"N_Cook" wrote in message ...

On 08/12/2015 18:55, Gareth Magennis wrote:
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)



Cheers,


Gareth.

Missing or inadequate ground line?



I have investigated that to a degree.

(Typical of these amps is to have the chassis ground connected to circuit
ground via 100 ohm resistor in parallel with a cap)

I've checked the grounding around the input socket and how the signal is
then transported to the board with the valves on it etc.
Everything appears to be OK. Nobody has fitted a non insulated jack socket
or anything like that.

But yes, I suspect something like the feedback is coming via the Ground, or
lack of, somewhere.



This amp HAS been messed with, however. The soldering on the footswitch
jack was appalling, resulting in disabling all channel change functions,
which I have now fixed.
There was other work that had failed that was equally poor quality.



Thanks,


Gareth.