M 50 repair

rsweet1 · Post by **rsweet1** » Mon Dec 14, 2009 7:42 pm

Well I bought a PG m-50 second hand. when i bought it, it was bridged to a
4-ohm, 15" Polk/Momo Sub. One day the sub turned on but had no signal. I
twisted the gain knob and if i twisted fast enough i would get a loud bump
from the sub...so I opened it up and found a blown 47 micro farad cap and
burns 460 ohm resistor. well i changed them and tried again....it blew
them. so i changed the rail caps and a few more questionable resistors. and
tried it once more. I thing i need to change the potentiometers and i want to
change some transistors. But i don't know anything about identifying blown
transistors or pots....can anyone help? Schematics would be awesome but
any info at all would also be greatly appreciated...

I tried to highlight the parts i think i'm having trouble with in green....

Thanks,
Ryan Sweet

Post by **Eric D** » Fri Jan 01, 2010 7:37 am

Chances are there is nothing wrong with the pots. They are probably just showing you symptoms of another problem with the amp.

Do you have a DMM (digital multi-meter)? You will need one to test parts of the amp.

The 6 devices mounted to the heatsink, which you noted in green are a good place to start. More specifically the 4 on the right. If you can verify they are in working order, you can then move on to the 4 transistors mounted under the heatsink. Those might actually be your problem.

rsweet1 · Post by **rsweet1** » Sat Jan 02, 2010 7:58 pm

Thanks. yeah i have a dmm, but when i test what am i looking for? the transistors should only show impedance when they are presented with 12V right? other than that they should not conduct? does this mean they work?
without power the right four on the front all show 31 mega-ohms.
the ones on the back are a little different.
2 show 16 M-ohms and 2 show 12 M-ohms.

Post by **Eric D** » Sun Jan 03, 2010 11:43 am

You need to check the transistors with no power to the amp, or anything else connected. You put the DMM in diode check mode and test all 6 possible lead combinations on each FET. Since there are 3 leads on a FET, you can connected the DMM to any two pins in two different directions for a total of 6 possible checks.

What you are looking for is a short, or one FET being quite different from the other FETs.