exxos's Atari, Amiga & retro forum

It was the power connector!

I'd had the machine running for a few minutes and decided to check the chip temperatures, but to do this I have to remove the power supply. As I pulled the connector I noticed that the +5V wire was getting warm.

Some Electrolube contact cleaner and a few replugging cycles and...

OK, there seems to be a PSU problem. It seems to not be outputting +12V and the +5V is 4.37V. I’m surprised the system even works at all.

This morning, without touching anything, I turned the machine on and the CPU isn’t running, but I do have +5V up at close to 5V and +12V!

The CPU is getting warm and it’s getting at least one clock. Practically all the other pins are floating. I’ll have to print out the pinout.

I wonder what could have died after just sitting over night.

I've a sneaking suspicion that it's probably the PSU. I'll get the bench power supply down from the loft later and see if that makes any difference.

OK. No difference.

The machine's pulling 2 amps, which seems more than it should. I think that the next step is to get a whole load of axial capacitors and replace them all.

So, it seems that the CPU is trying to do the first instruction fetch. The ROMs seem to be enabled and outputing bits and the address buffers are passing them through but the CPU isn't doing anything and just sitting there.

I s'pose it could be a bad ROM (though I've read both in the TL866 OK) or bad sockets on either the ROMs or the CPU.

It's not the ROMs. The data read from them runs in an emulator and writing the data to 28C256 EEPROMs in the high half gives that same symptoms.

At this point I'm not sure where to go. I could look for another CPU on-line but I can't guarantee that it will not be fake or faulty. I could replace one by one every 74LSxxx chip on the board which interfaces directly with the CPU. As I know that the memory sub-system works, as does the video sub-system.

Discovered the remains of a label, obviously from a repair shop, with CPU written upon it and also possibly "BAD" too. So maybe it was a fluke that it ran last night. I've ordered a "new" 8086 from Cricklewood Electronics, so we'll see if that is the issue.

In the meantime I've been glueing the case together. There's only one significant part of the case missing so it should actually be usable.

Initially I'm using liquid plastic adhesive which melts the plastic to give the bond. In places I'm going to have to supplement this with epoxy resin, though I have to be careful not to get any areas where the motherboard slides into.

Very cool find I wonder if a nec v30 cpu would work in it

Tom

Tomswork wrote: 09 May 2021 22:07 Very cool find I wonder if a nec v30 cpu would work in it

Tom

Was that a full 8086 compatible then?