When I was doing the recent DMM calibrations it became apparent that trying to calibrate a 4.5 digit DMM (8600A) with another 4.5 digit DMM (8050A) was not going to yield the most accurate results. So I decided to upgrade the bench 1 reference DMM to 5.5 digits. Off to Ebay. I wanted to try to get one that at least had some NIST traceability in it's past. I found a reasonably priced 8800A from a surplus seller in NJ from whom I've purchased before. It was advertised as “powers up but no further testing and parts only”. It looked to be in good shape so I bought it. It arrived yesterday and unfortunately it did not fully power up but I quickly found the problem. The -35V bus was at -25V due to a shorted 50uf/50V cap. Replaced it and at the same time ordered a complete set of caps to do the PSU. After replacing that cap it powered up OK. I had this EXACT same fail with the same capacitor on another 8800A so apparently this is a common fail with these DMM's.
In the following pix the 5.5 digit DMM's are stacked with the new 8800A on top, followed by an 8810A, and then the Siglent SDM 3055. They have all been powered up 12+ hours and they are all connected to the AD584-M reference calibrated at 9.99691V. Upon initial power up the 8800A read 9.9970V then slowly settled and stabilized at 9.9968 to 69V as shown.
The 8810A isn't even a player. At initial power up it read 9.9974V and has drifted down to 9.9964V and appears to be still drifting. That's unacceptable so this guy needs to be investigated. What's surprising about this poor performance is that it's almost identical to an 8800A. But it's going into the repair cue.
The Siglent upon initial power up read 9.9960V then over the course of 4 to 6 hours settled and stabilized at 9.9965 to 66V as shown. I'm not really too concerned with that warm up time but I am hoping that Siglent will release a method to tweak the cal as indicated by Tautech. It just needs a very minor tweak upwards.
So how do I know that the 9.99691V from that AD584-M is accurate? Well, here's the 2nd AD584-M. It's referenced calibration is 10.00096V. The 8800A is reading 10.0009 – 10V. The 8810A is reading 10.0005 – 6V. And the SDM 3055 is reading 10.0006 – 7V. So my conclusion is that the new 8800A is dead nuts. And further proof that my assumption is correct is there is a damaged (long story) 8800A in the TEA closet. Before it was damaged it read within 1 minor digit on both references as this new 8800A is doing. So I'm very confident that both AD584-M's are stable and accurate.
So why did I go for an 8800A and not something like an 8840A? Several reasons. First, 8840A's are popular and demand a higher price. 8800A's less so. This one was less than $70 USD including shipping and tax. Next, I don't feel like dealing with the issues with the VFD that WILL arise. Third, I don't need the 2nd display and the other bells and whistles. Finally, I can't get enough of ruby red LED's. They are soooo sexy.