Yet another Fluke 87 question

[dherron]

Got this for free and turned it on to check it out. DC voltage didnt work. opened it up and found this.
I desoldered the crispy resistor.  The remaining resistor is 906k and the fried one was 3.somethingk. Isnt the 909k supposed to go in the outermost position?

[Kim Christensen]

There are five different versions of the 87... Which one do you have? The circuitry is quite different in some of them.

[dherron]

it just says 87.  i know its from 1996 or before. board says rev 4G. best i can do.

[Kim Christensen]

Does the 909K connect to E1? (Spark-gap)  From your picture, it look like it might. Maybe they've just mixed up R1 and R2 on the silkscreen drawing:

[dherron]

I saw those schematics and that is what is confusing. in the parts listing it says the same thing as the board diagram but the actual schematic it shows the 909k going to the spark gap.  furthermore, ive looked at pics of other 87s and its the opposite of what i have.

[Kim Christensen]

The other thing that seems wonky on yours is that E1 and RT1(missing?) appear to be swapped also. (According to the manual)
I wonder if someone tried to repair it prior and messed things up. Does the 909K appear to have been replaced in the past?

[dherron]

more pics.
It doesnt look like like someone messed around in there but i cant be sure.

[Kim Christensen]

You have other problems on your PCB. Looks like the red RV1 has a burn mark on it as well. Therefore RV2 might also be bad.

It could be that yours is an earlier variant of the 87ii/iii where the R1 & R2 are arranged in a similar fashion:


https://www.eevblog.com/forum/repair/fluke-87-iii-misbehaves/

[dherron]

well this is interesting. think ill go find a suitable thermistor and a 1k resistor. i got 0 resistance on that damaged varistor.
the one you posted has a lot more smd components. I think this was 1996 or prior because i met the previous owner that year and he had it then and i wanted to borrow it for something i was working on.  guess he is looking down/up(depending on where he went) at me and laughing ass off. 

edit. the asic has 96 stamped on it and the serial number on the back of the case starts with 6.

[floobydust]

That multimeter had a major input overvoltage, it takes a few kV to arc over to the adjacent part and wreck that resistor. It looks like it arced to the missing outside resistor.
If it carbon tracked the PC board then the damage is too severe to repair IMHO. Must be a microwave oven job.
Because the parts are old and have a safety function, they are not easy to substitute.

Fluke p/n 831594 RES, CERM, 909kΩ 1% 2W 100ppm
I recognize that as obsolete IRC GS-3 series, now Welwyn/TT Electronics GS-3 thick film metal glaze resistor, 1,000V surge rated, 5.7mm D 13.1mm L.
This is the brown resistor you have that is damaged from the arc.
Older 87's used (blue) Vishay Dale FP69 3W MOX 150ppm flameproof.

Fluke p/n 822015 THERMISTOR, RECT, POS, 1.5kΩ 30%
The PTC thermistor needs to be 1,000V rated. All I am aware of is Vishay PTCCL07H100VBE but it has no UL approval for some reason.
Second contender is G.E. now Amphenol YS4020 but it is 1.1kΩ.

Fluke p/n 1275713  RESISTOR, CERMET COMPOSITION, 1kΩ, +-10%, 1W, -1300+-300PPM
Original KOA Speer PCF-1C102K, possible substitute Ohmite OX102KE.

Fluke p/n 1277360  THERMISTOR, POSITIVE, 1.1kΩ, +-20%, COATED, RADIAL
Original G.E. YS3961, possible substitute Amphenol YS4020.


The missing resistor is either 3.5kΩ or 1kΩ surge-rated so check your service manual.
Not sure but 1st gen 87 Mr. Modemhead repair the resistors are switched in position.

edit: added the 87V 1k surge resistor and PTC for people doing repairs on others.

[dherron]

good info floobydust.  everything works except the dc V/mV. the one i removed was the 3.5k that had a chunk blown out the side of it.  the charred yet working resistor is 906K. I dont know what happened to that thermistor. This guy was an hvac tech. i dont think he would have messed with it. there are no parts of leads in the hole where it would have been.  like it was desoldered. im just want this working and reasonably accurate. im only using this for some microcontroller and robotics projects.  its never going to see anywhere near the kind of juice that did this damage. 

[floobydust]

What's wrong with DCV? I see a red MOV is also blown, I would replace both.  If they are shorted DCV will read way low.
You can temporarily pull them and see if DCV works, otherwise the overvoltage made it to the protection transistors.
 

[dherron]

since ive got it, it reads 0v on dc V.

[floobydust]

With another multimeter you'll have to check ohms on the protection transistors, and the MOV's to see who is shorted. It could even be the multimeter IC because the 909kΩ part is not supposed to ever have drama across it. You've got three failed protection components so far which is not good.

[dherron]

I went out to the parts stores and bought close as i could find components. works fine now. just had to replace the resistor, ptc and one of the movs. thanks for the help guys.