HP ET 5188 (in-house) Voltage Standard - Page 1
Forum section: electronics / general

author: cellularmitosis
author_rank: Unknown
timestamp: August 20, 2017, 03:47:11 am
content: I recently came across a mysterious "in-house" voltage standard on ebay.The seller had this to say about it:QuoteHe 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.

author: cellularmitosis
author_rank: Unknown
timestamp: August 20, 2017, 03:52:09 am
content: 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.

author: cellularmitosis
author_rank: Unknown
timestamp: August 20, 2017, 03:55:51 am
content: The "main" board:

author: cellularmitosis
author_rank: Unknown
timestamp: August 20, 2017, 03:58:06 am
content: close-up shots of the main board:

author: cellularmitosis
author_rank: Unknown
timestamp: August 20, 2017, 04:08:07 am
content: 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).

author: cellularmitosis
author_rank: Unknown
timestamp: August 20, 2017, 04:10:15 am
content: vref board continued...

author: Vgkid
author_rank: Super Contributor
timestamp: August 20, 2017, 04:19:16 am
content: 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.

author: cellularmitosis
author_rank: Unknown
timestamp: August 20, 2017, 04:20:00 am
content: 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.

author: cellularmitosis
author_rank: Unknown
timestamp: August 20, 2017, 04:24:08 am
content: A logic board. This is connected to the front-panel decade push-buttons via ribbon cables.

author: cellularmitosis
author_rank: Unknown
timestamp: August 20, 2017, 04:27:03 am
content: 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.

author: TiN
author_rank: Super Contributor
timestamp: August 20, 2017, 04:29:47 am
content: CLARE "chips" are relays. What's the lamp for, is it forwien oscillator?

author: cellularmitosis
author_rank: Unknown
timestamp: August 20, 2017, 04:30:09 am
content: 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.

author: cellularmitosis
author_rank: Unknown
timestamp: August 20, 2017, 04:31:34 am
content: Super high-res photos are being uploaded to xdevs.com -- stay tuned!

author: cellularmitosis
author_rank: Unknown
timestamp: August 20, 2017, 04:34:53 am
content: Quote from: TiN on August 20, 2017, 04:29:47 amAh, 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).

author: cellularmitosis
author_rank: Unknown
timestamp: August 20, 2017, 04:37:30 am
content: A few more misc. shots of the chassis with the main board removed.

author: cellularmitosis
author_rank: Unknown
timestamp: August 20, 2017, 04:41:48 am
content: Oh, wait, it appears the metal-can "op-amp" on the vref board is actually two transistors -- I'd guess a matched pair.

author: cellularmitosis
author_rank: Unknown
timestamp: August 20, 2017, 05:02:00 am
content: 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.

author: vindoline
author_rank: Unknown
timestamp: August 20, 2017, 01:29:43 pm
content: 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.

author: alm
author_rank: Super Contributor
timestamp: August 20, 2017, 01:57:40 pm
content: Quote from: cellularmitosis on August 20, 2017, 03:47:11 amAre there any scopes that use a 1 / 2.5 / 5 progression rather than the standard 1 / 2 / 5?Quote from: cellularmitosis on August 20, 2017, 04:34:53 amLamps 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.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 20, 2017, 02:15:03 pm
content: 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.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: August 20, 2017, 03:27:33 pm
content: 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

author: technogeeky
author_rank: Frequent Contributor
timestamp: August 20, 2017, 03:57:37 pm
content: Quote from: cellularmitosis on August 20, 2017, 04:20:00 amThe $75 price was probably a good deal for those resistors alone. Nice find.

author: nisma
author_rank: Regular Contributor
timestamp: August 20, 2017, 04:53:55 pm
content: The lamp probably is part of the oscillator for the square wave output.

author: manganin
author_rank: Regular Contributor
timestamp: August 20, 2017, 06:50:55 pm
content: Quote from: cellularmitosis on August 20, 2017, 03:55:51 amHP (Harrison) 6116A Precision power supply:http://literature.cdn.keysight.com/litweb/pdf/06116-90001.pdf

author: cellularmitosis
author_rank: Unknown
timestamp: August 20, 2017, 07:37:03 pm
content: 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!

author: cellularmitosis
author_rank: Unknown
timestamp: August 20, 2017, 08:11:26 pm
content: Here's an edited version of that pdf, which just has the circuit analysis, schematic, and parts list (attached).It looks like the main reference zener is a 1N825.The heated reference "can" might make a great training playground for gaining experience with building and tuning an oven and current driver for the 2DW232 zeners.

author: cellularmitosis
author_rank: Unknown
timestamp: August 20, 2017, 08:41:59 pm
content: Oh wow, hparchive has a gorgeous copy of the 6101A manual (which has the same schematic as the 6116A), which appears to have been digitally reconstructed / recreated! Perfectly legible!http://www.hparchive.com/Manuals/HP-6101A-Manual.pdfI've attached a similarly condensed / edited version.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 21, 2017, 04:59:09 pm
content: The HP 61xx supplies use a 3 stage differential amp in the oven part. The unit shown here is just a 2 stage differential amp in the oven. Also not very many transistors outside. So the circuit seems to be even simpler, but i could very well follow the general structure.The temperature regulation looks like a simple proportional regulator. Not sure the oven part is that good to reuse it in a more modern application. The circuit is still from the pre OP era - such circuits today are usually OP based and thus look much different. Today a transistor couple is usually more expensive than a complete OP with in many respects better performance - so precision circuits today prefer OPs over the old style pairs.

author: cellularmitosis
author_rank: Unknown
timestamp: August 24, 2017, 05:29:52 am
content: Quote from: Kleinstein on August 21, 2017, 04:59:09 pmThanks for your insights here. I've been wanting to develop a better understanding of how op-amps work, so this is a useful circuit to consider. Another thread on this forum advised studying the GAP/R P65 circuit, which was a primitive op-amp which only used 6 transistors.

author: manganin
author_rank: Regular Contributor
timestamp: August 24, 2017, 01:01:36 pm
content: Quote from: cellularmitosis on August 24, 2017, 05:29:52 amIt doesn't need to be that complex. See the circuit attached: 3 transistors and 3 resistors (if you replace the current sources with resistors). Works fine in applications like audio if you remember the drive capability, gain and PSRR limitations in your circuit design.

author: Zucca
author_rank: Unknown
timestamp: August 24, 2017, 01:54:23 pm
content: cool, can't wait to see how this ends...