So I looked around here for some detailed information regarding the pinout of vintage API 525 compressors, and how to modify them to get them to work properly in modern API / other 500 series racks. There's one page out of API's service manual that was linked above (copied here as well) that shows the proper pinouts for both the 550 eqs and the 525 compressors, and how to change them. Aside from that there seems to be nothing out there on how to do this, so I'm going to post some pics of the mod in case future gear / geek slutz need the info. The mod was pretty easy for me, but I am somewhat experienced with a soldering iron. I'd say the hardest part was trying to bring myself to start molesting and changing the circuit on these beautiful vintage compressors that I just spent a lot of money on!
Here goes. Of course modifying your own vintage gear comes at your own risk. I'm just a moron on the internet, so please do your homework and don't take my word for anything!! Also my circuit boards are Revision F, so if you have a different revision, your circuit may be different! Check and refer to the sheet from API if in doubt!
1. Remove the Yellow / Brown Wire that connects to pin 3 at the edge connector. This is the only thing actually connected to pin 3. Also remove the Blue and Yellow wires from pin 10 & 11. These were for the VU lamps, and used to work on +24vdc but they will run on +16vdc and we will hook them back up at the end of this process.
2. Cut traces going to Pins 4 thru 9. Cut them as high up as you can, you don't want to be soldering directly to the pins as the module may not seat properly if you get too much solder on a pin. IN Circuit revision F, you do not have to cut the trace from pin 3 or the pins 10 & 11, as we just disconnected them above. If your circuit is NOT revision F, please refer to API's documentation to make sure this is still the case!!
4. Drill holes. I used a long pointy drill / awl attachment for the Dremel, and it worked great, although I'm sure a tiny drill bit would work fine. Don't make the holes too big, as you want to be able to reverse this if anyting doesn't work! You need to drill holes in the card side (on the inside of the split traces) for pins 4, 5, and 6, and 12 (12 is for the lamps, optional). You'll then need to drill holes on the pin side of the split for pins 4, 5, and 8. You may have to expand the hole in pin 10 as well to get your jumper to fit.
5. Next, take the yellow / brown wire coming off of the transformer that was attached to pin 3. Again, this is revision F, so the color wire may be different on your unit, just make sure you're getting the wire that was attached to pin 3. Solder it into the hole in the pin 4 connector pin, being careful not to get too much solder on the pin.
6. Put the first two jumpers in place - from pin 4 (on the card side of the spilt traces) to pin 5 takes about 1/3", from pin 6 (card side) to pin 8 takes about 1/2"... Then put the last jumper in place, from pin 5 (card side) to pin 10, which is about 1" in length. Solder them in very carefully!
7. Next you'll want to make the lamp work, even though it's just going to be 16vdc, and they aren't extremely bright. This step is optional, and it does not affect the compressor's operation. To do this you will insert the blue wire that came from pin 10 into the small hole you drilled near the top of pin 12. Then take the yellow wire you took out of pin 11, and insert it into the already present hole in the trace coming from pin 13.
Next, test them out and see what happens, you should be good to go at this point!! Below are some pictures of the process, hope it is of help to someone in the future.
API's Instructions in a nutshell:
OLD Pinout ---> NEW Pinout (cut traces on pins 3-11)
3 ---> 4
4 ---> 5
6 ---> 8
5 ---> 10