Amp cutting out, dependant on "attack"

[zepromz]

Hi, I'm new to the forum, but I hope someone can help.
I have had this problem on a few solid state amps in the past, the current one is a Marshall "Reverb 12" (Model 5205)
This is with a standard humbucker equipped guitar going direct into the amp. Guitar and cables verified 100% good. This problem happens at any volume or gain setting.
Plug guitar in, play for a few seconds gently, everything is fine. However, if I hit a big chord or start playing aggressively, the amp has a fraction of a second at normal volume, then the volume drastically reduces when I smack it, and limps along with hardly any gain. Unplug, plug in again, its fine until I hit it hard, then it happens again. Something is cutting out under stress, regardless of whether the amp has been turned on 10 seconds or 1 hour. I have plugged into the headphone output and the same thing happens, so it is presumably a preamp issue. I have checked and cleaned the input jack surfaces and checked for dry joints, had the PCB out and gone over that really carefully for obvious problems, there were burn marks round the zener diodes on the +/- rails (had this before on small combos), so I changed them for higher wattage ones to be sure, but the problem still persists. Caps look good, no bulging or obvious deterioration. This is probably something stupid, but I have limited experience with solid state stuff, and would really appreciate some pointers as to what might be causing this, as I have had quite a few solid state amps do this to me over the years. This Marshall is an incredible little bedroom amp, and I'd really like to fix this myself if I can. Is there a common cause for this sort of behavior?
Many thanks, and sorry for the long post!
Pete

[Enzo]

Hi.

First a note on zeners.  They do run warm, and their dropping resistors too usually.  If you replace a 1 watt zener with say a 5 watt zener, your dropping resistor may not allow sufficient current to insude the thing "zenes."   It probably will never bother you, but zeners rely on a certain amount of currne flowing to maintain a steady voltage.  The higher the wattage the more current needed for this.

Make it do it.  Now you have the symptom.  The symptom is the most valuable troubleshooting tool you have.  Don't turn the amp off.  We know power cycling will bring it back, but leave it in this state.  Ball; up your fist and whack the top of the amp.  Does that make it wake up?

Isolate the problem.   When it is like this, turn the reverb up partway and rock the amp to crash the reverb springs.   Is that loud and normal or is it also diminished?   The reverb return is after the preamp, so it tests the power amp.

SInce it does this in the phones, I am not sure how you decided that was a preamp problem.  The phones are driven by the power amp.  Whatever comes out the speaker should also come out the phones.  The problem could be anywhere.

My schematic is fuzzy, but in the power amp, bottom center, is R38 (I think) and C24.  Whatever number resistor joins C24.  It is 10 ohms.  Make sure it is not open.  If it opens, then the amp can break into oscillation.  And that will sap the power in large manner.  Those two parts are your stability network or "zobel'.

Is the reverb up inside the chassis or is it a regular reverb pan with the two jacks on the side?  You can play into the input and see what shows up on the reverb drive to the spring unit.  Likewise the return from the pan is a convenient point to inject a signal.  Divide and conquer.

Got a scope?   If not, got some other amp handy?   Look up "signal tracer" and make one.   It amounts to a probe with a series cap plugged into some amp.  You then probe a circuit with signal running throuhg it, and the tracer lets you hear what is going on at that point.

And look up "RF probe" - a very simple thing made of a diode, a small cap and a resistor, and you use it with your meter.  Your volt meter will not respond to high frequencies, but this little circuit lets you measure levels of RF voltage.  If the amp is oscillating, that is what you have at the output - RF.