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Old 10th August 2008, 06:52 PM   #1
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Default MOSFET thump and panic!!

Old Studiomaster 1000W MOSFET amp is the basis of my small PA rig. We set up at the gig, 200 miles from home. Plugged it all in and on channet went
"BUMP.!.........."
a short pause. then;
"bump, bump bump, bump, bump," Until I shut it down.

A little investigation fount that it did it with no input or speaker attached. The speaker relay was clicking it and out and the LED power indicator was maxed out.

We got by by removing the power fuses from that side and running in mono. Its really anoying because the last thing I did before loading of for the trip was a speaker check.

Any clues?
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Old 10th August 2008, 07:09 PM   #2
nikwal is offline nikwal  Sweden
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any shorted mosfets on the output? I'm guessing that the input or output transistors should be most possability of error.. or if there is a prestage opamp that blew.
(theoretically, even if one is protecting the gate from overvoltage externally , it can still break from internal oscillation)
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Old 10th August 2008, 07:19 PM   #3
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If there was thunderstorm with lighting anything can be damaged, if you didn't disconnect the mains before you leave the house!
Here in the past i founded a led exploded in lighting storm.
You should start to check first from the PA and go back to the input (preamp).
But first check the power supply, and voltages.
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Old 10th August 2008, 08:30 PM   #4
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No, there was no thunderstorm, and I had to disconnect it to move it 200 miles. I think it was the long drive that killed it. I will open it up tonight.
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Old 10th August 2008, 08:46 PM   #5
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That's a good idea.
Sorry for my bad understanding, so when you open the amp, first check all connectors if there is any connectors, second try somehow to visually dee any dry solder point but that is difficilt to see with naked eye, perhaps you can re soldering some part of amp, or just take a measurements ( all power supply voltages and also on amp. pcb, all fuses.
I hope that nothing is burned up, but just a bad contact.
I wish you good luck and fast solving of the problem.
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Old 11th August 2008, 02:48 PM   #6
burbeck is offline burbeck  United Kingdom
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hi,
which model studiomaster do you have?
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Old 11th August 2008, 04:03 PM   #7
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Default Maybe protection entering...the relay entering in operation.


When you listen the bump is because one of the rails may be present into the output... result of a damaged output power amplifier.... this produces the bump when fast cutted out by the protection.... the protection releases and bump again....maybe the consumption when you have the bumb are lowering the supply voltage, and because of that the relay switches the speaker "on" back again...so.. this results in that sequence of "bump", silence and another "bump".

This can continue, alike a very low frequency oscilation...the relay "oscilates" between on and off.... and the result of that is supply voltage present into the speaker terminal, and a small time latter, no voltage at the speaker.

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Old 11th August 2008, 04:38 PM   #8
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Hi Burbec
Its a studiomaster MOSFET1000. One of the second generation from about 1985 - the one with slightly better cooling.

PM me and I can send a PDF of the schematics
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Old 11th August 2008, 08:49 PM   #9
burbeck is offline burbeck  United Kingdom
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email sent
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Old 12th August 2008, 03:43 PM   #10
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Diagnostic update - back at base:
So I switched it on without input or output connected. The relay isn't shuttling any more.
OK:-
1/ Lid off:- 4ohm 15 inch speaker connected - BUMP........ (my trousers fold backwards) and the relay flashes, but it only does it once, at start up. Lets not do that again!

2/ Disconnect speaker and measure voltages. Power rails are supposed to be 73V but both sides are at 79V. However a voltage has appeared on the sources of -73V (with respect to ground.)

3/ With a 10ohm dummy load, the stray voltage has fallen to -66V. Measuring between source and drain on the K135 side reads 154V. Between Source and drain on the J50 side is only 1.5 volts.

So...... I conclude that a J50 has let go. Actually, I just rebuilt that side a few running hours ago so maybe I created a short and the 200 mile car ride to Brecon found it.

Now:- I have 5 x J50 TO3s to choose from. None of them got noticably hot. How do I test?
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