Fluke 8840A Faulty CPU

[cheapskate]

I am glad you got the CPU working giosif, perhaps there was some mismatch between the 8840A and 8840AF ROMs. I am surprised that these two have different ROMs at all, since I though the AF model didn't have any hardware changes aside from the ADC board being included as standard. I have previously tried using my UB8840 adapter in my 8840A multimeter and it worked without the 4.0 hardware. I didn't test it rigorously, but it did work. 

The existence of a continuous 8k ROM that also works as a split 4k image is very intriguing. Looking at the documentation, external memory is mapped into the same address space as the internal ROM. It may be possible to use a ROMless Z8 variant like the Z8681 with an external 8K ROM since U222 socket supports 2864 devices. This would eliminate the need for an additional PCB since the Z8681 is a drop in replacement for Z8611.

[Kjo]

The 8840A & 8840AF have to use different ROMs because the rear terminals on the 8840A are just duplicates of the front 4 kelvin terminals. But the 8840AF measures 2 different voltages at the rear pair and computes the ratio of the two. I have 1 AF out of 24 meters but never have used the ratio function. The F/R switch position is sensed via S301 to pin 32 of the Z8611 on all meters. Normally the front or rear 4W sense terminals are routed to the scaler & ADC via Q303 when 4 wire ohms is selected.
I have never seen anything that indicates that the AF motherboard is significantly different. The whole meter has better noise immunity ( more screws in bottom of cover and some upgraded components) but that is it. They just figured a way to use the rear sense terms to measure a second voltage and calc a ratio. The FW measures the volts term then the reference term. The GPIB doesn’t know that there is anything different going on. The ratio is shown with DC volts units if I remember correctly.

I had a guy in Germany with a bad AF. He swore it was the U222 ROM that was bad and had tried every combination of ROM in the wild with no success. But AF ROMs weren’t wild. I looked through my collection and noticed a AF among them. I never really paid that much attention to the marks on the front. I ripped the U222 ROM for him. Took 3 weeks to get there, but he sent me a picture of the working display!

I wish I had a Z8681 for I would try that experiment. I have a bunch of other variants that all work. The V4.0 F ROMs work fine with Z8613 piggybacks (split), Z86E21 OTP 8K, Z86E21AF1 8K UV windowed and the UB8840M clone (split). There is a lot about these Z86 devices that has become obscure with time. Thankfully the hardware is robust and fairly well documented!

Kjo - KO3Y

[cheapskate]

After reviewing the schematics, it seems that using Z8681 not possible. Unfortunate, because they seem reasonably common on eBay. U202 is capable of using all 8k of the external ROM, but not in the way Zilog intended. The Z86** series can do 8, 12, or 16 bit addressing whereas an 8k device requires 13 bits. To avoid wasting 3 pins in 16 bit mode, Fluke used 12 bit mode and an extra pin that is then controlled manually. This means that it is impossible to read all 8k without the software being aware of it. At least that's what I think, based on the schematics.

I do remember seeing some activity on U222 address pin 12, device pin 2 / U202 port 06, device pin 19 while I was testing my adapter so I wonder if Fluke intended for the software to be 8k aware. I don't have a Z8681 to test with either and it is pointless to try if there is no activity on that pin, but I will investigate tomorrow.

FOLLOWUP: There is no activity on that pin.

[Kjo]

Options for a bad CPU.

I have had a number of private inquiries on this subject. I thought it worthwhile to bump this thread with a consolidated set of reliable? options.
Given the previous comments in this thread I want to make it clear to the uninitiated that the 8840A, 8840AF and 8842A ALL use different
ROM sets. You cannot interchange them! And there are multiple versions of each in the wild.

• Find a working Z0861108PCS mask ROM CPU with the correct ROM for your meter model.
  But be cautious. These mask ROM chips have a habit of failing or faulting when hot. You might
  get one that works initially but fails later.
  
