On 11/12/2015 22:38, Gareth Magennis wrote:


"Gareth Magennis" wrote in message
...

This is doing my head in.

Marhall JCM 600.
http://www.classictubeamps.com/schem...jcm600_60w.pdf


Turning the distortion channel's gain/volume/master volume combination up
too high will break into oscillation (a few kHz) above a certain gain.
This doesn't happen unless the pre-amp is unmuted by inserting a (shorted)
jack in the input socket.

As these 3 series gain controls approach the point of oscillation, you can
hear the inpending frequency that will feed back rise as the gain is
increased. (i.e. before feedback, the boosted frequency is gain dependent)

(Under certain test conditions it will oscillate massively ultrasonically.
It's probably best not to do that very often)


Another amp repairer has been inside this amp and has attempted to fix the
problem by the looks of it. He's put small caps across some
electrolytics, and there was a resistor piggy backed over the top of R4,
feeding VR5. Not sure why.
I haven't found any other "mods", but that's not to say there aren't any.


Anyway, I've tried to isolate various things to discount them, but am going
round in circles and need some ideas my head doesn't have right now.

There's some frequency dependent positive feedback going on somewhere, but
since it's a complete loop broken by muting the input, it's kind of hard to
isolate anything really.
(Actually the input jack mutes both the input and the signal at CN6)


Any quick hints or tips from anyone? (It's not the Prescence feedback
circuit, or the valves)








Come on Phil, give us a clue, I'm stuck here.


Gareth.


I assume you've swapped out valves one by one, in case there is an
internal electrode problem , not picked up in a valve tester.
Another idea, moving a piece of mumetal around in likely areas?
Do aluminium valve shields actually screen from stray, to any great
extent, rather than just for valve retainer function?