Electronics > Repair
Fluke 8840A Faulty CPU
cheapskate:
I used an 8k AT28C64 ROM. Despite having 4 extra pins the package is extremely similar to the 2732. Look at how U222 (external ROM) is wired up since it is compatible with 8k chips but ships with a 4k chip. My PCB design should also be compatible with both. I don't think it's possible to (easily) combine the ROMs onto one chip since the Z8613 only supports 4k internal ROM so you would have to move everything to the external ROM which would require a different CPU and probably software changes.
giosif:
I see it now: pins 26, 27 and 28 are all tied to VCC and the rest of the overlapping pins have the same layout.
Thank you!
cheapskate:
Turned out that the problem I was seeing with bad measurements wasn't electrical, the front/back selector switch was bad. |O The switch is actually made up of two switch sections connected together with a small plastic piece. That coupling was damaged and the switches wouldn't move together as one unit. I superglued the pieces together and everything seems to work! Meter passes all self tests and measurements seem reasonable. I still need to replace the power line filter, electrolytic capacitors, and the screen is a bit dim, but for now I'm just glad to have a working unit.
giosif:
Hi,
I am having issue with this adapter: I made 4 such boards and, out of them, 1 is not showing any signs of life (I need to investigate this one), but the other 3 kind of start, and the display is showing some readings, but then the display is periodically updated with random data.
Also, switching to resistance measurements sort of works but, then, if I try to switch back to VDC readings, the meter hangs.
I've created a clip showing the above behaviour:
https://youtu.be/Y1eGM1Da-GU
I am sure it is the adapter boards that have an issue and not something else in the meter because I've tried them on two 8840A and one 8840A/AF (which work fine with original CPUs) and the behaviour is the same as above.
It is true I cannot be certain these UB8840M ICs are 100% working, but having 3 of them behaving the same way is very unlikely, in my opinion.
Any suggestions where I could start looking for the problem?
Thanks!
cheapskate:
Are you sure you burned the firmware correctly with no errors? I don't have a 2732 to test with so I think my board ought to work correctly with it but I can't verify. I have assembled 3 boards, one of which came out defective because the CPU is dead. On closer inspection, I could see probe marks that weren't caused by me. Evidently that chip was damaged decades ago and put back on the shelf. The manual includes detailed troubleshooting procedures that include waveforms one can expect from the CPU. Maybe you have a short somewhere?
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