"James Sweet" bravely wrote to "All" (31 May 04 07:07:00)
--- on the heady topic of "Pioneer M-790 amp adjustment"

JS Reply-To: "James Sweet"
JS From: "James Sweet"

JS I've got a Pioneer M-790 here with all 8 power output transistors
JS blown. I found a number of cracked solder joints, is there anything
JS else I should check on this particular amp before I install new
JS transistors and try it out? Also how do I adjust the bias? I only see
JS one pot in the whole amp.

Yes, check the bias circuit "very" carefully and use resistors to
limit current in the power transistors during testing. The value of
these protection resistors depends on the supply and current but for
say 40 volts with 100mA, something like 100 to 330 ohms at 7W+.
Determine what is a safe value for this amp but it is very advisable
to use them for initial testing just in case.

Bias is often permanently set in some models and thus one must
use oem matched transistors for these or they won't track thermally.
The single pot is likely to adjust a regulated supply voltage: don't
touch it. Do some tracing starting from the wiper and determine its
function.

Be very careful to check the driver circuits as well because when the
output transistors go they tend to inject a heavy current back there.
It's like a chain reaction which propagates backwards from the output.

Well, this is probably not enough to help but I do wish you good luck!

A*s*i*m*o*v

.... A fail-safe circuit will destroy others.