Marshall 6101 problems tracing a short

[Alex R]

I'm a bit stumped - can anyone work this one out? I'm getting circuit blindness and I must be missing something obvious (or not...)

This amp blows fuses and shows a short - the light bulb tester lights up brightly.

I traced it down to the HT and bias supply areas at the bottom of the diagram, around the bridge rectifiers DB201 and DB202. Lifting the black wire from the mains txr puts the light out.

With R213, C210 and the + connection from DB202 to the CT and the HT supply lifted, the light bulb still is bright showing a short.

With C212 lifted the light bulb indicator shows no short

With everything reconnected but with EITHER DB201 OR DB202 removed, the light bulb shows no short.

Both DB201 and DB202 test good on all four diodes.

Any ideas?


6100___6101_Power_amp.pdf

[Steve Conner]

I'm going to guess C212 is shorted. It might read OK on a meter but break down when voltage is applied. Try replacing both of those Class-X caps.

If that isn't it, the only other possibility I can see is a short between the HT winding and the core.

[Alex R]

Thanks Steve I will give the caps a go. In fact they've been replaced once, have 211 and 212, and not with Class X, so it looks likely. I wonder if a 300vAC 1250vDC rated class x will do?

If I disconnect the secondaries there's no short indicated. even if I just pull the black one from the board.

Everything is so tight inside these Anniversaries, it's like a mesa in there!

[g1]

I'm a little confused that removing only DB202 gets rid of the fault. How could this be if the fault is related to those caps?
The standby disconnects one side of the AC from DB202, what is the bulb doing on standby?

[Alex R]

All of this happens with standby off and mains on.

I'm a bit hampered in commenting due to the fact that I don't fully understand how this cap-fed bias supply works - to be honest...

[Steve Conner]

Draw wires in place of the caps and trace the circuit out. You should find there's a short circuit path across the PT secondary, via the diodes in the rectifiers.

The circuit works the same as the 48V supply on Soundcraft mixers, if that helps. :/ except the caps are smaller so they drop some voltage across their capacitive reactance. This circuit is a hard job for a capacitor, I suggest using the highest quality, highest voltage rating you can find.

[Alex R]

thanks Steve, the new supercaps arrive on Monday, I will report back

[Alex R]

Yep, replaced both the feed caps and the short went away. Thank you Steve, that one was a bit beyond me .

[g1]

Glad to hear you got it sorted.

In fact they've been replaced once, have 211 and 212, and not with Class X, so it looks likely. I wonder if a 300vAC 1250vDC rated class x will do?

So classX is probably a good idea here. Out of curiosity, what was the voltage rating of the caps that shorted?