HP 3456A calibration procedure - Page 1
Forum section: electronics / general

author: Rax
author_rank: Super Contributor
timestamp: October 11, 2025, 08:37:26 pm
content: Hi all,I'm embarked on calibrating this 3456A which I just repaired - a bit more on that below - but I'm not sure about steps "h" through "v." Maybe someone here has been through this and has a good recollection of steps, or just has an easy grip on this procedure.Steps "h" and "i" seem to just subtract any offset that would be present on the input when a short is applied. In my case this is barely registers (.1-.4uV). I'm not convinced this step is needed.Steps "j" through "t" seem to essentially apply 100mV to the input, which then gets adjusted from pot "D"If I got this correctly above, I think all this "h" through "v" business can be done by applying 100mV from my F5440B and adjusting pot "D." Do I have this right?The repair involved some odd behavior from keys "1," "2," "5," and "6" (scattershot, multiple actuations of those keys under even brief depressing of those buttons). Initially, I cleaned the heck out of the Bill West switches, but that provided zero progress. I was very suspicious of a non-mechanical issue from the beginning, but then I realized those keys are all tied through Y3' to one of the inputs of the MC14068BCP. Replacing it with a CD4068BE (where I removed pin1 which is not used) returned the instrument to full functionality. Hopefully this tidbit helps others repair if the same symptoms occur.Thank you all.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 12, 2025, 08:39:57 am
content: The whole section with the reference divider and null-meter is a way to provide the ref. voltages. It can be replaced with a modern calibrator, that can provider the test voltages like 0.1 V , 1 V, 100 V . The F5440 should be OK for this, though the accuracy can be a bit different from the old style Fluke 750, Nullmeter and 10 V ref.With a repair only at the keys and the digial part one may not even absolutely need a new adjustment. It depends on the cal history and cal status / qualtiy of the F5440 that would be used as the reference if a new adjustment makes sense. The zero at least seems to be stable.

author: Rax
author_rank: Super Contributor
timestamp: October 12, 2025, 02:21:57 pm
content: Quote from: Kleinstein on October 12, 2025, 08:39:57 amI agree. I've made many repairs where exactly that happened - once those auxiliary modules were brought back into full functionality, essentially the unit is in the same condition it was when it was last on the shelf. In this case, though, the meter was about 18ppm off at 10V - which is certainly still within spec (going by 90 days - though in this case, we're probably taking very many years).Quote from: Kleinstein on October 12, 2025, 08:39:57 amThe 5440B was just calibrated to a 17025 traceable standard.The zero seems to be very stable, indeed. In fact, the whole unit seems to be rock solid in all ways I was able to check this far. One thing I am seeing is a bit of tempco - something like 3ppm over about 6C variation - which is also nothing out of bounds.I'm still not clear on why they require the "null math" step during the adjustments.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 12, 2025, 02:38:18 pm
content: AFAIK the analog null is relatively crude with exchanging a resistor. If there is a provision for digital zero correction there is not penelty in using it. It is just the math to subtract a constant (may be per range). The analog part is OK for the coarse part, but inconvenient for the fine part.With a fresh calibrated calibrator it would likely make sense to calibrate the old meter.

author: Rax
author_rank: Super Contributor
timestamp: October 14, 2025, 11:17:16 pm
content: Quote from: Kleinstein on October 12, 2025, 02:38:18 pmThank you very much, Kleinstein. - always willing to pitch in. The meter looks rock solid at this point.