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Soundcraftsmen PM860 / PCR800 power-amp restoration

Very nice work! The 3D printed fan shroud and fan speed resistor are nice touches, and the paint job looks fantastic!
 
Racingh11 said:
I bought this amp in rough shape as a project to match my other PM860 beater amp. I should have taken before pictures. The inside was coated in heavy dust that was also oily. And all of the black sheet metal parts had the paint pealing off. I opted to fully disassemble the amp. Ultrasonic cleaned everything and stripped the metal parts down and repainted. I installed a handle and additional rubber feet to match the other PM860 I have. Makes them real handy for mobile use.

I replaced all electrolytic capacitors and all small signal TO92 transistors throughout. I also installed one of my own input boards that makes it bridgeable and switchable low-pass filter with bass boost for a subwoofer, and a turn-on muting delay. It also got gold RCA inputs to replace the 1/4" phone plugs

After everything was assembled and settings were all adjusted, it was tested on the distortion analyzer. It seemed fine until I approached high power and distortion rose before the rated power. After connecting the oscilloscope to the high voltage transformer connection to the power board, I could see it was only charging the caps on every other phase. I traced this back to a bad solder joint at a diode used to sense that phase. This is why the previous owner installed a 15A fuse instead of the 10A stock. Because these amps have to charge at twice the amperage when it happens half as often, and they blow the fuse. It now has a maximum of .0113% THD at 205W 8 ohm. I also tried bridging to 8 ohm, tests .006% THD up to the 300W my loads could handle. IMD is even lower at .0033%.

Another thing I did on this amp was install a C13 power cord socket. This makes it much easier to work on and gives cord options. This is the second time I have installed a heavier awg power cord. Stock is only 18 awg. The last amp I tried had increased 1% THD clipping limit with a heavier cord. But I didn't test before to verify that was the reason. On this amp I was able to switch between a 10' 14 awg cord and a 6' 18 awg cord. I have 6' 14 awg ordered.

The 14 awg cord powered the 1%THD RMS clipping level all the way to 260W per channel 8 ohm, both driven at 1khz. The 18 awg cord brought it down to 220W per channel. (rated 205W) The cord alone was worth 40W per channel increase in RMS power!

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Beautiful....What paint did you use?....I have just acquired 8 MTX800s all paired in custom brackets...these have supposedly upgraded caps and better Hitachi mosfets than the PCRs that preceded them....they have just a couple hints of surface rust on tops only...the rest is super clean...After a light sanding I would like to try and match as close as possible?....thanks in advance
 
psychmonster said:
Beautiful....What paint did you use?....I have just acquired 8 MTX800s all paired in custom brackets...these have supposedly upgraded caps and better Hitachi mosfets than the PCRs that preceded them....they have just a couple hints of surface rust on tops only...the rest is super clean...After a light sanding I would like to try and match as close as possible?....thanks in advance
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Hi, I use Rustoleum - -Canyon Black. It give it a couple of coats, then a dry spray from a distance for the texture.
 
Hello, has anyone found a replacement for the K135 J50 mosfets? I know they will have to be in a TO-247 case,
Thanks in advance for any info
 
 
thank you so much for the info, I just ordered some Exicon 10P20 and 10n20 , these have slightly higher power handling then the originals. The issue I was having was one channel in a 450x2 M was pegging itself to clipping with no input. I pulled the mosfets and a number of them under diode check the voltage keeps creeping down, so i think under real conditions this keeps happening until a short occurs, at least this is what I think, I have checked all the other signal transistors and they all check good, and most of them have been also replaced in the past any ways. So we shall see, once the new fets arrive and are installed.
 
Has anyone witnessed this issue on these mosfet amps, the creeping up on the output meter then going into protection and then falling back down to zero? And this happens with no input or speakers connected?
 
kenmcc999 said:
Has anyone witnessed this issue on these mosfet amps, the creeping up on the output meter then going into protection and then falling back down to zero? And this happens with no input or speakers connected?
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What do you mean by protection? Is there DC voltage or oscillations on the output and then the fan turns on high and possibly lowered supply voltage?
 
while sitting at idle the B channel output meter slowly creeps up until all the LEDs are lighted, the clipping indicator for that channel illuminates and then the Power meter slowly reterns to zero
 
kenmcc999 said:
Problem solved, I suspected some of the outputs as they where testing weird, no shorts , but low and falling diode check voltages. I replaced that channel with some Exicons, all is good in Soundcraftmen Universe now.
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Nice, those MOSFETs do weird things when they fail.
 
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