Radial Bones Twin City ABY - faulty output

[Zounds!]

I've got a Radial ABY pedal here that is having problems. Channel A passes signal, but it is weaker and thinner sounding than Channel B. All of the functions work as they should otherwise (and Channel B sounds fine). I cleaned the jacks and reflowed the soldering on the Channel A output jack to no avail. Are there any test procedures you would recommend or components to check? I don't have a schematic for this one and it's all SMD  For what's it worth, the pedal used to work flawlessly! Not sure what happened, as it's very solidly built and only ever used with my usual 9v power supply. This is one of the transformer isolated ABY pedals with ground and polarity switches (tried flipping all of these already of course!).

[ElectricDruid]

Have you tried swapping the inputs over just to be sure?

I know it's obvious, but that hasn't prevented me from forgetting stuff like that...

Tom

[Zounds!]

Do you mean swapping cables to rule out a bad one? I did try that, though that's the kind of thing I would overlook!

[ElectricDruid]

Yeah, I meant Channel A sounds bad so you spend ages trying to work out what's wrong with Channel A, and then you discover that when you plug whatever you had in Channel A into Channel B, *Channel B* sounds bad and you suddenly realise it was nothing to do with Channel A at all!

I can't remember how many times stuff like this has happened to me beyond saying that it's far too ?&%£$! many...

T.

[Zounds!]

Ah, gotcha! Yeah, it really is just channel A! Channel B sounds just fine.

[Zounds!]

So this pedal has two footswitches. One switches between channel A/B and the other is for Y mode (running both A and B together). Turn the Y switch off to run A or B solo. I noticed that in Y mode, I can get an identical signal from both outputs if the channel toggle is set to "B." If I switch to "A" while in Y mode, then the A output goes thin again, just like it is when I try to solo A.

[ElectricDruid]

Do both channels have the polarity switch? Someone else posted this schematic in a recent thread:

http://postimg.cc/image/xt74epxf1/

This is for an Orange ABY, apparently, but it uses a different circuit for one channel to the other (slightly different). Perhaps yours is similar?

[Zounds!]

Thanks for the Orange schematic - it's a start! And at the very least, I can say that channel B has switches for polarity and ground in its path, while channel A does not. Time to audio probe I guess...

[Zounds!]

I emailed Radial about this and they were nice enough to write me back with a tip, though no schematic. They said that both channels use the same input buffer, but Channel A goes through two MMBFJ175 switching FETs (Q6 and Q7) before the output, so the problem might be there.

I know this is hard without a schematic, but maybe someone with more knowledge of P channel switches could tell me if my voltages are reasonable (I googled it to get the basic idea of how this works, but not enough to sort this out).

The power supply reads 9.14v.

Q6
D - 0v
S - 0v
G (foot switch set to Channel B) - 0.6v.
G (switched to A) - 6v, before quickly falling back to 0.6v.


Q7
D- 0v
S - 0v
G (foot switch set to Channel B) - 0.3v.
G (switched to A) - 7v (no drop after hitting 7v).

[Zounds!]

Bump!