PDP-12 Restoration at the RICM

Dan (the donor) brought more manuals, diags on paper tape, and his very nice color oscilloscope. Dan's 'scope displays RMS, PTP, Average, and other voltage values on the side of the trace window, so it makes power measurements very easy. After reading the manual and getting instructions from Warren, we found that some of the misbehaving front panel was due to operator (me) error. The Mode switch to change between LINC and 8 operation only has an effect after you press the I/O Preset switch, so that is working correctly. When you press I/O Preset the INST FIELD is set to 1 and the DATA FIELD is set to 3. I though that this was wrong for an 8k machine. If you look at the front panel you will see three bits and an additional 2 bits for the INST FIELD and DATA FIELD. So part of this is for the 8 and all of it is for the LINC, and it is working correctly. We recabled the VR14 and TU56. The Local Forward and Reverse switches on the TU56 do not get the correct behavior from the motors. The Line fuse on the VR14 blew when we turned it on. We will replace the fuse and try a slow power up with a Variac. We received lots of comments on reforming the capacitors in the power supply and strong recommendations from experts to just replace them. Since new caps are a different physical size, and would cost about $250 we decided to continue with reforming the originals. We measured the voltage ripple on the backplanes near the power connectors. - +5.0V = 5.01V, 200 mV PTP ripple - +10.0V = 5.9V, 200 mV PTP ripple - -15.0V = -14.2V, 800 mV PTP ripple - -30.0V = -31.2V, 800 mV PTP ripple The 800mV of ripple on the -30 is a sign that the caps in the power supply were not working well, and is probably too noisy for the core to work. We tried to measure the capacitance of the power supply capacitors using an ancient, but very nice, GenRad capacitor meter, but unfortunately the caps were too big to measure. We thought that more power on time for the caps might improve their behavior, so we started debugging the processor. We found that bits 4 and 11 in the Program Counter were always on. We looked at the flip-flops on the M221 modules in the processor that make up the PC register and they the contents matched what was loaded from the console switches. We need to determine why the indicator lights on the front panel do not exactly reflect the internal state of the registers in the processor so we can continue debugging. After running the system for about four hours the ripple on the -30V was down to 180mV, so the capacitors are getting better. More run time will hopefully reduce the ripple to an acceptable level. If not, we will have to replace them. -- Michael Thompson