Fluke 87-I repair

[staze]

No, I haven't. I hadn't really thought that could be an issue since I wouldn't think it would be in the signal path in DCV...

I'll give it a check tonight though, thanks!

[lowimpedance]

I think you have reached the point where you need to cut some traces as Modemhead suggested to see if the dc is coming from U4.
It looks likely that U4 is toast.

[staze]

I think you have reached the point where you need to cut some traces as Modemhead suggested to see if the dc is coming from U4.
It looks likely that U4 is toast.

or at least, part of it is. since the higher ranges for DCV work, as well as DCmV, Ohms, etc.

Yeah, I'll just have to bite the bullet and cut them. Just for completeness sake I think I'll pull one of the MOVs too just to make sure they're not bad it some weird way.

Guess the real question is, if U4 is bad, do I keep the meter and use the ranges that work, or do I turn around and sell it on eBay as a parts unit?

[staze]

So, pulled the MOVs, no diff (as expected).

I'll see about cutting that pin tonight. Is there any chance it's the op-amp (AR1)? I just can't figure why the outputs were pegged like they were... but I guess cut the pin if nothing changes?

ugh.

[staze]

Pay real close attention to the APV0, APV1, and APV2 lines.  You might break one at a time and see if the voltage on the meter input jacks disappears.  Maybe start with APV2, since it seems that the 40V range is the screwiest?

Cutting APV2 (96) resulted in just killing that range (the meter goes into auto-range loop, or just OL on all but 4000V), and still voltage on the jacks. Cutting APV0 (99) oddly killed the voltage on the input jacks, but still shows -35V on the meter's reading. Cutting APV1 (98) resulted in no change.

Anywhere else I might want to look before I call it a goner? To satisfy my own curiosity, I cut the VDD and VSS rails for AP1, and that make no difference.

So... is U4 toast?

[staze]

So, thought I've had. Would I gain anything by re-cutting APV0 (pin 99), then checking for current/voltage on each side of the cut? Or am I just paddling against the reality that U4 is toast?

[lowimpedance]

Its a goner, keep for the parts on the off chance you get another, or just move it on as a parts only.
You win some and loose some, look forward to the next repair job  .

[Dave Wise]

Some years ago I managed to kill a Fluke 87.  I hooked a service-grade high voltage probe to it (the type that uses the meter's 10 Meg as the bottom leg of a 1000:1 divider) and probed 8kV without setting Manual Range.  There was no noise or smoke, but the meter started reading about +0.5V offset.  After reading this thread I probed around - using my Fluke 895A DC Differential Voltmeter which goes down to 100uV full scale - and found that APV0 is sourcing some current.  It's definitely APV0, not APV1 or APCC, even though in DCV mode and 4V range the chip connects them together.  About 40nA cold, 150nA warm.  Vdd and Vss are okay.  The board is factory-fresh clean so it must be the main chip.  Pity.