Pdp8 8k core repair

Repairlog PDP8/f

Our PDP8/f seems to have problems with its two corestacks. bank 0-3 are taken care of by a Keronics 812075A, a onr board/two slots memory card. Bank 4-5 is a DEC MM8-EJ 8k cardstack of three cards. As we have the docs for the 8k MM8-EJ stack, we started troubleshooting that one first

MM8-EJ

Adresses ending on xxx4 or xxx5 all return zero data, so something is wrong in the adress decoding part or in the stack itself. checking continuity on the X-Y lines of the stack is the first thing to check.

a test with our DSlogic analyzer reveiled a broken 7442 on position E59

wrong logic results

As you can see output F4 goes low even if another output is low. and sometimes F6 and F7 also. to get these results, i put the address counter just before the offending address 0003 and started pushing EXAM as fast as possible after pressing start on the DSlogic on 16Mhz samplerate. The pulses are very short so in an previous attempt with a lower samplerate i did, nt se anything coming out of the chip.

after desoldering the suspected chip

New and old chip

Replacing the chip with a 74ls42 did not inprove matters and checking the chip on a breadboard reveiled that this chip was faulty as well. Fortunately i found a other NOS sn7442 and that worked.

That was easy...