PDP 11/24 - A Step Backwards

Toby Thain toby at telegraphics.com.au
Sat Mar 26 17:06:35 CDT 2022

On 2022-03-26 5:20 p.m., Rob Jarratt via cctalk wrote:
> 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
> whatsoever.
> 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

I think the next step would be to inspect the PSU and see if the ripple 
can be eliminated, perhaps by replacing whatever blew up :)


> 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?
> Regards
> Rob

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