Thank you for replying so quickly! 😀
Yes, that picture of the PSU is very similar to mine. It has no mention of 3.3vdc, just like that one.
Then that's one worry less: the lack of 3.3V is by design.
My PSU no longer does anything. It made a dying rattle, and choughed and smoke came out of it.
OK, that's pretty clear then 😉
It didn't take the motherboard with it when it died as the the motherboard worked fine with another PSU for about a week before the board stopped posting. I did connect it just now with two different good PSU's and the same thing happened.
Maybe, but I'd say the PSU blowing and the motherboard dying so close together is unlikely to be coincidence.
The CPU fan starts, the floppy drive light is constantly on (perhaps because I have the floppy cable the wrong way around?)
Light constantly on sounds like cable the wrong way round. In any event, try with no floppy connected.
but it makes no sound or beeps. I have tried several different ram sticks without luck, all of which were good when I stored them, but I will test them in another machine! I have no known good CPU that is similar, but I do have another socket 7 CPU. An AMDk6 II-450, but can I use this on this board? Will it just clock down to whatever the FSB allows?
FSB isn't an issue, it's the motherboard that determines the clock, not the CPU, so it can always run slower than specced. Bigger issue is voltage. A K6-2 wants 2.2V but your Pentium MMX get 2.8V and I don't see any voltage settings on the board. It's possible that it might autodetect or something, but more likely it would feed that K6-2 2.8V as well. Not a good idea.
What I do see: you say that's a Pentium 200MMX in there, but if I look at the dipswitches it's set for 1.5x, which an MMX reads as 3.5x, so the CPU is set for 3.5x66=233MHz. Now usually you can overclock a Pentium MMX with ease (I had a 200MMX running at 350MHz once), but in case of doubt, set it back to original speed or - even better - lower. I'd recommend setting it at 120MHz for testing, so 2.0x 60MHz. See here, now switch 1,2,3,4 are set to on,off,off,off. Try off,on,on,off instead.
I have connected the board to a PSU, with only RAM attached, but no post. I did notice a faint etectrocution when I touched both the board tower and the PSU at the same time (the PSU was laying on the desk next to the tower. I will follow your directions and do it properly with tested RAM and with the board taken out of the tower.
That shock sounds like an earthing/grounding problem to me. Is the PSU connected to an earthed power outlet? And the monitor? In itself that's not likely to be the cause of your problems, but it's definitely something to look into.
Another thing to rule out: that COAST cache module. It's unlikely to be the cause of the problems, but it's easy enough to remove just in case.
The hardware looks good to me. The capacitors look great, if fact, but I've learnt that looks can be deceiving when it comes to caps. All of them look flat and clean! I don't know how to measure ESR, but I will read up on it!
I had forgotten the spectacular build quality of Siemens-Nixdorf stuff. Massively thick boards, huge heatsinks, high-quality components - and those silver coloured legends. Beautiful. No guarantee that it all still works, but I'd trust this stuff more than most.
It's a D990 rev E11 motherboard by the way, the name is in the little white field above the dipswitches.
I don't have a post debug card either, but will order one right away. That sounds useful 😀 It's one of those PCI cards that is hooked up to a small LCD screen, right?
Yes, although I suggest getting a PCI/ISA combo card. Frequently the POST stuff is only output on a boards' oldest bus - and of course you have at least one system with no PCI at all.
I've had a look on the board and the bios does not seem to be socketed. I will look again. BIOS recovery procedure sounds like a good and easy step. 😀
Yes, I'm surprised by that. Most Siemens-Nixdorf boards I've seen have nice socketed stuff. I have a BIOS EEPROM here from a D1192 board. Does make me wonder what that PLCC32 socket at the rear of the ISA slots is for...
Regarding Overmann. I am that same idiot, and this is that machine! 😀 I don't know why I have a different handle on this site. I was looking for this machine for years and when I finally get hold of one I screw it up after only two weeks. 🙁 Btw. the 486 Motherboard that I bought from you is working like a charm! 😀
Talking to yourself probably isn't going to help much no 😉
Good to hear the 486 board hasn't let you (or me) down 😀