EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: Nogtail on February 12, 2021, 01:43:52 am
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I got a broken Fluke 8840A from eBay with the hope of repairing it. The display flickers like crazy and shows different random junk each time I power on the meter. The display/keyboard controller seems to be working but I haven't seen any life out of the CPU.
There's a bloke on eBay who sells CPU replacements (https://www.ebay.com/itm/Fluke-8840-8842-U202-CPU-Replacement-/184631109930 (https://www.ebay.com/itm/Fluke-8840-8842-U202-CPU-Replacement-/184631109930)) but only offers them with v2.3. My unit has U222 labelled as v2.5 so I'd imagine they aren't compatible.
Does anyone have a dump of the V2.5 CPU ROM, or is there a replacement I can use for U222 and flash it with v2.3?
Thanks!
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You should ask the seller to be clear, but it appears to me that you just plug this replacement in and remove the existing ROM if there is one. V2.3 would have had a Z8 labelled processor and a separate ROM in the U222/U211 slot, later versions used a version of the Z8 with built-in mask ROM, these typically had a Fluke p/n and sticker. I don't have a list of versions, but V4.0 is certainly like that. There have also been at least two piggyback setups where a ROM chip sits on top, but I don't have one apart at the moment.
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You might be right - I assumed the EEPROM on the replacement CPU was just replacing whatever was in the mask ROM/piggyback. It looks like the lower 4K are stored on the CPU and the upper 4K are in the external chip. Looking at the replacement CPU it has an 8K EEPROM so in theory could fit everything. I'll see what the seller says.
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Take care if you have the GPIB Option. The Version of this has to match yours main Version as i know.
Habe you looked at the page where all the old eeprom stuff is loaded up? (Cant remind the Name currently)
Did you try with the service manual the CPU test / pattern?
Are you sure that there is something in the CPU when its the piggyback Version?
My assumption was that you have to get a (hard to find) piggyback CPU und write the eeprom / a new eeprom with the matching Version in top?
I think there was a project anywhere where the (hard / expansive to find) CPU was replaced with a diy option here in the forum?
or try to Google..
Edit: OK the seller sells this diy mod. I think you can find more information for that i found it some time ago it was a open project..
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Thankfully I don't have the GPIB option so I don't have to worry about that.
I've gone through the service manual, but the CPU doesn't seem to want to enter the troubleshooting mode. Shorting TP205 to the shield then powering the unit up doesn't seem to do anything productive. It seems to keep TP205 driven high for some reason. The only activity I can see is the 8MHz crystal pins and TP201/pin 40 which is a 4MHz square wave.
As far as I can tell the lower 4K are in the CPU mask ROM or piggyback ROM and the upper 4K are in U222. I don't want to use the original U222, but the AT28C64B seems like it should be compatible, so I might try and downgrade to 2.3 and write it to that.
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Check this thread:
https://www.eevblog.com/forum/repair/fluke-8840a-faulty-cpu/ (https://www.eevblog.com/forum/repair/fluke-8840a-faulty-cpu/)