Fluke 8840A Faulty CPU

[SilverSolder]

^LOL Essele, you have to be one of the most stubborn + smart guys I've seen at work in a while!

[dtxy101]

 8)I have replaced the CPU of fluke 8840A with z86e21af1 of ST company.
Lt-48 program tool.
The bin file address should be written after offset.

[dtxy101]

z86E11

[SilverSolder]

8)I have replaced the CPU of fluke 8840A with z86e21af1 of ST company.
Lt-48 program tool.
The bin file address should be written after offset.

Sounds interesting, got some pictures of your project?

[Kjo]

Any update from essele?

[essele]

Hello ... I thought I'd summed it up nicely...

"I'm going to get my three 8840A's up and running and then I never want to see another one ;-)"

I got them up and running and then sold them.

... and I've resisted the temptation a few times when some 8840A's and 8842A's came up on eBay .. I'm working on some other things at the moment and don't plan on doing anything further with this, ever, even if my life depended on it. ;-)

Lee.

[cheapskate]

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.

[giosif]

[...]
I had to design and order a custom PCB but I made it fit and it seems to pass self tests.
[...]

Would you be so kind to share the PCB design files?
Thank you!

[cheapskate]

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.

[coromonadalix]

will you share you code to the eevblog community ?   is the ohms measurements problem(s) hard to resolve ?

thks

[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]

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.

Thank you very much!

[giosif]

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.

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?

[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.  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?

[giosif]

Assuming you are referring to the FW on the adapter board (i.e. U202), yes, I am certain I burned it correctly.
That's assuming the U202.bin file from the Ko4bb web site is good in the first place (just to check, the MD5 hash of the file I have is 79cebebdf156a9b1219a6c9f26da09e0).
Just to be on the safe side, I burned the FW once again, this time on a different 2732 IC.
I also bought an AT28C64B, burned it and tried with that.
In fact, I also burned the U222 FW again (on another 2732 IC).
I tried with these new ICs in their corresponding places, but the result is exactly the same.

Another thing I did was to order another batch of UB8840M, this time from a different seller.
I put in one of these newly arrived ICs but, as expected, the behaviour is the same.

In terms of shorts, I very much doubt it as I did both a visual inspection and continuity checks.
Also, I now have 3 boards that behave the same way, so it's unlikely I shorted them the same way.

To summarize:
1. The rest of the 8840A meter is working fine (confirmed with using an original Fluke CPU IC & associated U222 IC - true, those are FW 2.5, but I don't think that matters).
2. U222 IC (i.e. 2732 burned with corresponding U222 FW 2.3) is fine.
3. On the CPU adapter board:
   3.1. UB8840M IC is good.
   3.2. 2732 IC with U202 firmware is good.
   3.3. I checked the adapter board connections are according to the schematics and it all checked out.

The only remaining parts which are left are really the 3 capacitors on the adapter board (I am using 1 x 10 uF electrolytic, 1 x 100 nF film and 1 x ceramic cap whose value I don't remember now, this last one on the 2732 IC power rail), but I doubt it they have any influence in this.

Any other suggestions, please?

[cheapskate]

I briefly checked the circuit I uploaded against the boards I got manufactured and I don't see any differences in wiring. The only change between them is the cutout on the left side for access to test points. Here are the UB8840 datenblätter I used when designing the board.

https://hjs.lima-city.de/DDR/
http://www.blunk-electronic.de/datasheet/GDR/datenblaetter_ddr.pdf

[View[+]Finder]

Hello ... I thought I'd summed it up nicely...

"I'm going to get my three 8840A's up and running and then I never want to see another one ;-)"

I got them up and running and then sold them.

... and I've resisted the temptation a few times when some 8840A's and 8842A's came up on eBay .. I'm working on some other things at the moment and don't plan on doing anything further with this, ever, even if my life depended on it. ;-)

Lee.

A disadvantage of becoming a 'recognized expert' in any domain is that one can never retire . . .

So, this question is for anyone other than Lee: is there a possibility of extracting hidden digits from the ADC? The quality of the ADC and voltage reference in the Fluke 8842a show stability nearly equal to 6.5 digit meters like the Keithley 6500 and Keysight 34465a in tests I have done. (Depending on the choice of DCV input of course.)

[bdunham7]

So, this question is for anyone other than Lee: is there a possibility of extracting hidden digits from the ADC? The quality of the ADC and voltage reference in the Fluke 8842a show stability nearly equal to 6.5 digit meters like the Keithley 6500 and Keysight 34465a in tests I have done. (Depending on the choice of DCV input of course.)

Probably not, at least not directly.  The Recirculating Remainder method is different than dual or multislope integrators which often do generate extra digits internally since they measure time.  If I understand it correctly, the ADC on the 8840A/8842A models generates the equivalent of either 19 or 20 bits, which is about 500K or 1M counts, and then displays 200K counts--so there's not enough for another digit.  An indirect method would be to use an averaging algorithm on the readings and synthesize another digit--the older 850x monster meters do exactly that to generate either a 'CAL' digit (used only for adjustment) or an extra digit in the 10 volt range (8505B/8506B).

The 8842A does have remarkable stability, especially long term because it relies on aged and characterized references and resistors rather than an oven.  However I think short term stability, tempco and noise might not be quite good enough to merit another digit.  Of course there are plenty of actual 6.5 digit DMMs that probably don't merit their last digit either.

[Kleinstein]

The 8840/8842 still use the slightly odd reciculating reminder ADC. In theory with 5 passed and 4 bits each + maybe 2 bit extra from the last pass, there my be just 6 digit resolution. The slower conversions use the average over many such conversions - so in theory they may be a 6th digit, maybe even more. However it is not clear if the linearity and noise is OK all the way to the end - DNL may be limited by the adjustment of the ADC.
I would not consider the ADC especially good - it has some merits at high speed or with sampled readings, but not that great with slow measurements and mains hum suppresion can also be tricky.
With the computer one may use medium speed and get one more digit by averaging at the computer (e.g some readings).

The reference is very good - noise wise better than the usual LM399 found in 6 digit meter, though without a oven stabilization and not as good with the TC.