Repairing an Ampeg PF-500

I recently got the urge to buy a backup or easy travel bass amplifier so I didn't have to lug around my whole rack kit if I wanted to jam loud. I found a broken Ampeg Portaflex PF-500 on eBay and figured I could probably fix it, or at the very least, get hold of the replacement parts from an Ampeg service center. The following is the adventure:

The amp arrived with one of the two IRFB4227 power transistor cut out of the board and the holes for it desoldered. I ordered a few of that replacement as well as some of the main switch-mode power supply transistors and the main bridge rectifier, just in case. Some folks on forums like talkbass have suggested that the IRFB4127 could be used in place of the 4227, as it has higher RMS and peak current specs, but the turn-off delay is longer by quite a bit. Either way, the replacement transistors arrived and I set about replacing the known-bad one and figured I'd go ahead and desolder the other one while I was in there.

This is not a good picture of the power MOSFETs on the far left side of the chassis. They're a bit of a pain to get to in situ, so the easiest way to deal with it turns out to be trying to loosen the hold-down bracket and removing the heat sink. That may require removing the big heat-shrinked capacitor right in front of the middle screw; the amp came for me without the middle screw in place, so I don't know how the PO or his service tech dealt with that problem, as it also didn't look as though the cap had ever been desoldered. Maybe needle-nose pliers? An about 5mm deep 90-degree PH1 bit? I don't know.

Here's a shot of the switch-mode MOSFETs. The bridge rectifier is behind that. These are hard to get to for unscrewing, but a 10mm right-angle driver should be fine. These are actually working well in my board; the power amp disaster did not affect the power supply's outputs.

Worth noting - my transistor tester reported that the still-installed output MOSFET was still good! I'm not sure how that happened, but I saved the part just in case.

After installing the new parts, the amp powered on and passed its self-test. No sound output, though. Next step came testing to make sure the power supply was actually working. I got a case of the apprentice shakes and blew out both replacement output MOSFETs and let the smoke out of one of the diodes nearby.

That little dude right there is what died. Nobody around here has SMD parts like that in stock, so another order to Mouser. ER1D diodes are reasonably close to 1N4004s, so I bodged one in to continue testing while the correct part is on its way. I also swapped back in the one known-good 4227 and the one remaining new 4227, and got back out of fault land into an amp that just doesn't work right. At this point, I plugged headphones in and was surprised to hear my bass coming through them! So now at least I knew that the input board was not totally dead, the power supply was good, and both of my output MOSFETs were good.

Next came testing the output board. The red wires are the outputs from the power section, and the blue wires are the returns to ground. A simple check with a multimeter made plenty of beeps across, and hooking up a signal generator and oscilloscope revealed a good clear signal transmission. Okay, so now we know the input board is good and the output board (at least as far as the power amp was concerned) is good.

Next comes trying to trace the signal through the head. I did NOT feel like trying to tear out the input board, so I started at the beginning of the main board.

Into the input on a hacked-together speaker cable test rig was a 2kHz 0.2V sine wave. Whatever happens to that in the input board, it comes out on pin 7 of that connector. On goes the oscilloscope and hey! 2kHz on pin 7. Great, so the problem is now isolated specifically to the audio section of the main board. I turned off the amplifier and checked continuity from the big resistor next to the big toroidal inductor (L7, obscured under hot snot) near the output jumper (visible in both of the previous pictures on the bottom edges of each). I had continuity from there to the output jacks, so at least I knew that the traces were intact. Next came oscilloscoping at each marked test point on the main board schematic.
Input of C33: couldn't find it.
Pin 7 of U18: success! 2kHz right there.
Pin 1 of U18: 2kHz once again.
Output of L7: a small DC voltage offfset. No sine wave.

Okay great, that's a big swath of components that could have failed, but at least the two dual op-amps were good. In reviewing the schematic for potential test points, I noticed that there is only one more place that the signal actually passes through on its way to the output MOSFETs, and that's an IRS20957 audio IC.

I can't tell for sure that the IC under that "heat sink" is U1, but it looks to have the right number of pins to be the part.

That's where this repair job is right at the moment. I suspect that people who have PF-500 heads that have sound that "cuts out" but no fault light to go along with it are probably dealing with this IRS20957 IC failing intermittently. What's interesting about this amplifier's self testing is that it isn't failing on whatever is broken in that chip, at least not consistently. About half the time I had my bodged test lead connected to either the main instrument input, the preamp output, or the power amp input, the amp would fail to start. I never had any problems with it starting without the test plug inserted, and the preamp board was clearly okay with the 0.2V input amplitude, as it passed it undistorted to both the main board pin 7 input and the tuner out jack on the output board. Is it possible it was faulting because of additional load on the IRS20957? Who knows. I do know that that heat sink is hilarious. I also know that, at least according to the product example picture at Full Compass, the revision J board (mine is a revision C) doesn't even have a heat sink on U1.

I ordered a replacement IRS20957 and a few more IRFB4227s just in case I pooch up this board again in testing. The solder pads for the outputs are getting very tired, and the last time I tried to do a SMD (installing a pair of THS3092 opamp ICs into an MHS-5200A signal generator in place of the AD812s that came in it), I managed to destroy three of the sixteen traces on the board and had to do some very careful scrape-and-bodge work on the board; thus, I don't have a whole lot of faith in getting to where I can call this rev C board a finished, working, and perhaps most importantly, reliable project. Fortunately, that Full Compass link up there exists, so I have a (hopefully rev J) replacement main board on order to put in once I've finished screwing up the one currently installed.

I think this'll be a nice amplifier once it's done. It has a speakon output which should prevent future short-circuit damage while plugging or unplugging the speakers, and the rest of the device is in really good shape. I plan on giving it a good workout and comparison to my SVT-3PRO once the servicing mission is complete. In the meantime, I wait patiently for some delivery person to bring me a box of lil' black boxes...