Before VCF East 2012 I turned on the PC04 to punch some tapes to bring. The punch started punching blank tape and I noticed the smell of toasting electronics. When I got time later I investigated and found the M044 that drives the punch feed had an overheated resistor. You can see it second from bottom between the black transistors and silver transistors.
You can follow along on the M044 schematic. The silver transistor (Q7 2N2904A) next to the burned resistor had shorted base to collector. That put -36 volts on the base. That voltage then fed through the emitter to base of Q3 (the black DEC3009B) which pulled excessive current through R5 and burned it up. It also killed the 7401 output that drove Q3 and Q3 itself. I found another DEC3009B transistor was shorted when I checked out the card. It was on an output that wasn't used.
Since I didn't have any 2N2904A I used a 2N2905A which is a higher gain versions on the 2N2904A. I was almost out of the DEC3009B so used a 2N4401 to replace it. The specs aren't quite the same but for the low speed this card operates at it should be OK. I'll save the proper transistor for more sensitive locations. I checked with my scope when I was done and the waveform where I replaced the parts looked the same as where I hadn't.
Feel free to contact me, David Gesswein email@example.com with any questions, comments on the web site, or if you have related equipment, documentation, software etc. you are willing to part with. I am interested in anything PDP-8 related, computers, peripherals used with them, DEC or third party, or documentation.
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