1 Mar
2007
1 Mar
'07
12:54 a.m.
Tony Duell wrote:
One of the most annying faults to trace (and we've
all had them) is the
fault that disappears when you connect a 'scope probe _anywhere_ on the
device under test. Most likeht it's a bad conneciton somewhere, but
finding it will drive you (in)sane...
Ah, the very first valve amp I repaired, which I bought from a friend at
school...
It would get progressively quieter and quieter over a period of a few
minutes, but turning the volume right down and unplugging my guitar
would make it work again. "Hmm", said my Dad, "You could try checking
the biasing around the output stage, because when it goes quiet there's
not even any hiss".
Fair enough.
Plug my cassette player in so I've got some drive, starts to fade,
increase volume, fades some more, increase volume... Measure bias
voltage between the grid of the EL34 and ground, then. Let's see, pin 5
and ground... and *LOUD*!!!
Yup, the grid leak resistor had gone open.
Gordon