PDP 11/24 - A Step Backwards

[Rob Jarratt]

> -----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:
> > 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 :)
> 

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.




> --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
> >