PDP 11/24 - A Step Backwards

Rob Jarratt robert.jarratt at ntlworld.com
Sat Mar 26 16:20:14 CDT 2022

I had the H7140 PSU in my PDP 11/24 repaired a little while ago and I posted
about it here: https://robs-old-computers.com/2022/02/10/pdp-11-24-progress/

I have since had the PSU fixed again and it came back a couple of weeks ago.
When I installed it and applied power to the input, I heard a reassuring
relay click.

So I powered it on. The fans turned, but there was a crackle and I smelt
something burning. I couldn't locate the smell, there were no lights on the
CPU board, but the fans continued to turn.

I had to leave it a few days and today I went back to it to check things a
bit more carefully. All the power outputs of the PSU appear nominal.
However, the ripple seems quite high, with an amplitude of 600mV on the +5V
output: https://rjarratt.files.wordpress.com/2022/03/pin-1-5v-ripple.jpg.

The DC ON light comes on, but the M7133 CPU LEDs show no activity

There is no apparent damage to the CPU or to the M7134 that was also
installed. So, I guess the component that blew up must be inside the PSU.
Presumably, whatever the part is, it is stopping the CPU working, because
previously the CPU did appear to show some activity, although of course it
could still be a failure on the CPU. I am not sure what other outputs the
CPU might depend on. There is the LTC signal for the line time clock, but I
don't know if its absence would stop the CPU working. I have not been able
to test the LTC signal as yet.

Can anyone suggest what else the CPU might need? Or is it LTC?





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