Trying to repair Fluke 8840A/AF

[giosif]

Hi,

I am working on a Fluke 8840A/AF meter which has a rather puzzling problem when using the rear inputs.
A few details:
1. When using the front inputs, everything works just fine.
2. If I switch to the rear inputs, DCV readings are out of whack and nothing else really works - e.g. when I press 2W resistance or 4W resistance, I get error 31 (which, according to the manual, should be displayed if I tried to enable current measurements, either DC or AC, with the rear inputs enabled). 

Things I've checked:
a. Power rails - all check good, either with front inputs selected, or with the rear ones.
b. F/R switch - checks good.
c. F/R sense signal to the CPU (i.e. U202) - checks good (i.e. pulled low when rear inputs are selected).
d. CPU - I've swapped the CPU with another one (as well as the associated U222 ROM IC), but the problem remains.
e. Swapped U212 with the same IC from a donor board (no change).
f. Swapped U219 with the same IC from a donor board (same situation).
g. Socketed ICs - re-seated all socketed ICs, with no difference.
h. Intermittent contacts - poked the board and a few components, but no change.
i. Checked U222 contents and they are good.

The only lead I have right now is the strange behavior when pressing 2W or 4W resistance buttons - i.e. getting error 31.
That would make me think there must be some logic fault somewhere, so the CPU interprets the 2W and 4W button presses as mA DC presses.
But, then, why does this happen only when the rear inputs are selected?
What else changes when the F/R switch is set for the rear inputs (other than the path of the input signal)?

Any suggestions are appreciated.

Thanks!

[giosif]

Just an update on this: turns out it was the CPU, after all.
I am not sure what I did first time around that I didn't see a difference, but I now swapped the CPU (and associated ROM IC) again and the readings are the same between the front and the rear inputs.
So, I am guessing the initial CPU doesn't like its pin 32 (F/R sense) pulled low.
Strange...

[giosif]

The adventure continues with this meter:
* self-tests 16, 17, 18 and 20 fail almost all the time
* self-test 10 fails occasionally (maybe 12 as well)
* self-tests 8, 9 and 19 fail sometimes

Going through the troubleshooting steps in the manual, I noticed something strange: pin 12 on U305 reads -25 V or so, while the manual says it should be -5.5 V.
Investigating a bit more, I noticed the location of CR302 and CR303 differs between the manual (which is, in fact, for the 8840A model) and my meter (which is the 8840A/AF model).
Not only that, but the anode of CR303 is shown in the manual as connected to the -15 V power rail (and I can confirm this is indeed the case for the 8840A model, with an 8840A meter I have) while, for this 8840A/AF meter, it is connected to the -30 V rail.
Also, upon careful inspection, the CR303 installed in this 8840A/AF meter is a 1N748 zener (3.9 V), while the schematics in the manual show this as being a 10 V zener (a 1N961B, for the 8840A model, anyway).

Does anyone have the manual for the 8840A/AF model to take a screenshot / picture of the schematics and share here, please?

Or anyone with a working 8840A/AF that could confirm:
1. The anode for CR303 is indeed tied to -30 V rail.
2. CR303 is a 1N748
3. The voltage on pin 12 of U305 sits at about -25 V.

Thanks!

[giosif]

In the meantime, I traced down the problem with the failed tests: an intermittent CR402.
The diode kind of works, but it sometimes decides not to and that's when issues appear.
I've replaced it with another 1N4007 and all tests complete fine now.

This also means all previous observations about CR303 are correct and intentional.

Hope this helps someone in the future.

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I believe the 8840A/AF is a designation for a 'military' version (A/AF = Army/Air Force), probably to use 400Hz AC power and filter accordingly. There might be some 'mil spec' components as well.