Electronics > Repair
Fluke 8840A Faulty CPU
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essele:
I picked up a couple of faulty 8840A’s a little while ago and have just got around to having a quick look.

Both were completely blank screens, with one doing absolutely nothing, and the other clicking a relay fairly regularly.

Focusing on the first one I did a bit of prodding around and came to the conclusion that the CPU was faulty ... a quick swap with the second CPU and it all sprang into life and looks good.

So the main CPU is a Zilog Z0861108PSC and these seem available online from China at least, but a bit of googling shows that they have a 4K write-once prom, so I’m assuming I can’t just replace it?

There are some comments about potentially reading and re-writing the prom, but I can’t find anything that makes me think anyone has been successful.

Does anyone know if this is possible? Or a source for a properly programmed one?

Thanks.

On a side note, both units came with the AC option (which was a surprise), but both of them seem to drag the power rail down and cause the blank-screen relay clicking, I’m hoping it’s as simple as a shorted tantalum, but I’ll get to that later!
bdunham7:
In a sane world your best source for a replacement Z8 CPU would be a parts unit.  I doubt they fail often.  However, as you may know--and I know from looking for parts for my 8842a--these go for stupid money.  A wrecked unit that looks like it fell out of one lorry only to be run over by another and was then scraped off the road by a vagrant goes for $50-75 USD here. 

I can't answer your question as to reading out and writing the PROM, but what did you test to determine the CPU was bad?  I had a similar problem with a different brand DMM and the "dead" ADC chip turned out to be an instability in the crystal oscillator, presumably caused by too high impedance of the crystal.  It is possible that I could have replaced the chip and the new one would shift things enough to start the oscillator, but it turned out simply replacing the crystal with a lower impedance "full-sized" AT cut model solved the problem entirely.  I just wonder if maybe you have something similar going on.  In my case, one the tests I did was to connect a 10M oscilloscope probe to a signal generator putting out the correct clock frequency sine wave  (8 MHz in your case) at 1V P-P or so and probing the 2 XTAL pins on the chip.  When I got the right one it started right up.  Just an idea to try.

essele:
Thanks for the reply ... I initially followed the troubleshooting guide looking for display related signals on the various display chips but saw nothing.

Eventually I started looking at the CPU, I could see a valid clock, but no activity on any of the data or address lines, so it wasn’t even starting to try to read memory, so I then switched it and all was fine.

It has crossed my mind that it could have been not properly making contact with the socket, so I will try it again, but I suspect it’s dead.
ignilux:
I have no idea if this will help you or not, but I have an 8840A that needed some TLC before earning a permanent place on my bench. Mine would be working perfectly fine for 15 minutes or so, then the screen would go blank and sometimes a relay would click. After some time of combing the forum, I found someone who had different erratic problems (I don't have the link saved unfortunately). Turns out that it's somewhat common for the solder joints on the main transformer to be cold / incomplete / cracked. I cleaned and redid all of the pins, and it's been rock solid ever since.

I'm not saying it's your problem, but it might be worth a shot before spending some cash on a new CPU.
essele:
Thanks Ignilux, but the clicking relay issue is a separate one and that only happens when the AC module is plugged in, my guess is it's either a power supply not being up to the task (although they all check out ok) or a fault on the AC board. I do have two AC boards exhibiting exactly the same problem, but also the same problem in two different 8840A's .. so there is some investigation still to do. I will check out the transformer joints though, thanks.

The CPU issue (I think) is pretty clear cut ... absolutely nothing going on, replace the CPU and it all works. I do need to just stick the old one back just to make sure it wasn't a seating issue, but my money is definitely on a dead CPU.
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