Fluke 343A repair

[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]

in fact, the last makes no sense to me after looking at the schematic...?

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: 500V

I 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]

I do have to decipher why so many legs for the Sprague 68D in there...

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").

[Rax]

Anyway, this far no smoking gun. I typically pulled one leg of any part I wanted to test. All the diodes I tested (those mentioned before) seem OK. From the caps I wanted to look at, one tested bad (I think it was C23), but all others fine (not great). I went ahead and replaced all of those tested.

TP4 and TP5 are still too low and the "ON" mode still trips the protection.

[Rax]

Well, as one progresses with their troubleshooting in the succession of steps in the SM, one may (fore)see a smoking gun?... In the POWER-ON TEST, at the FAULT ANALYSIS section they mention the HV fuse F2 (which I didn't realize can trigger the protection circuit if going open, but, I guess... duh!). Checking it renders it... [drumroll] open. So there's a smoking gun for yours truly. And humbly. 

I don't have any 1/16A fuses handy - nor they seem easy to source - so I guess I may have to wait for it a little bit before testing if it's the cause of all this hardship. Or, tempest in a bottle, depending on your vantage point.

[wn1fju]

I fixed a couple of these years ago, but I don't remember the details and so I didn't have any useful information to provide.

But I will say that the 343A is only one of two pieces out of the hundreds of test equipment things I have repaired that zapped me real good (the other
was the HV section of a CRT display board).  I had the 343A on and stuck my hand where it shouldn't have gone, not realizing of course that there is
about 1,000 VDC lurking inside the unit.

I don't know if you are planning to calibrate the 343A after you have fixed it as it requires accurate voltage sources, differential voltmeters, null detectors or a mix of those with a real good voltmeter (like an HP 3458A).  I found the cal procedure to be an extreme test of one's patience.

[Rax]

In waiting for the 62.5mA fuse ( ) I've replaced most of the caps I think absolutely needed that. I left the big reservoir ones in there (they measure OK), and some of the better quality Spragues.

Given about a third of the caps tested defective, no wonder the fuse popped... Once I have a fuse at hand and can establish this works again, I may go ahead and finish the recap job including what I skipped this time. Or maybe I change my mind tomorrow and do it then. The fuse order should be delivered on Thu the soonest.

[Rax]

But I will say that the 343A is only one of two pieces out of the hundreds of test equipment things I have repaired that zapped me real good (the other
was the HV section of a CRT display board).  I had the 343A on and stuck my hand where it shouldn't have gone, not realizing of course that there is
about 1,000 VDC lurking inside the unit.

I got my training years ago with exposure to about 550V from the B+ of my monoblock amps... Heck, on thinking about it, I may have done that in early childhood, testing the effect of inserting a bare copper bus into a 220V outlet... I just recall a time/space gap (I suddenly found myself not where I remembered I was just prior... ).

But more seriously, my other unit with this issue is my 332D, which uses a more careful insulation crate around the innards, which is then covered with the actual enclosure. I didn't manage to shock myself with that, but there's always another opportunity....

I don't know if you are planning to calibrate the 343A after you have fixed it as it requires accurate voltage sources, differential voltmeters, null detectors or a mix of those with a real good voltmeter (like an HP 3458A).  I found the cal procedure to be an extreme test of one's patience.

I'll reassess its condition after it's back up and running by using my Prema 6048, and my 5440A. I'll also probably by then have a couple of LTZ1000 kits back from a cal lab I plan to send them to. I got a lot of other useful things around here, such as a Keithley null meter, 732A, etc. I am not convinced the baroque procedures of those times are a must; they didn't have the luxury of 8.5 digit meters... But any attempts to adjust this though are quite a bit down the road and I'll assess things at that time. I have a lot of DCV references/calibrators, this 343A will not very likely even end up as a backup unit.

[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]

• TP6: 1.98k. Not compliant?

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.

[Rax]

This unit needed its HV rails electrolytic capacitors replaced. With those replaced, the unit works without a hitch.

Interestingly, in circuit all those caps seemed to measure correctly. I guess the lesson to be learned here is to take those in circuit measurements with a grain of salt. And the SM fault finding procedures as well. But it is important to observe that the common from your meter needs to go to + SENSE, which internally is ground for this instrument.

Before I got those HV caps delivered, I've also replaced all electrolytic capacitors everywhere else. This unit is now ready for very long trouble-free service.

[Rax]

Well, that success was short lived. I had it running for quite a while with no issues, so stable that I started considering adjustments, and just now I turned it on and it seems to output 150V-200V regardless of the "voltage out" setting. Right when this issue started occurring, it's switch to the LIMIT ON mode, now it's not, but the unruly output is still there.

Back to the drawing board.

[bastl_r]

Hi
Did you changed al of the HV-caps?
In my 341, the 450V electrolytic capacitors were all almost at the end and when the 250V electrolytic capacitor C3 died, the calibrator failed.

bastl_r

[daisizhou]

You need to take some pictures and videos to let everyone know about this 343A so that everyone can help you

[Rax]

Hi
Did you changed al of the HV-caps?
In my 341, the 450V electrolytic capacitors were all almost at the end and when the 250V electrolytic capacitor C3 died, the calibrator failed.

bastl_r

I actually replaced all of the electrolytics. The job is relatively easy and having the unit open, I thought why not give it many years of future service.

[Rax]

I've not yet taken any measurements to start diagnosing this, but one thing I'm sure is this is a new, or possibly intermittent issue. Differently put, the power supply work was necessary, and it brought TP4 and TP5 within spec.

But there's more work ahead.

[bastl_r]

Hi
I am also very interested in the progress of the repair as I will soon be getting a 343 which is supposed to work.
If it works already I can also make comparative measurements for you.

Regards

[70bytes]

Hi,
my 343A stopped working after running it for 5 hours. The limit LED went on and the needle wanted to go far below 0V.
After investigating the problem i found that the 63mA fuse has blown.  I replaced the fuse with a new one and turned it on. The limit LED turned off and immediate turned on again. This time the needle wanted to go to the other site over its max limit. I measured 200V+ at the outputs in the 10V range.
The cause of this problem in my unit was a dirty standby/On switch. After cleaning the switch the problem was solved.

I few moths ago i had a problem that it wouldn't tun on. No relay clicking sound. The cause of this problem was diode CR31.

Regards,
Maurice

[Rax]

I measured 200V+ at the outputs in the 10V range.
The cause of this problem in my unit was a dirty standby/On switch. After cleaning the switch the problem was solved.

Regards,
Maurice

Thank you, Maurice. This may be exactly what I have here.

One question I have is how does the cover masking the switch come off, if you recall. There's of course two, one on top, one on the bottom. They're attached with two screws, but they won't come off if I remove the screws. Does this need the handles to also come off, or something like that? The manual seems to indicate they should just come right off.

[70bytes]

For the top one you need to pull op from the middle. It will fold a bit and come loose. It has slots on the sides.
The bottom one i think is the same.

I have a very early one. Serial# 183
I don't know if later units are build the same.