Analogic AN3100 DC Voltage Standard

After picking up and repairing two Keithley multimeters I found it necessary to have a way to calibrate them. I have a DMMCheck Plus voltage standard but that is set to a fixed 5VDC, not very useful when the calibration procedure calls for 1.900VDC. Luckily I was able to pick up a used Analogic AN3100 DC Voltage Standard, a 0.005% of reading accuracy reference standard with a voltage range of 0 to 11.000VDC. Of course this unit requires repair and calibration before use.

Analogic AN3100 DC Voltage Standard

Like a Broken Record, Electrolytic Capacitor Replacement

First order of business with all old electronic equipment of a certain age is to replace the electrolytic capacitors. I replaced the only two electrolytic capacitors in the unit, Mallory 500uF 50V, with Rubycon 560uF 50V. I went with slightly higher capacitance because the only 500uF 50V capacitor available from DigiKey at the time cost $13 each! By comparison the 560uF 50V Rubycon capacitor was $0.73 each, I believe mainly because 560uF is a standard capacitance value whereas 500uF is a nonstandard value.

Old Mallory 500uF 50V Capacitors

This would have been a super simple repair except for the fact that one of the pads lifted during desoldering requiring a little ingenuity to repair in the form of soldering the component lead on the other side of the board.

Lifted Pad Near Center of Photo
Fixed By Soldering On Other Side

Voltage Output Stability

I connected this unit up to my Keithley 2700 6.5 digit multimeter to check the output voltage stability of the Analogic and found that it had a peak to peak voltage of ~2mV. Since this unit has an accuracy of 0.005% of reading, at 1V a fully functional unit should not vary by more than ±50uV. A 2mVp-p reading suggests a damaged unit. Time to find a manual.

Recreation of mVp-p, Always Remember to Take Pictures of Everything

I found a usable manual on a hifi forum here. The manual and schematic are readable and seem complete. I also found another manual with a slightly better schematic by searching for the Data Precision VS8100, they are the same unit just under different names.

Following the troubleshooting flowchart in figure 4.2.1 of the manual I was able to determine that the 30VDC measured from the cathode of diode 6 to the anode of diode 9 measured 24VDC, below the required ±10%.

A low 30VDC rail causes the ±15VDC rails to fall well below spec. Now that I was sure I found the cause of the high output voltage ripple I need to determine what was causing the voltage rails to be so low. Looking around with my thermal camera nothing jumped out as a shorted or damaged component so I started poking around with a multimeter.

AN3100 Schematic

I was able to track it down to one of the AC rectifying diodes, CR6. With the unit unplugged and using a multimeter on diode mode I found that CR6 was open. When I removed the diode from the PCB and measured it again I found that its forward voltage would fluctuate based on the force applied to the component leads, the diode had failed.

AC Rectifying Diode CR6 Open

Replacement of diode CR6 with a 1N4007 diode brought the 30VDC rail up to 28.3VDC, well within the ±10% spec and also brought the ±15VDC rails within spec. Measuring the output ripple I found that at 1V the peak to peak voltage was 1.66uV.

Replacement Diode in Place

The unit was now fully functional and I was able to check its output voltage against a calibrated voltmeter to confirm it was still within its 0.005% spec. With this unit I can now calibrate the DC voltage range on my newly acquired Keithley 177 and 179 multimeters.

DC Voltage Standard in Use

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