Electronics > Repair

Fluke 343A repair

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Rax:
I got a 343A this weekend at the swap meet for a song and a dance and ran it, as I always do, for a day, continuously, to trigger any dormant for imminent issues that may it has.

It worked very well for about 24hrs, which readings reasonably close to spec in all ranges and outputs all the way to 1000V - the only issue I could see is some hesitance from the protection circuit (seemed to trigger a bit randomly for the first second or maybe a minute of running, but so little that I didn't give it much thought, especially as it didn't repeat for hours of play, and deemed it just what a unit having sat for years and maybe decades would do) - then, after running unsupervised for a while (frankly, I forgot it on while leaving the house), it seems to have triggered its protection and now it won't successfully switch from STANDBY/RESET to ON (protection would kick in).

I didn't yet get a chance to poke around too much, just did the resistance checks at 4.39 in the manual (p.4-8), and came out with this:

* TP2: 2.7k
* TP3: 8.3k
* TP5: 140k
* TP6: 2k
* TP7: 53k
* -SENSE: 2M
I don't put a lot of currency in these readings (in fact, the last makes no sense to me after looking at the schematic...?), given they were supposed to be done with an analog meter and I'm trying this with a Fluke 189. I assume the latter would interpret some of these very differently, if only given how a digital reading would fluctuate in ways that a needle would simply not see. But I will check the parts that are suspect for #2, #4, $5, and #6 anyway. Next, I'm thinking of doing the STANDBY/RESET voltage readings and hopefully there's a smoking gun somewhere. I can't do anything in ON state, as the protection system kicks in, apparently, every time I try now.

Although there's apparently some coating to the PCB, I may try a recap next, which I render mandatory given the age. I don't have any >=100uF/450V at hand, but they'll be in my next parts order. I do have to decipher why so many legs for the Sprague 68D in there...

Any thoughts and prior experience with this unit are very welcome.

Rax:

--- Quote from: Rax on May 27, 2024, 02:15:47 pm ---in fact, the last makes no sense to me after looking at the schematic...?

--- End quote ---

This is because I don't see how R169 could "show up" between +SENSE and -SENSE terminals. I also measured it and it seems to be reasonably close.

I've also checked the auxiliary supply voltages and they're also reasonably close, except the HV:

* TP1: 21.4V
* TP2: 14.996V
* TP3: -14.990
* TP4: 339V
* TP5: 500VI am tempted to just go ahead and rebuild the power supplies, including both caps and diodes.

Rax:
Before I dive in too deep into this project, I am wondering if anyone has dealt with the coating applied to the PCB (a polyurethane, apparently - "epocast") - and what were some of the solutions used. Judging by the manual, some of the darkened spots on the PCB may not be due to heating during operation (as I've seen some you tube videos assume), but to replacing parts and the desoldering/rework involved (p.4-5).

The other point I'd be curious regarding the experience of others is whether there's some sustained failure of the rectifiers. If I go by the manual's recommendations to curing some of the issues I'm seeing, they may have to do with 6-8 rectifiers. I have heard reports that many Fluke and Tektronix units from the time have some sustained rectifier reliability issues, but not something I've experienced personally.

Rax:

--- Quote from: Rax on May 27, 2024, 02:15:47 pm ---I do have to decipher why so many legs for the Sprague 68D in there...

--- End quote ---

Essentially, from a bit of probing and ("educated") guesses on the geometry of the package, it looks like the middle pin in the line of three is the positive terminal, and the rest of them (a square) are all connected to the case/negative.

Rax:
The other thing I'm not quite sure of is whether I'm supposed to keep the OUTPUT and SENSE connected during the tests. I kept them disconnected, because I am specifically required to connect to +SENSE, for instance (not "+," or "+OUTPUT/SENSE").

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