K8077 Subwoofer problem


I’ve assembled the K8077 subwoofer kit according to instructions, yet when connecting the speakers I just get a loud buzz.

With relay energised and ICs inserted, testing produced the following figures:

J2: +15.9v
J3: -15.8v
Left R33: +23.75v
Right R37: +5.2v
Cathode D11: +38.9v
Anode D12: -39.0v
Voltage across R60: 4.2mV - this does NOT change when adjusting the trimpot RV3! It’s the same right through the range.

Any ideas?

Thank you.

I think I had a bad transistor in the power section somewhere. I’ve replaced them all to be safe. Current status is that the loud buzz has gone, auto power-on works (which suggests the opamps are ok?), but I have no output from the speakers.

The bias adjustment works now, but the range is wrong - I read between 0mV and 0.9mV over R60 when adjusting it.

Can anyone offer any suggestions on how to proceed?

Well after having a real hard time trying to troubleshoot this for the last few days, I’ve found one major problem. The PCB is poorly etched resulting in a short to ground on pin 5 of IC2:

I’ve fixed this by scraping away the copper.

I now have an output through the speakers, but it’s a horrible, distorted mess of an output - probably because the biasing of the power stage is so far out of range.

If someone (anyone at all… please?!) can help me figure out this problem, I would be most appreciative. I’m not asking for a refund or replacement, even though the PCB was clearly flawed from the start and I’ve wasted loads of time on it. I just want a bit of help getting this working. PLEASE?!

hi there i just read your comments about K8077 is your Subwoofer Amp working
i will appreciate if you can contact me thanks