Fluke 343A DC Calibrator Woes - Page 1
Forum section: electronics / general

author: wn1fju
author_rank: Frequent Contributor
timestamp: January 04, 2022, 08:25:36 pm
content: I am having a tough time adjusting my Fluke 343A DC calibrator. Everything is well within spec except for when the first digit's knob is set to 10.For instance, on the 10V scale, one can achieve 10V output by setting the first knob to 10 and the remaining knobs to 0. Or one can get the same 10V output when setting the first knob to 9 and the second knob to X (carry-over). Similarly, on the 100V scale, one can get 100V with the first knob set to 10 or the first two knobs set to 9 and X. Similar for the 1000V range...But no matter how I try to do the various linearity adjustments, what I get is something like:10V scale: 1st knob to 10 gives 10.00050; 1st two knobs to 9 and X gives 10.00000100V scale: 1st knob to 10 gives 99.9950; 1st two knobs to 9 and X gives 99.9998So it is only when I am using the first knob set to 10 that things are out of spec. What I don't understand is that exactly the same set of resistors is used in both the 10V and 100V ranges (also the 1000V range). It is only the current input to the virtual ground of the control op-amp that is changed among the three voltage ranges. I can see if both the 10V and 100V ranges read low with the first knob to 10, but one range is high and one range is low. And every voltage on every range that doesn't use the 10 on the first knob is perfectly correct.Any ideas? Self-heating of resistors? I'm at a loss.

author: TheDefpom
author_rank: Frequent Contributor
timestamp: January 04, 2022, 09:11:25 pm
content: I have done some videos on the 343A repairs, my last one covered cleaning all of the switches as they were giving trouble, don't assume that just because the switches are gold plated that they cant misbehave, gold contacts get dirty too, it is just a lot more resistant to it.Wipe all of the contact plates with a cotton bud to clean off the worst of the grime, then use some deoxit gold cleaner and work the switches a lot, and then apply a small amount dielectric lubricant, something like deoxit shield.

author: wn1fju
author_rank: Frequent Contributor
timestamp: January 04, 2022, 09:38:21 pm
content: TheDefpom: thanks for the reply. I did test the switches earlier by isolating the resistance string and using an ohmmeter. But perhaps my meter didn't have enough resolution to spot any problem. I will try some cleaning anyway.As I said earlier, I can see that if the 10 switch is adding unwanted resistance, I would expect the voltages to be wrong in the same direction on the 10V and 100V ranges - but one range goes high and the other goes low. That is what is confusing me. If there is some unwanted resistance, it seems nonlinear with voltage.

author: wn1fju
author_rank: Frequent Contributor
timestamp: January 05, 2022, 05:25:31 pm
content: I cleaned the switches as well as I could, but I don't think that is the problem. I partially erred in my earlier description by saying the 10V was high and the 100V was low when using the 10 setting on the first digit. Actually, from a cold start, they are both low.If I put the first knob to 10, the range switch to 10V, then change the range switch to 100V, wait about 30 seconds, and then switch back to the 10V range, the voltage is now about 100 ppm high and very, very slowly (over a few minutes) will drift back down to where it started. This effect is perfectly repeatable.It doesn't happen on any other number on the first knob - for instance, going from 9V to 90V and then back to 9V returns in a few seconds perfectly.So my guess is there is something wrong with the two Fluke precision resistors and/or adjustment pot that are only used with the first knob set to 10. It doesn't sound like a switch problem on the first digit since I'm not changing it in the 10-->100-->10 experiment. And it doesn't sound like a problem on the range switch since the 9-->90-->9 experiment is fine. Unless the switch contacts are changing resistance with voltage? I'm guessing it's a temperature coefficient problem. Opinions?

author: Kleinstein
author_rank: Super Contributor
timestamp: January 05, 2022, 07:02:16 pm
content: The descibed problem with the 10 V - 100 V - 10 V sequence looks like a thermal problem for the resistor/trimmer used for the 10 setting.In this case there should also be some drift in the 100 V range, though likely more difficult to see, as the meters in the 100 V range are often less stable and make the measurement a bit more tricky.The 10 set as 10 x1+ 0 or 9x1 +10x0.1 use different resistors for the last volt. So this could be a thing of the adjustment.

author: wn1fju
author_rank: Frequent Contributor
timestamp: January 05, 2022, 11:02:54 pm
content: Yes, the workaround is simply not to use the 10 on the first dial and use the alternate of 9 on the first and "X" on the second (10).I have run through the adjustment several times. Fluke has you set the linearity by adjusting various pots to make a setting on the first two dials such as values of N and 0 agree with (N-1) and X. That works fine except when N=10. When I try that pot, it throws all my previous adjustments off. When you are all done with the linearity pots, then you adjust 10, 100 and 1000V using three other pots. But by then, my 10 digit is screwed up, so again that throws the lower voltages off. It's just that pesky 10 on the first dial that seems amiss.The way it stands now is that it seems perfectly in spec everywhere else I look (as far as I can tell with a 6-1/2 digit DMM). Since there is a simple workaround, I'm not desperate to fix it yet. But it is curious.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: March 27, 2022, 05:12:39 pm
content: on my Fluke 343A i also dont get exactly the same resultas if I dial it to deliver 10V in all 3 rangesI think it is designed to deliver 10V in the 10 V rangefor best performance.feel free to see my happy video from yesterday, where mine came back to lifehttps://youtu.be/sOS_bX-AlFE

author: oz2cpu
author_rank: Frequent Contributor
timestamp: March 29, 2022, 03:30:31 pm
content: any one knows how much a 343A costed when it was released,and when it was discontinued ?also first and last official sale year ?

author: oz2cpu
author_rank: Frequent Contributor
timestamp: March 30, 2022, 07:57:08 am
content: news:Fluke 343A DC Voltage Calibrator, Teardown-Repair-Part 3 (Performance Tests)https://youtu.be/9cKic1UaIlI0:00 - Power Supply Consumption 230V AC4:32 - Line Regulation "4-26" (10V Pass)6:50 - Load Regulation "4-28" (10V Fail)sadly load regulation fails, i am a little bit puzzled about that, looks like the voltage RISE when i load the outputany suggestions where to lookso far ALL capacitors are original

author: Kleinstein
author_rank: Super Contributor
timestamp: March 30, 2022, 09:03:27 am
content: The load regulation test should be done with the meter cables connected to the sense terminals and the lard resistor connected to the drive terminals.When doing it the other way around, like in the video, the voltage drop on the connection from sense to drive would add to the voltage. So it is not a surprise to see the voltage rise.Another possible problem can arise from the heat the resisor produces. So close to the connectors this can cause thermal EMF errors. The terminals and also many banana plugs can cause thermal EMF in the 10s of µV under such conditions.Depending on how the calibrator is build inside, the better of the 10 V settings may be on the 10 V range and the 10 at the first "digit".The test with the supply variations should ideally be done with the case closed and waiting a little longer. Part of the supply voltage effect can be thermal. Even for an old instrument I would not expect much effect anyway.

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: March 30, 2022, 03:11:15 pm
content: ...and those big HV caps DO go bad ! (well after 40 yrs or so, you would expect) I had one on mine (a 341A) dry out, and start arcing, and blowing fuses. Once out of the PCBA, the roll inside was bouncing around if shook !