Fluke 5100 meter calibrator - DAC module, 420380

[Excavatoree]

My "working and calibrated" 5100 stopped working a while ago.  I have another unit, but it has never worked in my possession, so I didn't want to try exchanging modules to see if I could troubleshoot the first unit. However, lacking the funds to ship the unit somewhere for repair, I figured I had nothing to lose, so by a process of trial and error, I determined that the DAC and High voltage control modules were bad in the first unit, and, fortunately, these were good in my second unit.  So I was able to get one working unit.

Knowing I had a defective DAC module, I searched for a replacement, and learned that these things have tantalum capacitors that are know to fail.  I removed the shielding, and removed the capacitors.  One showed "leaky" on my PEAK capacitor checker, and the rest had high ESR.

I replaced them all, and tried the module back in my working unit.   As far as I can tell, it's now working.   Now I finally get to my question:

I had the eight 10 uF, 20V capacitors,  and I had the two 1.0 uF 35V capacitors, but I didn't have the 6.8 uF 35V capacitor.   A quick look at the board showed this is apparently a decoupling capacitor for a power rail, as it went to the power pin of several ICs.   I installed a 10 uF 35V Capacitor, which worked. 

I have no experience with tantalum capacitors at all.  Usually, I wouldn't worry about changing a 6.8 for a 10 for this application, but should I buy a 6.8 to be safe?  Is there something I'm not understanding? 

I believe I should change the capacitors of the "good" module (the one I haven't touched) just to be safe, but, again, is there something I don't know that may bight me in the posterior?

All advice is appreciated.

[pelule]

Volatge is the main critical factor an Tantalum capacitors. By design you choose the smalles possible value (size & cost), but there is no problem to replace it with the next higher value, if the voltage is same or higher value.
BR
PeLuLe

[Excavatoree]

I forgot to say that the 10 microfarad cap did have the same voltage rating.  (I'll edit the post)

The 510032 High Voltage control module seems to have a different problem than that of the DAC module - I replaced the high ESR capacitors, but the symptom remains.  (Output voltage isn't stable.  For example:  20V goes from 19.96 to 20.01, back and forth, about .5 second each.)

[dom0]

Volatge is the main critical factor an Tantalum capacitors.

Tantals fail for two reasons
- Out of range voltage = overvoltage or reverse voltage
- Current too large

Both will make a tantal a dead short sooner or later (overvoltage, very soon) in ~99.7 % of cases. This is also the main reason why decoupling tantals fail so often ; the dU/dt and thus peak current into the tantal is too large.

[Smith]

These are quality devices. I almost repaired one years ago. The output was shorted while it was feeding 1kv. The device wouldn't boot anymore. It took Fluke months to repair the unit, until they sent it back telling us it couldn't be repaired. It took me an hour to get it up and running , most of the time was spent opening this beast. Everything worked again, the only thing what was still wrong was the HV transformer which was used for the >20V settings. So anything above 20 volts was unstable. They already bought a new calibrator, so we never replaced the transformer.

[Jacek_Paw]

Hi, I also have Fluke 5101B. I had similar problem with fluctuating output but on high current range.  I spent a lot of time trying to find out what is wrong. Finally I discovered that one of the relays was causing the problem. I wasn't able of course to get a replacement so I disassembled it and cleaned the contacts. It helped and my calibrator has been working without any issues since then.

[Excavatoree]

Hi, I also have Fluke 5101B. I had similar problem with fluctuating output but on high current range.  I spent a lot of time trying to find out what is wrong. Finally I discovered that one of the relays was causing the problem. I wasn't able of course to get a replacement so I disassembled it and cleaned the contacts. It helped and my calibrator has been working without any issues since then.

Would you tell me which relay and on which module it was?  My problem SEEMS to be on the HV control module (two circuit board, large transformer, pain in the something to take off)   Embarrassing, I can't remember if I've checked the current function.

I also remember that my Parts unit produced AC volts, but not DC volts.  I'm not sure what I should look for, but I want to get the two module I had to exchange repaired so I can get back to the original problem on the second unit. 

[Jacek_Paw]

To be honest it was more than two years ago and I'm not 100% sure, but as far as i remember the problem was caused by one of  two relays K7 or K8 on A17 board.