Dave wrote:
"Dave" wrote in message
news:m1jbh.6192$dX4.81@clgrps13...

blown pair blown pair good pair
|| || || || || || || || || || || || || || || || || || ||
----------------------------------------------------------
Tip41--c945--Tip42--Tip41--c945--Tip42--Tip41--c945--Tip42
| | | | |
|
c945 a733 c945 a733 2sc2245 2sa965
bad

Okay, I rec'd my TIP41/42 replacement pairs yesterday. Before I put them in
I figured I'd power up the unit with no output devices installed. Checked
the bias voltage on the base of each output transistor position.

On the two pairs that are driven with C945/A733 pairs, there is 300-400mV DC
at the base. At the 2SC2245/2SA965 driven pair the I read a whopping
15.5VDC at the NPN and -15.5VDC at the PNP output base socket. This can't
be good. As the pair uses a common bias transistor, I replaced the bias
transistor, no change.

I can see what you all mean about troubleshooting a DC coupled amp. I've
made a schematic from the outputs back, but I don't have anything to compare
my values to because it's a completely different amp channel than the two
other channels which are identical.

Going backwards from outputs, I see outputs - drivers - dual op-amp IC.
Behind the op-amps are more op-amps (quad op-amp IC TL074CN), another
half-dozen NPN's (all c945's), two diode half-wave bridges, all kinds of
good stuff. Is there any hope at all, given that I don't have an identical
working channel to reference, of finding the source of this dc voltage? Or
should I just bring it to the dump?

I don't mind troubleshooting this piece of plastic junk as a learning
exercise if there is a potential positive outcome (i.e. light at the end of
the tunnel). But, maybe I'm just asking for punishment...

Any and all responses appreciated

Dave

If its all dc coupled youre most definitely asking for punishment. I'd
dump it and get some other chuck out to work one, one youve got a
decent chance of fixing.


NT