Rejustage my Fluke 341A - Page 1
Forum section: electronics / general

author: bastl_r
author_rank: Regular Contributor
timestamp: March 11, 2024, 02:56:03 am
content: I have an older Fluke 341 here that has too large jumps in the second decade, which serves as the basis for adjusting the first decade, both in the positive and negative direction.It may be that the specified deviation is maintained, but the resistances of the third decade are consistently too high and I end up with a deviation of approx. +0.01%! Measured with my Prema 5017 and in parallel with an HP3457A.I would like to correct this without having to recompile all the resistances.For the third decade, I thought that it would be sufficient to add high-impedance parallel resistors to the existing resistors. This would have the advantage that any EMF voltages at both soldering points would be equalized i think.For the resistors in the second decade, which deviate too much downwards, I would try to install low-impedance SMD resistors in the tracks, which would have to be separated.Is there a NOGO or any hints as to what else I could do or can I do something different?Enclosed are two tables with which I have calculated the correction resistors and the results.I made the measurements in the 100V range, as recommended in the manual.Regards Armin

author: bastl_r
author_rank: Regular Contributor
timestamp: March 12, 2024, 02:49:22 pm
content: I know. The 341 is not the high-end device.Does nobody have an opinion on the project?Or are my thoughts on the procedure plausible?Regards

author: Kleinstein
author_rank: Super Contributor
timestamp: March 12, 2024, 05:30:59 pm
content: The weak point seems to be the 2nd decade, with quite some difference betwen the resistors.If one decides to adjust the 2nd decade, one would likely take the 3rd decade as the starting point / reference, as this part is staying as it is.From the data shown the 3rd decase does not look that bad, at least if it matches the 4 th.This could in most cases mean adding some series resistance added to the 2nd decade and than readjust the 1 st decade. I see not sensible way to get around readjusting the 1st decade. With a modern meter as a help this should not be such a hard part - easier than soldering trim resistors.The very fist part would anyway checking if the switches make good contact - added resistance from the switches is a very plausinble failure point and can be changing over time.Instead of low value SMD resistors one could consider getting some manganin wire. The wire can be trimmed to length can can have very low thermal EMF. For wire of some 0.28 mm nominal I got some 15 ohm per meter and this a managable length for the 0.1 - 0.5 ohm trim resistors.

author: coromonadalix
author_rank: Super Contributor
timestamp: March 12, 2024, 05:39:47 pm
content: on a 343a i had many capacitors to change, 1st it was in the chopper amplifier section, it was unstablerotary switch cleanup with deoxitcheck the shaft extenders if they are not broken or fissured

author: bastl_r
author_rank: Regular Contributor
timestamp: March 12, 2024, 07:26:49 pm
content: Are the switches that susceptible?They have sliding contacts which should actually clean themselves to some extent.Why I have hope: I have already read a lot of negative things about oxide cleaners and wouldn't have any here.I think I'll try using paper first. Never use sandpaper!The shaftextenders of the switches all still look very good and have no visible cracks.I have already replaced some of the smaller capacitors and all the large high-voltage capacitors. One of the latter died two years ago, causing the calibrator to fail completely.

author: bastl_r
author_rank: Regular Contributor
timestamp: March 13, 2024, 01:04:39 am
content: Quote from: Kleinstein on March 12, 2024, 05:30:59 pmManganinwire would be the best choice. I know.But if I calculate a 0.66Ohm resistor with TK100ppm/k, the value increases by 0.002066Ohm with a 10° temperature change.In series with a 20000.072Ohm resistor, its value increases to 20000.072066Ohm.This can only be seen at the last digit on an 8.5-digit multimeter.Thats good enough for me and my old Calibrator.Or do you expect a much larger error if I solder SMD resistors instead of manganese wires onto an interrupted conductor track?regards Armin

author: Kleinstein
author_rank: Super Contributor
timestamp: March 13, 2024, 08:25:20 am
content: At least 2 types of manganine wire are available in small quantety (some 8 EUR for 5 m) via Ebay in Germany. If one needs several odd values, this may be easier than buying SMD resistors, that can be hard to get for all values. The wire would allow easy tweaking of other values if needed in more iterative steps.The problem with the SMD resistors would not be the TC, but more possible thermal EMF and simply getting odd values. 0.56 and 0.66 (as 0.68) ohms may however actually available.For just 2 values the SMD reistors could still be easier.

author: bastl_r
author_rank: Regular Contributor
timestamp: March 13, 2024, 07:45:16 pm
content: HiThanks for your opijion.I saw 0,25mm manganinwire on ebay(from ukraine).For a 0,56Ohm resistor i will need a lengt of 5.6cm. For this i need some space an it must be more precice soldered than 0,5mm(1%). I don't think i can manage that.For lower resistances it will be much mor complicated.Regards Armin

author: Victorman222
author_rank: Regular Contributor
timestamp: March 14, 2024, 04:14:05 pm
content: Yeah it's hard with smaller wires. If a bigger wire is used then it can be adjusted by removing some material accurately with a file. Many old wirewounds i seen were made so that they have copper wire overlapping the manganin wire at the place where they are soldered, maybe that allows some fine adjustment by moving the spot where they are soldered (see picture). Also for smaller wires strain relief becomes more important.

author: TheDefpom
author_rank: Frequent Contributor
timestamp: March 21, 2024, 11:25:41 pm
content: I have done some videos fixing my 343A, and yes the switches do need cleaning, as well as replacing ALL of the electrolytic caps, I also had issues with some of the switch mounting posts being broken, so I 3D printed replacements (available on my thingiverse page).

author: Rax
author_rank: Super Contributor
timestamp: May 28, 2024, 12:27:27 am
content: Quote from: TheDefpom on March 21, 2024, 11:25:41 pmScott - I am embarked on a 343A project (https://www.eevblog.com/forum/repair/fluke-343a-repair/) where I may have to replace some failing parts (aside from recapping, which I just think is a need due to age) and am wondering if you've had any issues with the conformal coating that seems to be applied to the 343A PCBs. Was there a lot of charring? Or maybe you've used some solvent to take the coating off? Or maybe it just wasn't much of an issue to ignore?

author: TheDefpom
author_rank: Frequent Contributor
timestamp: May 28, 2024, 12:47:57 am
content: No issues with the coating on mine, apart from trying to get it off to solder the joints, I don't have any solvents to dissolve it, it is something I probably should have actually.If you have issues with the switch shafts being cracked/broken I did design a replacement 3D printed shaft which is on my Thingiverse page:https://www.thingiverse.com/thing:4824180I dont remember there being any major issues with mine apart from the capacitors, there may have been something else but I don't remember now, it was several years ago.

author: Rax
author_rank: Super Contributor
timestamp: May 28, 2024, 01:11:39 am
content: Quote from: TheDefpom on May 28, 2024, 12:47:57 amWas it just IPA you used to clean up after reworking the solder points on this:https://www.youtube.com/watch?v=X3MyAJw2Pxs/?