Humidity should not be a problem so close to the LTZ chip, as the temperature is higher than the normal air temperature. It is more that the high temperature could be a problem for the foam, accelerating aging. Quite a lot of foam foam from the 1960's does not look that good today and at something like 60 C aging could be 10 times faster than normal.
I recreated Dr. Frank's board layout (https://www.eevblog.com/forum/metrology/mx-reference/msg1297126/#msg1297126) in kicad and published the design on OSHPark, so that others could order a copy of the board if they want recreate his setup. (Dr. Frank, I hope you don't mind!)
I imported the jpg of his board into kicad and tried to match it as closely as possible. I hope I got it right!
The board dimensions are just shy of 80x50mm.
Modifications:
- The precision resistor footprints include 0.15" (Vishay) and 0.2" (AE) lead spacing.
edit:OSHPark order link: https://oshpark.com/shared_projects/aF98MB4w
edit2: github link: https://github.com/pepaslabs/dr-frank-ltz1000
edit3: hmm, Frank's design actually has the copper on the back-side. oops...
edit4: moved the copper traces to the back side, and used copper traces for the two jumper links. new OSHPark link: https://oshpark.com/shared_projects/RFzngdpy
Humidity should not be a problem so close to the LTZ chip, as the temperature is higher than the normal air temperature. It is more that the high temperature could be a problem for the foam, accelerating aging. Quite a lot of foam foam from the 1960's does not look that good today and at something like 60 C aging could be 10 times faster than normal.
I was thinking in more of an on/off situation like many's 3458A, but you're right on kj7e's case with it hot 24-7.
But I think the foam aging is a better point, as it ages it will slowly drop strength/push/compression against the LTZ can.
If you like to improve the circuit and simplify the PCB a bit, then you might skip D3, as it's not necessary to have two diodes in series.
That might even disturb the T.C. compensation, so I did not use that either.
There is a disagreement going on about R418 (a 2K67 resistor from -15V to ground)-- and what is it's purpose. I say it's not needed, others say "oh yes it is". If anyone can, please test your meter with and without this to see if it makes any difference-- if it doesn't, (test temp tracking also)-- then leave it out.
There is a disagreement going on about R418 (a 2K67 resistor from -15V to ground)-- and what is it's purpose. I say it's not needed, others say "oh yes it is". If anyone can, please test your meter with and without this to see if it makes any difference-- if it doesn't, (test temp tracking also)-- then leave it out.
It's R419, and its purpose is quite obvious.. it draws -5.6mA out of GND, exactly compensating all currents into GND from the reference circuit.
OK, you got me! Thanks for the correction-- yes, when I zoom in on my very low resolution copy of the schematic, it does say R419, and not R418. Oops!
Well, this is one of the theories that I have heard as to it's purpose, but I have never seen any experimental results proving such a claim. If this is true (and R419 is indeed necessary), then the value would have to be adjusted if we choose a better op-amp that might have a different quiescent current than the LT1013.
Well, if this is being done for the reasons that you state-- and what you are saying sounds reasonable, then why are they bothering when it's an offset, and as long as the offset does not change then it doesn't matter? Why in the heck would they use a very noisy carbon-film type resistor for this. There is really something goofy going on here-- I think somebody was desperately plastering band-aids all over this design-- with the hope of being able to "fix" a number of issues-- instead of just re-engineering the whole circuit and layout. The whole design looks as though they were flailing about without really understanding what they were doing. Those engineers are no longer at HP (or Agilent, or Keysight) anymore-- so we may never know what the hell they were up to. I guarantee you this-- I could design a much better Vref replacement for the 3458A than factory; and if I can do it, then why don't THEY do it?
Update the HP A9 reference board clone (03458-66509):
I figured out how to scale and align TiN's images (see https://xdevs.com/article/volt_xfer/) on top of kicad screenshots (using gimp) and dialed in all of the dimensions. They match up pretty closely! (see attached overlays).
Also, I've managed to squeeze in room for hermetic footprints on R413 and R415. The footprints are backwards compatible, so you can populate this board with original HP components, or upgrade the critical resistors to hermetic packages.
This version of the board is intended to mimic the original board as closely as possible, with the option of upgrading the resistors, but in a future version I'd like to explore diverging from the original design a bit, perhaps with the "R60" mentioned above, etc.
I was a bit surprised to find that the LTZ isn't perfectly centered between the two mounting holes. I decided to err on the side of reproducing HP's error.
v2 OSHPark link: https://oshpark.com/shared_projects/jP5xkfJy
github: https://github.com/pepaslabs/hp-03458-66509-clone
I have assembled and tested my 5 new LTZ1000 reference boards.
They all are based on Andreas schematic, providing additional EMI filtering for great immunity against RF and mains disturbances.
And there is a seller which is selling LTZ1000ACH, but I'm not sure if I would buy them (no affiliation):
https://www.aliexpress.com/item/5PCS-LOT-LTZ1000ACH-LTZ1000/32757679028.html
I have assembled and tested my 5 new LTZ1000 reference boards.
They all are based on Andreas schematic, providing additional EMI filtering for great immunity against RF and mains disturbances.
ive been playing with eagle, i wonder, has anyone been succesful in totally laying this entirely on 1 side? i must say i have no success.