Marshall Haze 40 shorting out?

[ricach]

Amp will play from 10 to 20 minutes then its as if something shorts out (like if you grounded out your guitar cable). The loud sound continues until the amp is turned off and is not affected by any of the volume/tone controls or switches. Shutting off the amp and back on, you then can only play about 30 seconds before it repeats. The effects out jack does not pass this sound, but the amp simulator does, so its happening in the power section. Tubes have been replaced. The amp can be on for hours without passing any sound (not playing guitar). But as soon as I pick up the guitar and play a few notes - there it goes again. So what would be suspect first?

BTW: Marshall manual sez that when the effects are turned off, and the effects loop switch is off - it does bypass the digital circuitry. So I don't suspect anything happening there.

[Jazz P Bass]

Try going in FX Return.
This should bypass the preamp.

[ricach]

Try going in FX Return.
This should bypass the preamp.

I did that but still same problem.

[Jazz P Bass]

I wonder if there is something going on in the mute relay circuit.

[tpawlowskis]

I'm not too great with tube amps, I'm actually fairly new so my advice is limited but luckily there's a considerable amount of clues here. After you play your axe for about 10 minutes it'll die, you turn it back on and itll die in about 30 seconds this time.. that was a considerable hint for me. It's obviously doing it while the amp is warmed up if i am understanding you correctly. Can a bad, warmed up tube cause this problem?

[tpawlowskis]

oh, and that's if it even IS a tube amp. Like i said, i'm very new to this whole scene still lol.. But if it is a solid state amp, i don't really know, I don't mess with that stuff most of the times unless i can see a visually defective part on a board seeing as how i find troubleshooting solid state stuff 10x harder than tube stuff. It def. sounds that if this is a digital amp your looking at taking the chassis out.

[52 Bill]

When I hear the phrase "grounded out" I assume that the signal stops or goes quiet. What you are saying is that the amp hums or buzzes?

If the amp can idle for hours quietly and the noise only starts when a signal is sent through the amp, I'll ask if the problem can be started mechanically. If you give the amp a good Enzo thump with your fist can you start or stop the noise?

[ricach]

When I hear the phrase "grounded out" I assume that the signal stops or goes quiet. What you are saying is that the amp hums or buzzes?

If the amp can idle for hours quietly and the noise only starts when a signal is sent through the amp, I'll ask if the problem can be started mechanically. If you give the amp a good Enzo thump with your fist can you start or stop the noise?

Yeah, I hesitated to use the word 'grounded' but just like when plugging in a cable and the "plus hits the minus" so to speak, you get that loud electrocuting hum sound. that's what it is, constantly, and isn't affected by volume controls.

As for the Enzo thump, tried that, including shaking the chassis - no start or stop. Although I mentioned I replaced the power tubes, for some odd reason I didn't think to change the PI tube. I plan to do that when I'm home tonight.

[tpawlowskis]

When I hear the phrase "grounded out" I assume that the signal stops or goes quiet. What you are saying is that the amp hums or buzzes?

If the amp can idle for hours quietly and the noise only starts when a signal is sent through the amp, I'll ask if the problem can be started mechanically. If you give the amp a good Enzo thump with your fist can you start or stop the noise?

i second this idea.

[Jazz P Bass]

What I don't get is the Emu Output & the FX Return giving the symptom.
At this point I'd be looking at the power supply voltages.
Because the only thing that Emu & FX R have in common is the Mute circuit.
If that part of the power supply is acting goofy, there you go.

[ricach]

What I don't get is the Emu Output & the FX Return giving the symptom.
At this point I'd be looking at the power supply voltages.
Because the only thing that Emu & FX R have in common is the Mute circuit.
If that part of the power supply is acting goofy, there you go.

I'm not sure about the FX return giving the symptom, only that FX send does not. Also, apparently if I let the amp idle long enough (30 minutes) it will start to howl on its on - no notes required. Replacing PI tube did not help.

Jazz, I'll have time this weekend to put the board on my bench and actually start checking circuitry - just needed to get a better idea exactly what's happening first. I'll look more into the Mute circuit and power supply that you have referred to.

This appears to be a 'heat' issue since the only certainty so far is that it takes a long time initially for the problem to surface - and once warmed up, surfaces continually.

[Jazz P Bass]

The reason that I asked you to try the FX Return is that would have narrowed down the problem.
If FX R works o/k, then the problem is confined to the preamp circuitry.
Divide the problem.
Twas mine to fix, I would flip the board over & carefully resolder the whole thing.
Intermittents are just that.
The amp was not designed to be that way.
The components , generally speaking, are not manufactured to be that way.
That leaves assembly.
I have seen more than my share of inferior solder jobs.

[ricach]

I did mention a few posts back that I inserted a plug into the FX return jack and it had no affect on stopping the howl. I did not, however, plug in before the howl started. But should be the same result and thus I feel it does originate in the power section.

[Jazz P Bass]

I did mention a few posts back that I inserted a plug into the FX return jack and it had no affect on stopping the howl. I did not, however, plug in before the howl started. But should be the same result and thus I feel it does originate in the power section.

'Power Section' meaning the power amp or the power supply?
I still maintain that EMU is separate from the power amp & should have no effect on it 'if' you go in the FX R jack.
Which still leaves the 'mute circuit' in the equation. (which appears to be a relay)
You really do need to get this thing opened up on the bench.
Inspect the way it was soldered.
Look at the components that have the thickest, bulkiest leads.
If the soldering looks thin then carefully resolder the board.

[ricach]

Yes, I'm sorry, I do mean power amp section. I think i see the culprit - R4 - it looks a liittle burned in one spot. Since it straps the OT output to ground, it seems reasonable that heat buildup could turn it into a shunt. Anyway, worth replacing - but not easy to get to.