HP ET 5188 (in-house) Voltage Standard

[cellularmitosis]

I recently came across a mysterious "in-house" voltage standard on ebay.

The seller had this to say about it:

HP/Agilent ET-5188 voltage standard, you won't find this in any HP catalog because it was made for in-house use, I don't know how accurate it is, and it may need calibration, so I am selling it as-is.

He was asking $150.  I decided to offer $75, thinking that even if it turned out to be a hunk-o-junk, it would at least make for a cool tear-down.  I got lucky, and he accepted!

(tear-down photos to follow shortly...)

Edit: my best guess is that this unit was used for testing / servicing oscilloscopes.

[cellularmitosis]

It supports decades of a 10, 25, 50 sequence, from 10mV through 100V.

The output is a BNC jack, which is a bit unusual.

It supports DC output, positive or negative polarity.  It also has a square-wave output mode, which can either be DC or AC coupled.

It looks like the square wave output is 100Hz, and there is also a "comp" mode which outputs a 1kHz or 10kHz square wave.

[cellularmitosis]

The "main" board:

[cellularmitosis]

close-up shots of the main board:

[cellularmitosis]

The voltage reference was interesting.  It was a small board inside of a heated and insulated metal can.

The inner metal can appears to be wrapped in resistance wire.  Inside of the can, there is a thermistor which is bent so that it touches the wall of the can, and a big dab of heat-sink grease keeps it in thermal contact.  Surprisingly, this goo was still gooey!  There are many date codes from 1974 on this board, and as best I can tell, I'm the first one to crack open the vref can (the foam was glued to the can, so I would have known if someone else had popped this open).

The vref board itself is pretty simple.  Two transistors, an op-amp, a handful of resistors, and what appears to be a "regular" zener diode.  I haven't yet traced out the circuit to see what the topology is (I'm guessing it's either the "bootstrapped" zener circuit popularized in our LM399 forum thread, or the classic two-transistor constant-current source).

[cellularmitosis]

vref board continued...

[Vgkid]

That is pretty interesting, the mainboard reminds me of the HP 61XX precision power supplies. I know the earlier HP 611x used a ovenized zener diode.
Thanks for the teardown so far...
I'm correct about the oven, the precision power supplies prior to the 61-04/14/05/15 used an oven.

[cellularmitosis]

On the underside of the unit is what I'm assuming is the "divider board".  It has a bunch of precision resistors -- this appears to be where they are making the 10-25-50 decades.

I'm not sure what the blue "CLARE" chips are.  I'm guessing either opto-isolators, or analog switches.

As for the precision resistor string, there are 2 x 20R in parallel, 2 x 30R in parallel, 25R, 50R, 150R, 250R, 500R, 1.5k, 2.5k, 5k, 15k, 25k, and 50k.  All of these are 0.05% and dated 1978.

It appears there are no individual trim-pots per-decade.

[cellularmitosis]

A logic board.  This is connected to the front-panel decade push-buttons via ribbon cables.

[cellularmitosis]

The "output" board.  This board is connected directly to the output BNC jack.  It has a mercury relay, which I'm assuming provides the polarity switch, and I think this board also implements the square-wave feature (the unit emits a buzzing noise in square-wave mode, and it sounds like it is coming from this board).

This board also has the main power-switch relay.

[TiN]

CLARE "chips" are relays. What's the lamp for, is it for wien oscillator?

[cellularmitosis]

Unfortunately, this unit is "sick".  There is a 120 Hz "shark fin" riding on the output.  I'm assuming this means the main filter caps have gone bad and the power supply is falling out of regulation towards the end of each sine-wave.

[cellularmitosis]

Super high-res photos are being uploaded to xdevs.com -- stay tuned!

[cellularmitosis]

CLARE "chips" are relays. What's the lamp for, is it for wien oscillator?

Ah, thanks!

I'm not sure what the function of the lamp is.  It starts to glow dimly when the unit it set to 25V output, then brighter at 50V output, and brightest at 100V output.

(if you look carefully you'll notice the 25V LED isn't illuminated.  This is because one of the front-panel ribbon cable pins broke when I was re-assembling the unit, before tearing it down further).

[cellularmitosis]

A few more misc. shots of the chassis with the main board removed.

[cellularmitosis]

Oh, wait, it appears the metal-can "op-amp" on the vref board is actually two transistors -- I'd guess a matched pair.

[cellularmitosis]

Here's why they chose the resistor values that way.  It looks like they are generating a very stable 100V reference, then dividing it to get all of the ranges.

[vindoline]

Very cool find CM! I'm looking forward to a successful repair. The heated reference board reminds me somewhat of the volt refs in my Power Designs 2005A and 4010.

[alm]

Edit: my best guess is that this unit was used for testing / servicing oscilloscopes.

Are there any scopes that use a 1 / 2.5 / 5 progression rather than the standard 1 / 2 / 5?

I'm not sure what the function of the lamp is.  It starts to glow dimly when the unit it set to 25V output, then brighter at 50V output, and brightest at 100V output.

Lamps were sometimes used as a qualitative voltage indicator if a panel meter was too large or expensive. Tektronix used this for some of their modular bench supplies like PS503A. If the lamp suddenly becomes dimmer, the supply is likely current limiting.

[Kleinstein]

The parts of that circuit look like they are rather different in age: die digital part is 1978 (or a little newer) according to the date codes. However the large board with the reference looks more like 1960's - still using discrete transistors (and only a few) instead of OPs. So it could be kind of an old supply design and the two digital boards added later.

The board in the heated box has a differential amplifier and the zener diode without any interconnection. Well possible to have the amplifier for the main circuit and not just the reference.

[Conrad Hoffman]

Way back (1966) HP made the 735A DC Transfer Standard,  similar in appearance and function to the Fluke 731. It used an 80C ovenized zener almost identical to the one shown here. The diode was specified as 6.8 VDC, but no part number given. The foam in mine turned to goo and the whole thing wasn't very good by modern standards, so I either parted it out or sold it, can't remember which. Manual here- http://literature.cdn.keysight.com/litweb/pdf/00735-90002.pdf?id=1870825

[technogeeky]

As for the precision resistor string, there are 2 x 20R in parallel, 2 x 30R in parallel, 25R, 50R, 150R, 250R, 500R, 1.5k, 2.5k, 5k, 15k, 25k, and 50k.  All of these are 0.05% and dated 1978.

The $75 price was probably a good deal for those resistors alone. Nice find.

[nisma]

The lamp probably is part of the oscillator for the square wave output.

[manganin]

The "main" board:

HP (Harrison) 6116A Precision power supply:

http://literature.cdn.keysight.com/litweb/pdf/06116-90001.pdf

[cellularmitosis]

manganin, thank you so much for pointing out where that board came from!

The 6116A was a 0-100V power supply, which coincides perfectly with the resistor divider values!