Allen and Heath PA CP power amp

Gareth Magennis · 2012-05-22T18:04:11+00:00

Hi,I'm a bit puzzled by this blown power amp, as the design of these things is not really my forte.(I have posted the schematic over on The problem is around the two Lateral Mosfets Q16 and Q21 (EXC10N20 / 10P20).R36 from drain to -70v is complete toast, R35 from the N channel to +70v is OK.R33 on the P channel gate is also fried, as is R30 on the other gate, but not quite as drastically.Now, R36 has been replaced before by someone, but I'm fairly sure they didn't replace either of the Mosfets or any other components.Also, none of the SCA / 2SC output devices or their emitter resistors appear to be faulty, though I have so far just done a cursory check on these and the usual suspects on a blown output stage.I have removed both Mosfets, and on a (low voltage) Peak Component Tester, and they show as both working, which may or may not prove a great deal really.Now, I had one of these in a few months ago, and came across a similar fault, but only R36 AND R33 were burnt to a crisp, indicating I thought, a blown P Mosfet.Again this checked out as working on my Peak tester, but I ordered another from Allen and Heath anyway, which actually took about 3 months to arrive.Replacing the two resistors and the Mosfet effected a cure, and the unit has not been brought back in again.The question is, what failure mode might be causing these symptoms? I don't fully understand what these Mosfets are actually doing in this circuit, or whether something else in the circuit is causing these failures. Why is the gate resistor on the N channel burnt?Another major problem is actually getting these Mosfets - you can't, as far as I can tell, and Allen and Heath no longer supply parts, instead having subbed out parts supply to various "service centres", who can sell you a complete amplifier PCB, but not individual components.Sorry if this is a bit long winded.Cheers,Gareth

May 22, 2012 22 Replies

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Phil Allison 14 years ago

"Gareth Magennis"

** You might have swapped the N and P channel mosfets.

FFS !!!

Always power up gradually with a variac and small value rail fuses after repairing a blown up power amplifier.

Monitor the AC supply current OR the DC supply current at the same time too.

** Yep.

Now answer my questions:

Are the DC rail fuses connected as shown in the schem ?

Are there really NO zeners on the mosfet gates ?

... Phil

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Gareth Magennis 14 years ago

The devices were the right way round. I marked the heatsink before removing the originals.
The amp was carefully powered up on a variac and 100W series lightbulb. Relays clicked and no limit LED this time on front panel.
Prior to that I had used a +-30v bench supply on the PCB, no current draw at all.
With lightbulb disabled, amp then blew a few seconds after power up. Yes, stupidly I did not fit 500mA fuses as I usually do. That'll teach me not to be so cocky. Doh!

The original fuses were intact and did NOT blow on power up - just that sides limit LED lit. They are blown now.

No Mosfet resistors are burnt this time, the Mosfets again read good on my tester, but that seemingly is meaningless.

I will check out your other questions later.

Cheers,

Gareth.

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Gareth Magennis 14 years ago

DC rails are as per the schematic.

There are no gate zeners.

Cheers,

Gareth.

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