Originally Posted by b-b-b-bass
Good job fixing the disabled PF-500! After all the moaning about PF-500 fails, someone actually had the moxie to fix one themself. Kudos.
Do you have any affiliation with Ampeg/Loud?
Disclaimer: Don't do this at home kids. Refer servicing to qualified personnel.
Moxie - LOL.
I have no affiliation with Ampeg or Loud, other than buying a few of their products. Long ago, I learned to fix things like stereos while I was a poor young engineering student that couldn't afford a stereo. Generally it was power transistors or power supplies that failed. In this case, both failed.
LumberjackTBird shorted out his PF-500 with a speaker jack that was halfway pulled out.
I bought the PF-500 as salvage from him, with the thought that maybe I could fix it to the point of at least using the pre-amp section.
Ampeg will send you the schematics to fix your unit, if you send them your model number, serial number and sign a disclaimer.
The PF-500 is designed to be serviced at the modular level. It is not really made to troubleshoot to the component level. They glue big capacitors down right in the way of getting to the screw heads on the transistors. It is almost impossible to get a screwdriver in there. I kept saying, there is more than one way so skin a cat. There is more than one way to skin a cat. Now, I'm replacing the screws with Allen heads.
The power transistors were shorted, so I isolated the power amp section by pulling the rails so that they got no power. I re-assembled the unit and put in another fuse. Stand back, turn power switch on with a pencil. [I call this the pencil test] and POW, the fuse went instantly with a big flash of light.
[Disclaimer: I know that I'm supposed to have a variable power source and an ammeter, but a fast blowing fuse does the same thing. I have experience with this, and have some stories to tell. I'm really careful, but it's still pretty scary. Don't try this at home. ]
So then I pulled out the switching transistors, and put it back together. Stand back. I braced myself for blowing another fuse, and hit the switch with a pencil. But the flash and pop never came. After a few seconds, the blue light came on. Sweet!!! I isolated the problem.
Some on the DIY audio website have noted that the IRS2092, IRS20957 and others in the family have too slow current protection to save the power transistors. They have two separate over-current protection schemes: high side and low side over current protection.
I replaced the pair of blown IRFB4227 power transistors. They are the smaller FETs shown in the picture. Pretty dinky.
The blown power transistors blew the switching transistors in the power supply. The switching transistors are the larger SIHG20N50Cs shown in the picture.
After fixing the unit so far, I still have a distortion at high gain. I may have to replace more parts to fix this. It is common when units fail like this that some other part may be compromised.