Hi Joe
Yes, its roughly similar to the old Dave video on the Fluke 27 input protection:
I do not know if the desoldered blue MOV is still fine, but given all the MOV are across the inputs, they were exposed to the same insult, and will be bound later to fail. However, I will destructively test them later and report back, Imcurious to know what state they are in; I'll also sacrifice one of the replacement MOVs to see what will trigger a failure.
When I first 'refurbished' my 87V 10 years ago, all the MOV appeared "blue" virgin from the factory.
Of interest, at this link, from another user back in 2013, my failure is the same two MOV.
https://www.eevblog.com/forum/testgear/fixing-a-fluke-87v/From another user, the native undamaged MOV area:
I could find no equivalent 575V varistor that was ~ 5mm in diameter, this a fairly uncommon part. At most it was 420V. Given the 87V failed at ~ 1300VDC with the 575 "blue" rated MOV, using a reduced working voltage 420V MOV could make it fail sooner, unless, there is something about the part Fluke uses.
Overall, I think Fluke made a safe choice, because 5mm MOV at ~14J will blow less violent: burning, heat and smoke. But the bad side, is it could be a bit sensitive as it has less reserve if sees a high voltage transient, which maybe my case. Regardless, Fluke's lifetime warranty won't cover usage damage as its for product build defects only.
In this video starting at 16:00, the 87V is shown repeated hit with high voltage transients, but while it appears to function, whether its remains factory accurate is unknown but suggested by Fluke literature. At most one can say it can take a transient and not fall apart. In my case, the meter was functional and appeared undamaged, but not factory accurate, which is useless from a DMM point of view.
To diagnose the issue, I desoldered one foot of one MOV and it returned to factory accurate, so the in series resistors/PTC were unaffected. It stands to reason that if a transient were to partially damage any input MOV, the Fluke would start to act 'out of factory' since MOV across the input would affect the input impedance. I suspect this was the case as since repaired it seems to be faster or solid reading that in the past, and I use it fairly often.
The replacement I used are 3-4x bigger and rated 65J, if they blow it can burn with greater energy and likely damage nearby non affected components compared to the more isolated placed MOV from the factory.
I don't remember what the meter looked like on the front end. Looks like the V input goes through the PTC/resistor before it hits the MOV. Looks like on MOV shared for the common return. What limits the current for the second leg, third MOV? Were the two black ones damaged and the blue fine? Had the back ones been replaced before? Maybe they were the wrong value? The last one I had ran, all the MOVs were blue.