Just repairing one of these, usual problems with heat and bad solder
joints. Did the recommended modification of replacing the MPSA56/06 pair
with BD139/40.

Question though, this has a Rev C preamp board which is different from
the Rev E which is shown on the available schematics.
U3 on this one, which is a BA4560 was running very hot and a 'scope
check showed it was oscillating.
It seems, as far as I can follow the traces, that it's being used as a
buffer with pin 2 (-in) connected to pin 1(out) which goes to one end of
a 20k pot. The other end of the pot is grounded and the wiper goes to
the power amp, as you might expect.
Pin 3 (+in) is being fed clean audio, via the preceding op amp, from the
input jack.

What I don't understand is the purpose of the 100n capacitor attached
from the output of this op amp to ground. Removing it stops the
oscillation so I've left it out for now.

The only thing I can think it might be there for is to stop high
frequencies from operating the AGC in the amp*, but otherwise I'm at a
loss. Any ideas?

Lee

*the output from this op amp goes to the CA3080 based AGC circuit on the
power amp board, signal then comes back to preamp board to go through
low pass and notch filter on preamp before going back to power amp.

On 16.04.14 18:41, Lee wrote:
Just repairing one of these, usual problems with heat and bad solder
joints. Did the recommended modification of replacing the MPSA56/06 pair
with BD139/40.

Question though, this has a Rev C preamp board which is different from
the Rev E which is shown on the available schematics.
U3 on this one, which is a BA4560 was running very hot and a 'scope
check showed it was oscillating.
It seems, as far as I can follow the traces, that it's being used as a
buffer with pin 2 (-in) connected to pin 1(out) which goes to one end of
a 20k pot. The other end of the pot is grounded and the wiper goes to
the power amp, as you might expect.
Pin 3 (+in) is being fed clean audio, via the preceding op amp, from the
input jack.

What I don't understand is the purpose of the 100n capacitor attached
from the output of this op amp to ground. Removing it stops the
oscillation so I've left it out for now.

The only thing I can think it might be there for is to stop high
frequencies from operating the AGC in the amp*, but otherwise I'm at a
loss. Any ideas?

Lee

*the output from this op amp goes to the CA3080 based AGC circuit on the
power amp board, signal then comes back to preamp board to go through
low pass and notch filter on preamp before going back to power amp.

use a resistor(1-10K) to connect the buffer to the cap.
1k would be my first choice.
That would block the HF oscillator loop(maybe).
Something like this should have been there in the first place.