uh ... current through when not fully cranked..?

That's not really a thing. But something that is a thing is that old Rat pedals use this (2nd diagram), which - in normal circumstances - connects its Gate to the effect output when bypassed:

http://www.muzique.com/lab/true_bypass.htm

I'm pretty starving ATM so can't think of anything beyond that. Maybe check to see the footswitch is functioning properly in the old Rat. You haven't really mentioned any LED issues, though.

[nF]

hi, tnx again.
no, the rat I own is the 1991 reissue, no led on that one (and usual old wiring, I think).



anyhow, I mentioned the behavior of the volume pot on the rat because I'm trying to isolate the issue to either one of the pedals. I started thinking the problem was obviously in the clone but the interaction with the level set in the volume pot of the rat makes me think that the issue is there.
There seems to be a sudden 'bump' in the last 5/10% of the volume pot of the rat where the volume drop in the clone disappears the more you open. So I was wondering maybe a corroded track inside the pot itself..?

[MrStab]

I'm hesitant to assign blame solely to the old Rat's volume pot when the issue doesn't happen when that pedal's plugged straight into the amp, but a spot of contact cleaner can't hurt.

If the clone works fine with other, known-good pedals on either side of it, at least you can probably rule out issues with that.