HP745 oven reference - Page 1
Forum section: electronics / general

author: artag
author_rank: Super Contributor
timestamp: November 24, 2020, 11:11:05 am
content: I picked up this device on ebay. If it still works, it should at least be well burnt in.I was planning to use it as a stability reference to characterise the drift of my other voltmeters (hp34401 and k2000).Have you got any thoughts about how that might work out ?Warnings about putting it into use?To save you looking up the manuals (there is a fairly dirty copy online), it's a 6.something zener in an oven. There are a few other circuit blocks in the same oven, mostly pairs of transistors for diff amps.

author: dietert1
author_rank: Super Contributor
timestamp: November 24, 2020, 04:51:11 pm
content: Can't you give us the link of the document? Do you know how old it actually is? Maybe tear it down and see the datecodes.

author: artag
author_rank: Super Contributor
timestamp: November 24, 2020, 06:30:39 pm
content: There's a noisy copy here, but you have to sign up for access.https://elektrotanya.com/hp_745a_ac_calibrator.pdf/download.htmlI've bought a much nicer scan from artekmedia - a fragment is attached. But really, although it's worth me having the manuals, I'm interested in more general comments here. Such as whether it's worth me setting up, or is just a curio.The label is, I think, a production label from 1981 showing the burn-in drift of the reference and the whole calibrator during a 6 week burn-in. The reference shifted about 6ppm during that period, mostly during week 4. In the last 2 weeks is was drifting about 0.5ppm per week.There are some oven control circuits but the reference itself is just a diode and some SOT resistors. The voltage used would presumably be dependent on the 3K load resistance.I'm interested to know if I'm still likely to see that drift as low 0.5ppm/week, assuming I set the oven running. I don't have any other parts of the calibrator, only this unit.

author: pigrew
author_rank: Frequent Contributor
timestamp: November 24, 2020, 07:10:47 pm
content: Quote from: artag on November 24, 2020, 06:30:39 pmI don't have the data to back it up, but diodes generally become more stable over time. My experience was buying an ancient (1960s) DC calibrator and the reference was very stable but had developed popcorn noise (on the order of many ppm) and its zero TC point had shifted a little. If your particular reference diode is low noise, then it should have performance comparable (or superior) to modern references). It being ovenized will greatly reduce (or eliminate) hysteresis. You likely will see your own burn-in effect for the first few days.The entire assembly would be adjusted (resistors specially selected) in order to maintain a low tempco at the oven's operating temperature.The original design used a bipolar power source, with the -15 V rail referenced to the zener voltage itself. Setting up a similar power supply could be tricky (although you could design a supply with a separate, modern reference, and get good performance).

author: dietert1
author_rank: Super Contributor
timestamp: November 24, 2020, 07:55:51 pm
content: Your image seems to show the A1 assembly, and the zener appears in A1A2A1. Now they don't have a parts list for A1A2 nor it's subassembly A1A2A1. You could try and connect +/- 25 V to your box and see whether the oven works. The +/- 15 V supplies on A1A1 should start up and maybe the thermistor bridge works, too. You can measure the heater output. Then you will have an ovenized zener.The basic accuracy of that AC calibrator was about 200 ppm. Will be an interesting exercise, otherwise i don't know.Regards, Dieter

author: artag
author_rank: Super Contributor
timestamp: November 24, 2020, 08:52:51 pm
content: I think the oven is actually A2 of the section A1, or A1A2. Not A1A2A1.The drawings are a bit weird in that they show the circuits rather than the subassemblies - very helpful in understanding, not so good for locating the part on a board.However, I'm pretty sure the 3-transistor oven amplifier is on A1 and only the heater and the thermistor bridge inside the box A1A2. Pretty simple to replicate, though - yes, I will try running that.The 3K load resistor is hard to locate though. It should be on the A1 subassembly of A1, aka A1A1, but I don't think that shows it as it's already on A1A2 schematic. I'm not even sure if it's a single resistor or a network that defines the overall gain of the calibrator. This seems likely as otherwise it would surely be more stable to keep it in the oven.

author: dietert1
author_rank: Super Contributor
timestamp: November 24, 2020, 09:24:49 pm
content: The manual shows the zener in a section A1A2A1, which is a plugin module in assembly A1A2 labeled as "Reference oven". Assembly A1A2 is missing in the parts list - that assembly is one part, probably due to the tuning resistors. I thought your photo shows A1. Does it have the 37-pin connector at its bottom as shown on manual pages 7-7/7-8?Regards, Dieter

author: artag
author_rank: Super Contributor
timestamp: November 25, 2020, 12:55:16 am
content: No, it has a double-sided 10-way (20 pins) 0.156" edge plug (pcb fingers) in a recess at the end.The other end of the box is removable and inside is a pcb plugged into an edge connector, also 10+10 pin. So there is a short transition in the shell of the plastic box from the actual ref pcb to the exposed outer contacts.The inside of the box in insulated with yellow foam but the pcb is slid into an aluminium extrusion, capped with foam at both ends. I'll pull that out later but not until I've tested the heater and thermistor (which could be on the transition board) and the reference diode. Three 6-pin T05 style packages (the transistor pairs) and 2 plastic 3-pin packages are visible, as well as a square black package of a style I've seen used for precision resistors.So I guess A1 could be the first section of the main chassis, A1A2 the second module of A1 (being the box, oven and transition board), and A1A2A1 the first pcb of A1A2, being the pcb inside the oven.I could do resistance checks at the external pins but I'm waiting for an edge connector which will make it a lot easier.

