Tek 7904 scope

[Gordon JC Pearce]

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