Fluke 332D continued rejuvenation, could use insight

[valley001]

Hello all,

Some of you may remember the repair thread on this unit from way back:

https://www.eevblog.com/forum/repair/fluke-332d-repair/msg2239665/#msg2239665

Toward the end I indicated that swapping the chopper board fixed it but this did not last, after several hours the output would become unstable, spiking several hundred micro volts.  I became frustrated and put the project away.

I decided to give it another go yesterday and have achieves some success.  I have not attempted a calibration because I believe there are still repairs yet to be made.  Here are the things I had to do to get it mostly working:

1- Swap the auxiliary power board.  On the original I went around replacing caps that tested bad on my component tester, which resulted in WORSE performance out of the unit (output jumping around several hundred micro volts).  I installed the spare auxiliary power board, that I have not touched, and there is improvement. Obviously I disturbed some sort of balance on the original board but I dont know what. 

2- I adjusted the chopper per 4.65 in the manual.  This is a confusing process because the fluke procedure is not completely clear, and the waveform I end up with does not precisely match the picture in the manual 4-65 figure 4-5.  Nevertheless, the output settled further and has remained more stable once I attempted this adjustment.  Maybe someone could clarify this process?  I can post some waveform pics of that would help. 

3- I adjusted the ref supply and auxiliary power supply per table 4-15. 

Now, output stability has improved greatly compared to before where I would see spikes of several hundred micro volts reading with my hp 3456a (set to 10 PLS no filter).  I now see the output dither around a few micro volts, with the 332D dials at 1.000000 I will read 1.000135-1.000137 sometimes spiking a couple micro volts outside this range.  HP 3456a set for 10 plc no filter.

While this condition is a welcome improvement, I do not believe it should dither this much.  I wonder if someone may know where to check next?  Maybe further refinement of the chopper wave form?  I am out of ideas.

[TERRA Operative]

I found a bunch of out of spec resistors in my unit and a busted Zener on the Aux PSU board. It's the precision CR6 6.3V one. I'll replace it with a standard zener for testing then swap to a 1N829A once it's working.

Maybe try replacing the carbon composite resistors with metal film to bring them back into spec? I plan to replace the whole lot of carbon resistors in my unit.

[valley001]

I found a bunch of out of spec resistors in my unit and a busted Zener on the Aux PSU board. It's the precision CR6 6.3V one. I'll replace it with a standard zener for testing then swap to a 1N829A once it's working.

Maybe try replacing the carbon composite resistors with metal film to bring them back into spec? I plan to replace the whole lot of carbon resistors in my unit.

I am interested in the your results with resistor replacement on your aux board.  I may give that a go on my faulty board and see that brings it back to life.  I am hesitant to replace all the carbon comp resistors en mass because I fear it may induce other issues.

How is the output stability on yours, do you get any  noise at the micro volt level? 

[TERRA Operative]

Not sure yet as it doesn't work with the blown zener. It did work before the cap replacement, but it was a bit iffy, I think the new caps must have tipped the dodgy zener over the edge, I bought the unit cheap with intermittent output.
It did seem relatively stable when it was working, although I didn't have low thermal EMF cables then. I have a set of shielded low thermal EMF cables now though, so I'll be able to get a decent reading.


I'll be heading into Akihabara tomorrow to get parts, so I'll replace just the zener to start and see if it works. If so, I'll let you know how stable my unit is. That way I can see if the resistor replacement is worth while too.