Electronics > Repair
Fluke 8840A Faulty CPU
Dakkahun:
Hi.
The resistance issue is probably caused by the custom Fluke 700013 unobtanium analog switch with latches.
See these threads: https://www.eevblog.com/forum/repair/fluke-8842a-lm308a-supply-voltage-above-max-rating/msg1687820/#msg1687820 and https://www.eevblog.com/forum/testgear/replacement-for-fluke-700013-ic-(quad-spst-analog-switch)/
By the way, what would be the best way to read the ROM from a z8613 CPU? Both the piggyback and the external chip? Would a TL866II+ be sufficient?
coromonadalix:
the newest tl866II+ variant is a total crap, it doesn't program 21v devices, i would try to find the older series or a clone of it
or the mcumall gq-4x is a good one, sure its priced higher
cheapskate:
The eprom FLUKE used is the 27C32 so you should be able to read it in pretty much anything. I don't have a programmer so I rigged up an Arduino to do it. I got the files for the CPU and external ROM from KO4BB, both 8842A and 8840 files are available. I have both an 8842A and 8840 to test with, and I can tell that one of the several 8840 U222 files posted is incorrect. I don't remember which one, but it is off by a few bits. The 8842A files worked but I can't verify that they are correct either.
I haven't had time to look into the resistance issue yet. It just measures open circuit all the time. Also fails self test for AC ripple. Would suck if I have an unfixable unit after getting the CPU sorted.
giosif:
--- Quote from: cheapskate on December 24, 2020, 08:56:22 pm ---Sure thing. I added a cutout to left side of the PCB to give better access to some test points. I hope I configured the KiCad project correctly so that you can open it. I may be placing my remaining PCBs on eBay later as I don't really have a use for them.
--- End quote ---
Thank you very much!
giosif:
--- Quote from: cheapskate on December 24, 2020, 04:10:33 am ---Great work essele, I hope your project gets some attention from the vintage computing community as it seems these Z8 microcontrollers were used in all kinds of inconvenient places. I managed to fix my Fluke 8840 by replacing its CPU with the East German equivalent UB8840. I got the idea from http://john.ccac.rwth-aachen.de:8000/patrick/Z8emu.htm I had to design and order a custom PCB but I made it fit and it seems to pass self tests. Now I need to figure out an issue with resistance measurements.
--- End quote ---
One question, if I may: what IC did you use for the ROM?
The ROM sitting on the original Zilog CPU is 4 kB and I am not able to find a 28 pin 4 kB EPROM IC.
Or did you combine the IC ROM with the remainder of the code from the U222 IC and write that on a 8 kB ROM?
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