Marantz PM300 bias won't adjust down

[goldswimmerb]

Hey guys, I'm working on a Marantz PM300 (not the greatest work by Marantz but honestly not too bad either)

The dim bulb lights up bright, however both channels do produce output, testing I noticed that my bias for one channel was at 310mv at the lowest adjust point on the trimmer, and can only be adjusted upwards.

I went ahead and replaced the drivers, to no change, and changed the small transitor pushed against the heatsink (assumed it's supposed to deal with bias) and tested the trimmer to make sure that wasn't the issue.

Anyone got any ideas, or deal with one of these before?

Any ideas?

 

[MBuras]

I’ll let more qualified members take you further but first issue to address is the short indicated by the DBT. Until you solve that there’s no point moving on to fine adjustments like bias. Start testing the transistors at the outputs and move back to drivers, pre drivers, etc. And double check your new drivers cuz whatever is short might have taken one or more out again.

Not many members here work on the 1980’s stuff so probably not a lot of direct experience. My experience with this series is the bigger PM700, PM750, etc. Almost every time the issue has been one or more of the fusible resistors. Either extremely over spec resistance (+10k or more) or completely open. I understand that might not be your problem with a short but could be something to check while you wait on additional guidance. If they’re not trouble now they will be later.

 

[goldswimmerb]

I’ll let more qualified members take you further but first issue to address is the short indicated by the DBT. Until you solve that there’s no point moving on to fine adjustments like bias. Start testing the transistors at the outputs and move back to drivers, pre drivers, etc. And double check your new drivers cuz whatever is short might have taken one or more out again.

Not many members here work on the 1980’s stuff so probably not a lot of direct experience. My experience with this series is the bigger PM700, PM750, etc. Almost every time the issue has been one or more of the fusible resistors. Either extremely over spec resistance (+10k or more) or completely open. I understand that might not be your problem with a short but could be something to check while you wait on additional guidance. If they’re not trouble now they will be later.

Thanks, ill have to look further, not taking this thing off the DBT yet anyway. The reason I was checking bias was it seemed the output Transistors were not blown, as the unit actually produces sound while on the DBT, it's just horrifically over biased causing the bulb to light. Unfortunately I do not have a whole schematic and was previously banned from hifiengine after a dispute with the admin.

 

[Richs_Trains]

previously banned from hifiengine after a dispute with the admin.

downloaded it, PM me with your email and I'll send it.