Some excellent info buried in another thread, courtesy of
@Ike Zimbel
Background info: The PM-2000 channel strips plug into headers that are soldered to backplane boards. Each backplane board takes four strips and then links to the next one over with with bus wire links, about 1/2" (10cm) in length.
What's the issue? Sometimes the solder joints go bad on the links.
How does that show up when using the desk? Typically, a channel will not assign to a bus. So, for example, Channel-1 will not assign to PGM-1.
How to test for this? Pt-1: If, as in the above example, Ch-1 will not assign to PGM-1, the first thing to do is to test to see if Channels 2, 3, & 4 won't either. If they don't, that's a strong indication that you have a bad solder joint on the bussing for PGM-1. If they DO, that rules out a bussing issue and points to a problem on Ch-1 only, like a dirty assign switch, or possibly a soldering issue on its edge connector.
If things point to a bussing problem, the next step is to try and determine where the break is. To do this, move to the last four channels, 21-24 or 29-32 and try routing to PGM-1 from there. If that works, move down to the next four and try them (you just need to try one in each group of four). If it doesn't work, the break is between the last channel and the start of the PGM modules, OR, there is a fault with the PGM-1 module. Swap it with another PGM module and repeat the test. If it works in another slot and the other module doesn't work in the PGM-1 slot, the fault is in the bus wiring as already explained.
Eventually, you will find a point in the desk where above channel "X" PGM-1 bus works and below that channel it doesn't. This will
always be a 4th channel (4, 8, 12 etc.) if it's a bus link issue.
Pt-2: Once you have the back of the desk open (remove two very long screws in top of meter-bridge and tip meter bridge back to access) you can use a DMM to measure along the busses to see where the bad joint is.
TIP: Take a Q-tip dipped in isopropyl alcohol and clean all of the bus wire leads before your start metering them. This will remove the film of gunk that has built up on them over the past 40 years and make it easier for your meter to measure resistance quickly and accurately (doesn't hurt to give your meter probes a swab too). You should read between 0 and 2 ohms between most points that you measure. If you see much higher readings, or a reading that won't settle, you've probably found the spot. I went through and re-did all of the upper ones in the pic attached. By re-do, I mean first de-solder the joint, and then re-solder with new solder. In the pic, I'm talking about the row of silver wires down the center of the photo, with each one of those being soldered to the board on either side. Another tip is to use thin gauge solder, so you can quickly do each side without inadvertently re-heating the other side while you're doing the 2nd joint.