I'm working on a Phase Linear 400 Series II, which appears to be all original (for better or worse). It's the earlier version in the series with FPL909's and the RCA66546's. The issue: Even with the trimmer turned all the way down, the bias in one channel is still too high--around 320mV when first powered on, which means it's climbing over 400mV within just a few minutes of warming up. The other channel sits at about 180mV when powered on. I found a troubleshooting note in the service manual for the Phase Linear 400 (original) saying that the bias can sometimes be too high for the trimmer range, possibly due to Q8 having been replaced. The recommendation is to replace R17 (typo in the manual, I believe--should be R18) a 3.9K with a lower value until bias can be lowered enough. Nothing along these lines is mentioned in the Series II manual.
Any input appreciated.
Any input appreciated.
Reducing the value of R104 should do the trick. You want a more positive voltage on the base of Q115. Series I had a string of 3 diodes in that position so that's probably why they suggested reducing R118.
Craig
Craig