Marshall MA100H blows fuse

[themusic]

No the MA100 footswitch is channel and boost ...thanks

[Enzo]

IC3 is a good suspect for a bad 15v node, but don;t overlook C44 on the same page.

Don't just order a new one, remove the old one and power up the amp. Does R87 now survive? If so, then IC3 was indeed the problem, but if it still burns the resistor, no point in ordering the IC, at least not yet.


Once that is squared, maybe the silence will be cured. After all, without a positive voltage on the gate of TR45, which comes from the +15, it will stay in mute.

[themusic]

would this be the correct reading at R56 before C43 I'm reading .086v .
I'm reading 15.8v at the front of R55. What should I be reading at the legs of TR45...thanks

Also your readings on IC3 are correct as well as R50 and R51

[Enzo]

OK, so voltages are restored, amp still muted. You report the output of the IC at essentially zero. TR45 is across the signal path, it is turned ON and is shorting the signal to ground. There won;t be any voltage on the source and drain pins either way. But I explained earlier, we need a positive voltage on the gate of TR45 to turn it OFF. Assuming the new IC is good, or still good, then either the IC has no power, or the Tr45 has a gate to source short, or the inputs to the IC are bad.

So power off, measure resistance from the gate of TR45 to ground. Is it shorted?

Power on. I explained earlier the operation of the IC, so is there about 8.5v on pin 3, about 6.5v on pin 6, and about 7.5v on pins 2 and 5? Oh, and verify there is +15 on pin 8 of the IC. I am not concerned that 15 is really 15.8. As long as it is not zero. And those other voltages? The 7.5v only needs to be half of whatever the 15v really is. and the other two need to be about a volt higher and lower than that.


Oh, and I was wrong earlier. I said it was a reverb mute, it is not. It is just a mute for the amp in general, it happens that the point they use for muting the signal path is the point the reverb return comes back in. So I read more into the label "ReturnRev" than I should have. In fact that is a test I use on my trainees to see if they are thinking - I failed my own test. I show a small circuit and ask them to tell me what the thing does. One of the output lines is labelled "pulse", but the circuit actually never pulses, it is just a name for the output pin. People that tell me it pulses I know are not analysing when the circuit must do.

[g1]

Thanks Enzo for the IC3 explanation. Following that "reverb ret" line I see that it is the input to the power amp. So TR45 is muting the whole amp, not just reverb.
And now TR45 is muting the whole amp, as it is not getting a positive voltage at it's gate.
As IC3 is new, likely suspects are solder errors at the IC, or associated components may be defective or have solder issues.

Edit: Sorry Enzo, I didn't see your last post.

[themusic]

Resistance on TR45 power off. leg1 42.4ohm and 50.6ohm on leg2 3rd pin is to ground.

IC pin3 8.7v pin6 6.6 pin2 &5 7.6 pin8 15.3

[g1]

So it would seem either the Fet or C43 is bad. My apologies for not having you check these parts earlier, whichever one is bad is probably responsible for the failure of IC3.
Remove TR45 and then check of you still get the low resistance reading (42 or 50 ohm) across C43. If not, replace the Fet.

[themusic]

Well, TR45 was the problem...Thanks for everybody's help...It was a brute but I learned a lot...THANKS