Help debugging overdrive vintage fender amp

[StrummersOfThunder]

I have a bunch of decent replacement caps but I really don’t want to rip out the blue caps if I can avoid it.

Yep cap testing would be the ticket. I might look into that
What’s a megger?

I don’t mind lifting the ground leg tbh

A megger generates high DC voltage to test "insulation resistance". The old one's were hand cranked! My one looks like a multi-meter, runs off batteries and you can select testing voltages of 100V, 250V, 500V or 1000V. You don't exceed the cap's voltage rating. You test the cap out of circuit and if it's resistance breaks down the meter swings and gives a measurement in ohms. A perfect capacitor would have infinite resistance at it's rated voltage. It's handy for power filter caps which are usually connected to ground.

Excellent
I think my sparky mate has one of those !
So how do you go about testing them in circuit?

[MikeC]

In circuit:- with the amp powered on check for DC with a multi meter on the side where there shouodnt be any.

[StrummersOfThunder]

In circuit:- with the amp powered on check for DC with a multi meter on the side where there shouodnt be any.

So with that , and the megger test does an esr meter add anything important ?

[MikeC]

Have a read or Google capcitor testing. ESR won't help you find a leaky coupling cap.