Kenwood KA-907, odd biasing issue.

[saabracer23]

I have a 907 that had some pretty major issues. I had a thread some time back going over it, traces that had blown out, many shorted transistors. I got all of the issues addressed as far as I can see and I’m to the point of alignment.

When I first power the amp on I can adjust the biasing perfectly well and get it within the spec called for, but once the amplifier comes out of protection and the speaker relay clicks over the bias voltage jumps up to over 200 mV on each channel, spec being 20 mV. I’m sure it would be even higher, but I have the amp on a dbt and the bulbs go decently bright once it comes out protection.

Thoughts on where the issue starts?



The relay contacts go directly to a small board in the back of the amp that has four resistors and two additional relays for the speaker select.



Through those resistors it goes up to the tone board.



So the output of the relay goes directly into the filter that is made up of L5 and R61 (and the other channel) and goes off to the small board in the back. It’s only connected to the tone board through a 330 ohm resistor.

I do see that as it comes from the relay before it goes into that filter (L5/R61) that it runs directly back to the amplifier board where it goes into a 27k resistor (R53).

I’m a little stumped on this one right now…

Dan

 

[Broesel02]

The amplifier is oscillating. When the relay releases, the feedback changes, and the amplifier starts oscillating.
Most of the time, the transistors in the third differential amplifier are defective. Of course, all the electrolytic capacitors need to be replaced, and the black flags should be replaced as well. If you're lucky, that might be enough.
Sometimes, however, the output drivers are worn out. If the hFE values of the output drivers fall below 50, they are unfortunately no longer usable in this amplifier. In that case, you will need replacement types.