PDP-12 Restoration at the RICM

Michael Thompson michael.99.thompson at gmail.com
Sun May 17 08:30:59 CDT 2015

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

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