Back in July, I posted this note about a stability problem I was having with my Fluke 5200A. I didn't get much response, so I dove in with both feet.
The symptom was viewed on a trio of HP3456A 6-1/2 digit DVM's, when measuring the 10V 60Hz output of a Fluke 5200A. The 3456A's showed a 20mv offset, and an instability in the tree least significant digits. The instability was cyclical, with about a 1/3Hz period. The instability was gone when the range switch was changed from 100Hz to 1KHz. It clearly diminished as the frequency rose farther and farther above 60Hz. My first stop was the power supply, and checking and replacing about 1/2 of the electrolytic capacitors. They had ESR's on the high side of good, but still seemed to be functioning. Result: No noticeable difference. The next stop was to check all shields, grounds, isolation between various guards and isolated grounds, etc... Result, nothing found, no noticeable difference. I then went to the AC/DC board, which housed an AC to DC rectifier, various feedback paths from the sense terminals, the integrator, and the control amplifier... It should be accountable for any feedback irregularities. I found numerous carbon composition resistors that were between 20% and 50% high of their intended values, and replaced them. I checked, and found the 2uf mylar 100Hz range integrator capacitor had some leakage, and a bit of dielectric absorption, so I replaced it. Tested all the tantalums, they were good... Result, no noticeable difference... Hmmm? I took a little time out to make an extender board using my PCB mill. I couldn't find the appropriate connector, so I cut the middle out of a 100 pin 0.1" spacing card edge connector, and mounted the two halves to the extender. I tested the reference board, the oscillator, and the oscillator control, and found minor offset issues, a pernicious bit of digital ripple on the -15V(f) supply... that seemed like it should, but didn't really matter... and a couple of more carbon composition resistors that were well out of tolerance. Result, no noticeable difference... Hmmm? Hmmm? I checked out the power amplifier board, and found several electrolytics that were high ESR, loose screws on the output transistor's heatsink. Offset in need of adjustment, but nothing much else. Result, no noticeable difference... What's with this thing? After concluding that the instability had something to do with the 100Hz range, and checking all of the circuitry associated with the 100Hz range selection signals, and finding nothing, I finally did an experiment I should have done before: I made some test measurements at 110Hz on the 100Hz range, and then made the identical test measurements at 0.110KHz on the 1KHz range, and..... They were exactly the same! I can rule out anything to do with the 100Hz range selection, and just look at the output frequency as being important to the failure. I was puzzled by how I was supposed to measure mv signals on top of a 10V 60Hz sinewave, and got the vague idea to use my dusty old highly neglected Fluke 893A AC/DC differential voltmeter. I figured that it could show the instability in the galvanometer meter used for nulling the bridge.... On trying, I found that I could not see the instability on the 893A's null meter, and further, the Fluke 893A agreed with the Fluke 5200 calibrator as to the AC values being measured.... everywhere, every voltage, and throughout the differential voltmeter's frequency range! This can't be a coincidence. A differential voltmeter uses a completely different method of measuring AC and DC from a DVM, and it says the 5200A is working properly. So, on advice from my friend Tom B., I set my HP3336B synthesizer up to output 60Hz, and 7dBm, and plugged that into the HP3456A DVM, and there it was: The DVM can read voltages at exactly 60Hz pretty well, but fractionally higher, or lower, it shows a beat note in the reading... exactly what I was seeing on the Fluke 5200A. Having had problems with HP3456A power supplies in the past, I did a quick check on its filters, and found them all to be just fine... Doing something that comes hard for me, I pulled out the instruction manual for the 3456A, and read through the operating instructions. The HP3456A is not specified to operate below 400Hz AC unless the filter button (that I never use) on the front panel is pressed. I pressed the filter button, and the meter fell into line, and read a consistent, and fairly stable value. Wow! Such a ride to take for a simple case of operator error! An additional bit of insight was garnered, when I read the AC specs section of the HP3456A, and tried to understand them: 6-1/2 digit (>=1PLC) Filter OFF: 400Hz-20KHz: ±(0.07% + 730) Filter ON: 10Hz-30Hz: ±(0.47% + 450) 30Hz-20kHz: ±(0.07% + 730) Where PLC = Integrator's Power Line Cycles Given a 10.0000V AC 60Hz signal, the HP 3456A could read it as anywhere between 10.1430V to 9.8570V, and still be within specifications. Further, it could count wildly in that range, and still be in specification. Out comes the Fluke 540B, and a recalibration for my poor maligned 5200A. On the bright side, it was full of parts that had degraded over time... not that it seemed to matter... An so it goes... -Chuck Harris _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