author: artag
author_rank: Super Contributor
timestamp: November 27, 2020, 01:17:59 pm
content: Added pictures of connector and the pcb in place from the opened end.

author: Dr. Frank
author_rank: Super Contributor
timestamp: November 27, 2020, 01:41:01 pm
content: Quote from: artag on November 24, 2020, 11:11:05 amIf that's only a simple 1N82x in an oven, plus a differential buffer amplifier, its stability (noise, over time & temperature, etc.) will very probably not be sufficient at all to characterize 6 1/2 digit DMMs. For this purpose, you need a reference being more stable in all these parameters, or one on par, but being well characterized.It would only be nice and interesting to fiddle around with the oven, and - the other way round - characterize this assembly concerning the listed stability parameters.For characterization of these DMMs, as well as of the assembly, you definitely need a reference, or a DMM based on the LTZ1000, or the LTFLU, anything different (like an LM399AH based reference) makes no sense at all.Please also have a look into the schematics of the HP735A - 1V reference standard, which is also of that era, and shares the same, or a very similar oven reference assembly.Specification is e.g. 10ppm/month drift, that's about the annual drift of both DMMs.The HP740B DC calibrator / differential meter also contains such a diode reference assembly.From these schematics you can more easily deduce, how to apply your oven module.Frank

author: guenthert
author_rank: Frequent Contributor
timestamp: November 27, 2020, 06:27:06 pm
content: Quote from: Dr. Frank on November 27, 2020, 01:41:01 pmYou are pretty quick there with dismissing the temperature compensated zener 1N82x. The first commercially available 8.5 digit long scale DMM as well as the Datron 4000 calibrator used them, which I take as evidence that they can perform quite well. As far as I understand, the reason they aren't used much anymore is rather, that it is too time consuming and hence costly to pre-age and select them. Said that, it seems to me, that Datron and Solartron put considerably more effort into making the voltage references stable than HP did (many years earlier). The Solartron 7081 was even "calibrated for life"

author: Kleinstein
author_rank: Super Contributor
timestamp: November 27, 2020, 09:10:22 pm
content: The reference section of the Solartron 7081 is quite complicated, but I would not consider it an effective solution. The ADC part is only there to adjust the current - so kind of no better than jumpers and resistors. I would consider this way today with cheap parts, but at that time it's more like a. The HP solution may look simple, but individual selected resistors can be better.Similar even an only crude oven can easily be better than the compensation of the second order part. Sometimes a simple solution can be better. Selected and aged zeners can be quite good, but here can still be some bad ones.However one has the Problem, that one has no data on how good the old system actually is. No used of quite some time there could be a humidity effect for the first few months. So even if likely OK it is not a good reference to judge drift on the level of an aged LM399.I doubt the specs for the old HP unit are that great. The specs are probably the best we have about how good the adjustment and resistors are and how much selection / aging was done.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: November 27, 2020, 09:12:47 pm
content: I was also going to suggest looking at the HP735A, but couldn't remember the model number. I had one but the oven was a mess, so I parted it out or sold it. It wasn't very good compared to even the Fluke 731a/b units with no ovens. That said, if you select, you can do a good job with the 1N821 or similar diodes and if you put the right stuff in the oven, it reduces the demands on resistors and such. One problem today is the price of 1N821 diodes is so high you're probably better off looking for complete IC references.

author: dietert1
author_rank: Super Contributor
timestamp: November 28, 2020, 08:12:39 am
content: Yesterday when making some 15V power supplies for references i happened to use a LM723 with datecode 7840 for the first time in its life. It worked and was in spec but the output voltage would drift from 15.125 down to 15.024 V during the first hours, with about constant load and ambient temperature conditions. Old does not always mean well aged.The HP745 oven reference - once it works - will require kind of a calibration starting from scratch.Regards, Dieter

author: Andreas
author_rank: Super Contributor
timestamp: November 28, 2020, 10:07:24 am
content: Quote from: dietert1 on November 28, 2020, 08:12:39 amWas it a plastic housing or a metal can?Plastic housings suffer from humidity (can be used as humidity indicator).So after soldering or long time storage it needs to adapt for minimum 2 weeks.Hermetically housings like metal can or glass behave much better over time.with best regardsAndreas

author: dietert1
author_rank: Super Contributor
timestamp: November 28, 2020, 10:38:02 am
content: Yes, those were DIL plastic cases. We also got some old DIL ceramic ones and those did not exhibit any significant time drift (< 1 mV in 15 V). Some TO-100 ones arrived via ebay, but they don't have any date code printed on the metal can. One can spend arbitrary amounts of time on this.Regards, Dieter