Saturday, July 11, 2015

Pertec power issues - two steps forward, one step back, plus toying with P390


I attempted to set Control Register 0 bit 7 as this often helps with older OS running on the P390, but any of the available supervisors remain in a loop as soon as I load the CR. I don't have the source code or listing of the supervisor, nor any definition of what is configured in the six IBM supplied supervisors. If I had those, I could do address stops, single step and other debugging of the supervisor to figure out the specific cause of the loop and correct it.

My goal is to build a DOS/VS system from scratch, armed only with the IBM documentation and the distribution tape, just as I did it 42 or 43 years ago. There are prebuilt images of DOS/VS that I could fire up, having been uploaded by Hercules 390 emulator users. I have avoided this because it thwarts my objective of a ground up system generation. I may have to use the packs solely to get past this IPL loop problem, giving me the data to debug and the ability to produce and store a supervisor with the appropriate fixes.


Still chasing the cause of the high drain that blows the +20V fuse. Pertec has a couple of versions of the servo board. One has plugs on the board that can be pulled to isolate the regulators from all the other loads that use the power. It would have been very handy since my problems are likely caused by the consumers of the raw +20/-20 power.

I decided to isolate where I could - there are two darlington pair transistors on one of the heat sinks that are connected when I connect the +10V power transistor that shares the same real estate and cable. With those two transistors disconnected - they are the power to drive the positioner to move the arm in and out - I could bring up the power supply section without any fuse blowing.

Now, I traced voltages and zeroed in on failed parts. The +5V supply was running up at over 11V - not good at all. It would have fired the crowbar and blown the fuse, if I hadn't temporarily removed the crowbar SCRs. When I looked at the op amp that controls the voltage, I saw that the + input was the proper reference level of 5V while the - input was over 11V. This should have driven the op amp towards its negative rail (ground), but the op amp was pumping out the 11V into the power transistors.

I removed the bad op amp, put in a socket to facilitate further swaps if needed, inserted a new op amp and fired up the servo board. Now, everything was great. +10V, -10V, +5V and -5V were all correct and clean. Time to reconnect the two transistors that drive the positioner.

With them back in the circuit, I monitored the voltage sent to the positioner. When I turned on power, I saw +18V to the positioner, which swings between +20 and -20 (actually about 18 to -18) and the newly installed op amp for the 5V signal smoked as it died. Weeeeeelllllllll. Isn't that special!

Not so bad, because I have a socket there now and can quickly replace the +5V op amp. However, this flaw looks like the op amp for the positioner is generating an output at the top rail (+20). I still don't know why it would burn out the +5 supply, since the circuit mainly is connected to raw +20 and -20.

It tells me other components on the board are bad. I have read the adage that when the 'magic smoke' is released from an old system, things are quite dire. I never believed it, because I thought that if the failed parts are found and replaced, the damage being localized, the system could be repaired. In this case, however, it does seem that there are quite a few parts that died when my incident occurred.

1 comment:

  1. You might be Ble to use a prebuilt DOS system to get access to the source. Possibly the SYSGEN decks are also stored with the source, so you could see what the various options are.