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

Unfortunately, testing with no output transistors in place will give
erroneous readings. I even had an HK 570i receiver flame out on me when I
tried this back in the '80s.
The problem is that the base-emitter junction of each output transistor is
effectively a diode to the diving circuit, clamping at around .6 volt,
unless the outputs are Darlintons, of course...


Mark Z.