26 Mar
2022
26 Mar
'22
4:12 p.m.
-----Original Message-----
From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Toby Thain via
cctalk
Sent: 26 March 2022 22:07
To: cctalk at classiccmp.org
Subject: Re: PDP 11/24 - A Step Backwards
On 2022-03-26 5:20 p.m., Rob Jarratt via cctalk wrote:
Unfortunately, or fortunately, depending on how you look at it, the PSU repair is under
warranty, which means I can't do it myself without invalidating the warranty, so I
will have to send it back. I don't know if the ripple is caused by the blown part, but
I suppose it is likely. I may be able to inspect it without breaking the seals.
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 :)
--T
> 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
>