Electronics > Repair

Fluke 343A repair

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Rax:
In undertaking the recapping job, a word of warning is paying attention to the conformal coating. Not a thick or really hard to pierce/melt polyurethane coating - I wore a mask half way through and had the door wide open for ventilation - but it can mess with the desoldering tips and require constant cleaning and retinning. I clogged one tip, which is very frustrating,  given the cost of a Pace tip.

Rax:
Well, I wasn't aware of this, but these "small current" fuses have not negligible resistance. So my continuity test was likely not a proper test. I got replacement fuses, and both the replacements and the original fuse measure around 80 ohms... So I guess my issue is not the HV fuse having gone open. Live and learn.

So I'm back to the drawing board and head scratching. I may decide to continue my recapping job - my money's still on caps having failed during my "bake out" ("-in...?") initial run - or just take a closer look at the SM and see what my next troubleshooting steps should be.

Rax:
I decided to start over more slowly and methodically. I've redone the 4-39 tests, and got slightly different results. One thing I did differently this time is to bridge the OUTPUT and SENSE terminals, and that answered one of my prior questions (see below).


* TP2: 2.78k
* TP3: 5.3k
* TP5: >160k
* TP6: 1.98k. Not compliant?
* TP7: 680k
* -SENSE: 57
The last on that list answered my own question: the OUTPUT-SENSE bridging is supposed to be in place for these tests. Hopefully, this helps others that may run into the same dilemma I did.

Rax:

--- Quote from: Rax on May 31, 2024, 12:22:13 am ---
* TP6: 1.98k. Not compliant?
--- End quote ---

The SM calls into question CR26, CR27 and lamps DS5, 6, or 7. But I diode-tested the CRs out of circuit and the DSs are lighting when switching voltage range. The resistance from TP6 to +SENSE doesn't change at all either when operating the range switch.

Rax:
I rechecked the voltages at the test points, and I'm not seeing any significant variations.

Due to the non-compliance of TP4 and TP5, I am becoming suspicious of SINK SUPPLY + CROWBAR DRIVER and maybe the HV POWER SUPPLY. I've diode-checked CR1, 2, 3, 4, 5, and they tested fine.

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