PL-400, Q8 and Q9


New Around These Parts
Jan 11, 2022
I've opened up a PL-400 and discovered the 2 main caps were both open circuit and the meter lamps were dead. I replaced the caps (at great expense) and fitted new lamps. With the supply disconnected from the amp, I found the supply was sitting at around 74v ( both plus and minus). So far so good. Re-connected the supply and have very distorted sound in both channels. Off again and discovered a lot of mismatched output devices so I decided to replace Q11 to Q18 in both channels and am now going to replace all of the electrolytic caps and transistors on the PC board. I thought I'd gone this far so why not. I've now discovered that a some point in the past, possibly when the mismatching took place, that Q8 and Q9 in both channels had been removed. Why would anyone do this? I understand that these are the current limiting transistors. Has anyone experienced this before.