Bias and offset adjustments on PS 200 Delta

I have this amp on my bench for repair. It came in pieces, lots of missing hardware (always a bad sign) After locating a couple missing transistors by comparing the opposite board, I could set the offset for one side, the other won’t go below 1.6VDC. It may be the bias is way off, but without a manual there’s no way to tell. The side where the offset does work, it sounds gritty, sure sign of a need for bias adjustment. I can not find any service information in all the usual places. Any suggestions?

Though it has a PS Audio name on the amp, it was built at a time I was not involved. The design and the modules were from a company called Monolithic sound in Nipomo California.

Monolithic Sound Inc
Address: 515 Sandydale Dr, Nipomo, CA 93444
You might give owner Greg Shug or his brother Eric a call for advice or help.

Hi Paul, Thanks for getting back to me.I called and left messages and emailed the link to Monolithic Sound you provided, but no one has replied.

Later I found hand-drawn schematics on the net for the 250 delta monoblock and the 150 delta stereo amp, but nothing specific to my model. The 250 delta prints look closest in design to the 200 Delta, outputs use the same devices, very simliar topology but not the same input fets or pre-drivers. I don’t know if I can test the bias settings given. 10mv.

Comparing the working channel to the failing one, the positive pre-driver (nestled between the banks of output transistors.) was the wrong part, TIP112.

I changed it to the same device on the workinf side. it worked for 24 hours, then, while setting the offset, it failed again with the same symptoms as before. Bias and offsett at 1.6 VDC. The Bias and offset controls won’t move the levels.

nothing looks or smells burned, nothing gets hot, warm to the touch, but not uncomfortably so. The failing section noticeably cooler than the working section.

If I can’t get any leads, I’m going to have to return the unit to the customer unrepaired.

Best regards,

Steve M

Vintage HiFi Restoration.

I don’t know if this will be different than what you found, but I found an archived schematic on the Delta 200