Messy bogie DC3

[Wakculloch]

I'm back on the MB DC3, and i've not a scoobie.
Does its low volume distorted thing, then when you touch the anode pads on the board to the phase splitter it jumps back to life. It then works for a bit, until for no apparent reason the volume drops and the low end distortion starts again.
Stupidly checked the voltage on the PI anodes when i got it out the cab, and can't get it to stuff up again.
It's telling me something that i cant seem to see; or hear.

Ive swapped valves, re soldered joints, tapped, prodded and blasphemed!

[lowell]

have you replaced the anode resistors on the PI?

[Wakculloch]

Just changed the anode resistors and coupling caps but no improvement.
It'll come back to life with the touch of a pencil lead on pin 1 of the valve?
It's as though something is discharging, but cant thing of anything that could cause this.
Monitored cathode voltage and bias supply but they stay solid.
Will stare at the circuit diagram until I go crosseyed.

[Steve Conner]

I'd suspect the speaker mute switch that connects to pin 2 of the PI. Either that, or the 220k grid stoppers are bad, and one of them is going open. Or it's RF oscillations, but that's unlikely with such huge stoppers. Does disconnecting the NFB cure it?

But then again, Mesa have a reputation for developing carbon tracks in their PCBs. If all else fails, I buy junk amps for cash

PS, I wonder if anyone outside of Scotland gets the "Bogie" reference

[Wakculloch]

Would dearly like to give you this junk, but unfortunately it's someones pride and joy! Pretty confident its not oscillating.
Has been chugging away quite nicely for 6 hours now. Getting impatient on it going wrong.
Been giving that mute switch the evil eye for some time.
Cant seem to find a ground reference on pin 2, on the diagram or on the amp. This seems strange.

[Wakculloch]

For the sake of completeness, and as a warning to others who get this going on, at last i've tracked down the problem, (or ignored the bleedin' obvious for long enough).
Full marks to Steve;- it was oscillating at some frequency too high for my scope. Falsely i beleived that as this was a PCB'd, high volume production amp that these sorts of problems were not gonna occur. Also my testing methods were suspect, as only the owner could set it off with his screeming feedback and my clean sound, however loud, had no effect.
Simply cured with a grid stopper at the input to the PI, and a re-routing of the NFB line which snaked all over the PCB with gay abandon.