> -----Original Message----- > From: cctalk <cctalk-boun...@classiccmp.org> On Behalf Of Toby Thain via > cctalk > Sent: 26 March 2022 22:07 > To: cctalk@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 > >
