pdp8/e problems

Mark G Thomas Mark at Misty.com
Sun Apr 24 15:06:22 CDT 2016


Is there anyone with a good set of 8/e prints who could help me narrow
down some troubles?

My 8/e was working a few years ago, but now it's developed some problems.
It seemed to work fine for about the first 2 minutes that I tried it, but
then it developed these two symptoms:

- Most of the time, upon power up, LOAD-ADDR clears all the random 
address lamps, but will not take an address, just all 0s. Occasionally
when I power it up, LOAD-ADDR will permit me to load an address, and 
see it's contents as it should. I can then enter other addresses, hit 
LOAD-ADDR, and get consistent results looking around at random data in
different memory addresses.

- Regardless of whether it's in the broken load-addr state above or not,
hitting DEP or EXAM just turns on the RUN lamp, and it's then stuck with 
RUN on, and non-responsive to any other front panel actions. This happens
irrespective of the position of HALT. I then power-cycle it and it is
back in either broken or working LOAD-ADDR mode as described above, usually

So far, the only M8330 prints I have found are extremely fuzzy, so I cannot
make out the IC identifiers or match them up to the equally fuzzy schematic.
By board-inspection I identified the 74S74 "run" flip-flop, and in fact I 
can un-stick the RUN state by briefly grounding CLR, but still cannot 
DEP or EXAM from the front panel -- it gets stuck back with RUN on if I 
hit either DEP or EXAM again.

The power supply voltages are right-on. I've reseated and repositioned
the boards. The connectors are very clean. Wiggling boards doesn't change
any of the symptoms. Wiggling switches doesn't affect anything. This is 
a very clean 8/e.


Mark G. Thomas (Mark at Misty.com), KC3DRE

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