Wanted even out of order : Valhalla 2720GS DC VOLTAGE CALIBRATOR

[Djil]

Hello
I'm after a Valhalla 2720GS DC VOLTAGE CALIBRATOR.
Anybody have one lying around even out of order.
Thanks

[toastedcrumpets]

Just out of interest, why that item in particular? If you're looking for precision DC sources, the old flukes can be had at reasonable prices although their capacitors might need changing.
I note eBay has 5 for sale of what you want from the US at big prices ($5k) but they won't post direct, you would have to use a mail forwarder:
https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.com%2Fulk%2Fitm%2F142998820627

[jfphp]

Developped for the US Navy, the 2720GS is still the best DC calibrator on the world : unsurpassed temperature stability, 100nV resolution on 13V range, superb accuracy allowing calibration of HP 3458A, inboard nullmeter with 10nV resolution allowing calibration with the lab DC house standards, 6 (8 option) bank of zener references...
The zeners are only aged and selected by stability not for absolute value (it is impossible). The 2720GS mesures all its LM399AH during the calibration process based on a 10V and  a 1V external standards and its nullmeter. Then the OBRMS (on board monitoring system) mesures every 10 minutes the values of the 6/8 zeners and computes statistical data  : the value of each LM399AH can be read with a resolution of 10nV trough IEEE and the different values are, in fact, in each 2720GS, quite different (it is exactly the same in the bank of 4 Fluke 732A/B or in the Datron 4910 with its 4 zeners, apart the fact that you can adjust these standards : it is impossible on the 2720GS but there is in ROM, a band between minimum a maximum allowed values .Outside it, a zener will be excluded by the CPU). The four reference board option (apart from the high stability) results only in the plug in of a fourth board.
Only problems : EP900 and Xircor X2212 failures...

[toastedcrumpets]

Thanks! That's very interesting, will spend some time looking at the design, as well as revising my already long letter to santa this year...

[jfphp]

The 2720GS relies on some patents of Larry Eccleston and is full of brilliant and tricky systems : 30 bits ADC resolution but how ? Basic ADC on board is 16 bits. It mesures value and zero of the zeners 256 times. You get 20 bits. But how 30 Bits (the tenth of nanovolts for the 7V zeners on the IEEE bus) ? Still searching in the software... There is a black box - RN301 on the main CPU - compensating for the on resistance of FET, the circuitry around the error amplifier ICL7652 on the output board is ... non conventional and ...
Eccleston went to Fluke who, of course, killed the 2720 and 2723GS and the late Valhalla calibrators are a very very long behind. It is a systematic policy of well known companies to buy much better concurrents to kill their products and stand alone on the market with widespread but not very brilliant products.

[Echo88]

Can you elaborate on the patents and circuit-tricks used in the 2720GS jfphp?

[jfphp]

1. Patent US 5446371 (www.freepatentsonline.com) 29.8 1995. The heart of the 2720/23GS Funny : Larry Eccleston at that time was working for Fluke (2720GS released in 1982-1984 ! ).

2. Patent US4716398  29 12 1987  same observation
...