• Find a Z8613RS piggyback CPU and burn a LOW 2732 EPROM to match the HI U222 you already have.
  Currently Unobtanium.
  
• Find a Z86E11F1 (4K) windowed EPROM CPU and burn the LOW ROM into it to match the HI U222 you already have.
  You may have a challenge finding a burner for a Z86E chip and the algorithm is mfgr dependent.
  
• Find a Z86E21F1 (8K) windowed EPROM CPU and burn LOW & HI ROM into it. Be sure to remove the EPROM from U222.
  You may have a challenge finding a burner for a Z86E chip and the algorithm is mfgr dependent.
  
• Find a Z86E21F1 (8K) OTP EPROM CPU with the LOW & HI ROM already in it. Be sure to remove the EPROM from U222.
  These are super rare and were only used near the end of the 8840A production run.
  
• Find one of the several Z8613RS PCB based clones that work like the piggyback CPU. Use the ROMs of your choice for the meter you have.
  Here is the one I designed based on the UB8840M clone from East Germany:
  https://www.hollywoodcontrols.com/phpFluke/HCZ8611C.php
  Not cheap to manufacture considering all of the components.
  

[ignilux]

The posts in this thread are not absolutely clear on the ROM versions used. But it should be clear that 8840A, 8840AF & 8842A all use different ROM versions that are not cross compatible. My UB8840 clone worked with all 8842A ROMs, but not all 8840A ROMs. In fact, only the 8840A V4.0 ROMs worked flawlessly in the 8840A meters. I have a suspicion, but not confirmation, that there is a timing difference between the Z86 and UB8840 such that data transfer to the display controller is incorrect. There is likely timing differences in older ROMs that are improved in the V4.0 version.
(Attachment Link)
(Attachment Link)
As mentioned elsewhere, if you can find a Z86E21AF microcontroller with a glass window, it can be programmed with the complete 8K ROM, though I have only verified this with the 8840A V4.0 ROMs.

Sorry to bump a thread that's over a year old now, but I'm thrilled to find somebody else with an 8840A that has the Z86E21 windowed EPROM CPU and firmware V4.0. I've been pulling my hair out trying to figure out how to extract the firmware from these, but thanks to Kjo I no longer have to!

@Kjo, does your 8840A with the V4.0 firmware and Z86E21 appear to have a number of other hardware improvements over the service manuals readily available on e.g. BAMA and KO4BB? Having recently completed a recap of my HP 3456A I opened the 8840A to check for any suspect electrolytics. To my surprise it looks like most (all?) of the voltage ratings of the caps in my meter are over and above what is listed in the service manual. Not sure if somebody has been in here before me, or if Fluke revised the spec themselves. This meter appears to be from the late 90s, so if these are the stock batch of caps I might as well replace them for peace of mind. If they have been replaced in the meantime then I won't bother. Cheers!

[Kjo]

There are very few versions of the manual for the 8840A (12/91 revised 5/97), and it is incomplete in terms of schematics. But you can combine it with the 8842A (12/91 revised 7/96) which is complete.
The main PCBs are slightly different, and some of the option boards are jumpered  specific to the 8840A/AF (AC Converter) and/or have different ROMs (488 card) but the schematics are the same.

But I know of no material changes to the main PCB from 1984-85 to 1997-98. Your meter may have been re-caped, but of the 30 meters I have tested, I found only 2 that needed new caps (and not all of them).
I dont subscribe to the "re-cap" everything, particularly industrial equipment. I typically measure, with a pair of HP3457A scanning system meters), AC & DC values of the 6 main input filter capacitors
and the 10 regulated DC supplies. If these meet the Fluke specs I dont tough them.
The only thing I have found that changed is the supports under the main PCB in the transformer area. Meters up to about 1988 have only a white nylon hex standoff to keep the PCB from sagging. Later
models have a machined Delrin transformer support. I suspect a known failure was de-laminating of the transformer solder joints. (I always re-solder the transformer if I poen a meter.)

kjo  KO3Y