Well there was a twist in the tail of this repair.
Sorry this is a bit of a ramble, I had intentions of posting updates regularly but it was a fight against time to get as far as I could before the end of the weekend.
Well I replaced both of the diodes in the -8v rectifier in A15 - and it fired up happy as larry with just the post regulator and cpu boards in. The A15 PSU red LED went off and the green one was on which is good. The A8 postreg board was showing all but one green LED lit for the power rails. The LED which was off was just the front panel power for external probe/bias, and had a blown fuse, so I could ignore that for now.
So looking good and I started adding boards the back in again.
Unfortunately when I nearly had all of the boards back in, the A15 pre regulator reset and then blew one of the new diodes.
Tried again and I got it to the point where it would run, but as soon as I added more boards it would die. Blew another couple of diodes trying to figure out which board was faulty. Lucky I brought 10 of them...
I started using a multimeter with a temperature probe to keep an eye on the replaced diodes, and was able to nurse it for long enough to see the screen was good and the front panel was working - apart from the missing buttons of course and glitches caused by the missing boards.
I started looking for what was causing the problem assuming that something was overloading that rail, but couldn't get anywhere. The problem rail was -5.2V, and was used in several boards. The confusing thing was that the rail was fused on the A8 post reg board, and that fuse did not blow. So either the problem was prior to the fuse, or there was still an issue in the A15 PSU which feeds it with -8v. I don't have board extenders so there wasn't much I could probe with the boards in place.
Several dead ends later I was still confused by one thing. The -5.2v rail was fused for 4A and the fuse wasn't blowing - so perhaps there really was nothing else wrong. I thought I better find out what the current actually was. Easy enough to do by desoldering the -8v line from A15 to A8. And sure enough it was only drawing 1.6A with all of the boards installed.
The 6A1 diodes were overrated in both voltage and current - 100V vs 60v and 6A vs 5A and were physically much larger so they must be better - yeah right. I even tried using two in parallel, but that didn't help.
One of the original diodes was still working, I'd removed it along with the dead one. I measured it and noticed that its forward voltage read quite a bit lower than the new ones.... Maybe HP had used some flash harry diodes and my replacements were not up to the task? So google got a bit of a thrashing and I schooled up on how to choose diodes in a bridge rectifier.
This page was helpful
http://electronics.stackexchange.com/questions/48688/selecting-the-right-bridge-rectifierThe part number was readable so I found the datasheet of the original part [SB560] - (doh, should have done that sooner, instead I was working from the schematic for the older 8753A/B and I couldn't find those part numbers on the web):
https://www.fairchildsemi.com/datasheets/SB/SB560.pdfHere is the replacement I used [6A1]
http://www.diodes.com/_files/datasheets/ds28009.pdfVf = 0.67 vs 0.9
Thermal resistance = 25degC/W vs about 10-20degC/W depending upon lead length.
Then I saw that the thermal resistance of the new diode seems to require a ground plane of 25mm^2. This board didn't have that, so maybe the diode just couldn't dump enough heat out the leads into the PCB? And the physical package being rather large (9mm diameter) maybe not be as efficient at dissipating heat into the air.
The other difference was the original diodes were Schottky. I don't know if that makes any other difference in power dissipation apart from the lower forward voltage?
So off I went to the old mates at Jaycar again and came back with pretty much every Schottky diode they had over 3A.
First cab of the rank was MBR735 - 35V 7.5A
https://www.fairchildsemi.com/datasheets/MB/MBR735.pdfAaaand we have a winner.
I plugged all of the RF cables back in and it has been solid ever since.
Well apart from one loud bang complete with smoke from the CRT enclosure.
The CRT - which is colour and quite dim - turned bright green.
I quickly turned it off, thinking that was good while it lasted....
Crossed my fingers and turned it back on and happy days it still works. I expect was due to dust on the tube.
I've been able to test the source and it looks OK - unlocked as I need to feed the R input to lock it and I've only got one N-SMA adapter.
Here area some board screenshots TiN, sorry just using a phone camera and I haven't yet taken the RF shields off.
I will try and do better ones, and I will do the RF modules as I need to clean them anyway - they are full of dust.