In article ,
Steff wrote:
Well just lost a channel at my Harman Kardon 930 Vintage geek as I am I
want to keep my great Reciver and repair it..

What I done sofaar

Check Fuses ( They are OK)
Moving the speakers between the channels left-Right And A and B speaker
outlets.

With this info I understand that the left Channel is ALMOST dead turning
the balance to Left channel I can hear a faint OK sound. (Like the tiny
transistors trying their best before the end power transistors do their job.

My conclution.

1 Not enough power to the end transistors on the left channel.
2 Broken End Trasistors (two 2SC897 according to the scematics)
3 End of preamp broken on left channel.


Most likely End transistors..

Is my thinking OK or did I miss something ?

Those are possibilities. Here's what I would suggest for further
troubleshooting. [Turn the power off before changing any cable
settings, of course.]

First: the manual and schematic show that the preamp and amp are
linked together via a set of jumpers on the back panel, going from
"pre amp out" to "main amp in". First thing to do is remove and
re-insert the jumpers, to see if you simply have a bad connection in
the connectors there. If removing and re-inserting fixes the problem,
then clean the connectors and jumpers with contact cleaner, reinsert,
and you should be OK.

Second thing to do is disconnect those jumpers, and replace them with
a standard red/black RCA-connector interconnect cable. This time,
swap the two channels - connect the left channel "pre out" to the
right channel "amp in", and vise versa.

If the problem remains in the left channel, then the fault lies in the
amplifier section.

If the problem moves over to the right channel, then the fault lies in
the preamp section.

If it's in the amp section: it might be the fuses (I'm not sure if
these are among the ones you checked). It could be bad speaker on/off
switches, but it seems unlikely that you've had the same failure in
the A and B channel switches (but it is possible... the speaker-B
switch might have failed long ago and you might not have noticed it
until now).

If the switches and fuses are all OK, and you're confident that the
amp is getting a valid input signal, then you'd need to trace the
signal through the amp section with a 'scope and DMM - see if your
B1/B2 power supply voltages are OK, and see how far the signal gets
through the left-channel amp before it vanishes.

If it's a failure in the preamp section (if the dead channel "moves"
when you switch the cabling between preamp and amp) then I think the
big two suspects would be:

- Bad, or dirty selector switches. In particular the "tape monitor"
switches for Tape 1 and Tape 2 are both right in the signal
path... if one of these has an open contact you'd lose a channel.
It looks as if the "tone control defeat" switch is also a suspect,
for the same reason.

- An electrolytic interstage coupling capacitor which has failed in
an "open" state.

Here, also, signal tracing is your friend. Feed a known-good line
level signal into the "tape 2 in" jacks, turn on "tape monitor 2", and
see if you get a good signal... this is the last input "in the path".
If that one works OK, try "tape 1 in" and the "tape monitor 1" switch
setting. If that works OK, use "aux". With this approach you're
working "backwards" from the preamp/amp interconnect, looking back
along the circuitry a step at a time to see how far back you can go
before the signal disappears.

I've seen enough older receivers/preamps with dirty-switch problems
that this is my lead suspect in cases like this. There's a reasonable
chance that cleaning all the switches will fix your problem. Put a
small squirt of DeOxIt D-5 into each switch, operate the switches a
dozen times or so to break up any old grease and dirt and oxide and
tobacco-smoke tar and etc., use another small squirt to flush out and
lubricate the contacts. Put a small squirt into each RCA jack, then
plug and unplug a cable a few times to break up the oxides and
crud. Let dry for a few minutes, then test again.