Allen and Heath PA CP power amp

[Phil Allison]

"Gareth Magennis"

Changed Mosfets thursday, but has now blown 4 out of 6 output devices on
power up. (no load)

** 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.

It looks from your analysis and the schematic, that the amp should power
up
into its quiescent state with no 2SC/2SA devices present at all.

** 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

 

[Gareth Magennis]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:a2j55jF5u5U1@mid.individual.net...

"Gareth Magennis"

Changed Mosfets thursday, but has now blown 4 out of 6 output devices on
power up. (no load)


** 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.

It looks from your analysis and the schematic, that the amp should power
up
into its quiescent state with no 2SC/2SA devices present at all.

** 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

1. The devices were the right way round. I marked the heatsink before
removing the originals.
2. The amp was carefully powered up on a variac and 100W series lightbulb.
Relays clicked and no limit LED this time on front panel.
3. Prior to that I had used a +-30v bench supply on the PCB, no current
draw at all.
4. 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.

 

[Gareth Magennis]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:a2j55jF5u5U1@mid.individual.net...

"Gareth Magennis"

Changed Mosfets thursday, but has now blown 4 out of 6 output devices on
power up. (no load)


** 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.

It looks from your analysis and the schematic, that the amp should power
up
into its quiescent state with no 2SC/2SA devices present at all.

** 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

DC rails are as per the schematic.

There are no gate zeners.



Cheers,

Gareth.