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Mark D. Zacharias

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

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.

then youve still got as set of output trannies yu can use in a
project. Not often I'd say forget it, but with dc amps I wouldnt
bother.

NT

Practically all the amps I work on are dc coupled. Not a problem in most
cases.
Virtually all your Kenwood, Pioneer,Sansui, etc going back to 1970 or so are
DC coupled.
Modern amps like Denon, Yamaha, Onkyo, etc are all pretty similar in
topology. Certain components tend to go out along with the outputs
(resistors and driver transistors for example) and symptoms such as
premature clipping, DC offset etc have predictable causes.
As to the OP, it does seem his piece is quite possibly a "piece" all right,
but I'm not at all sure I'd classify it as such a difficult repair simply
because it may be a "DC" amp. As I say, I fix such amps all the time. No
problem.

Mark Z.