Dean Markley Overlord not working

[TarheelTechinTraining]

Actually I already have both Q1&2 removed and ic2. I was trying to isolate any culprits but no luck yet. Do ya think I've got a problem in the tone stack?

[52 Bill]

Actually I already have both Q1&2 removed and ic2. I was trying to isolate any culprits but no luck yet. Do ya think I've got a problem in the tone stack?

If the tone controls all work, there probably is nothing wrong with it.

Whenever something oscillates, it can be caused by a number of things. Off value parts, bad grounding, input wiring too close to output wiring, etc. I know that you have replaced the filter caps as well as the audio IC. Have you tested the values of any other components, like the gain pot or the feedback cap?

[TarheelTechinTraining]

Well I went ahead and replaced VR4 and C5 to narrow the suspects. Still no love then like you suggested I started looking at the next components. I found R14 to be 24k though the schematic shows 10k. So I swapped in a 10k and we are headed in the right direction. The squeal is almost gone but it might just be noisey, drive at 10, but not like before. What does R14 do and would a lower than 10k value improve that hum? Thanks again Bill for your patience, Greg

[52 Bill]

What does R14 do and would a lower than 10k value improve that hum? Thanks again Bill for your patience, Greg

R14 is basically a grid stopper for the first triode section of the tube.

What hum are you talking about?

[TarheelTechinTraining]

Well when I swap R14 to 10k, it was 24k, the squeal stopped but now its a hum. The hum isn't to loud slightly annoying, probably picking up AC noise.

[52 Bill]

If you turn down the volume control on your guitar, does the hum go away? If it does, then the noise is coming from outside of the pedal. If it still hums, then the pedal itself is creating or picking up the noise.

[TarheelTechinTraining]

Well the hum was from the transformer. I twisted the secondary to the pedal and no hum. Different question, if I lower the value of C5 what'll happen? I've seen a couple schematics of this type of pedal and a few show a 12pf at C5, mine is 120pf. Any thoughts?

[52 Bill]

Different question, if I lower the value of C5 what'll happen? I've seen a couple schematics of this type of pedal and a few show a 12pf at C5, mine is 120pf. Any thoughts?

That cap is a high cut filter. It is wired between the output and the inverting input of the op amp. As soon as the gain pot is turned up from zero it sends hi frequency signals back to the input like a negative feedback loop to reduce the chances of oscillations in the stage.

Too small of a value and it has no effect, too large a value and it will change the tone of the audio signal.