The 3456A is a very nice meter, and I think the only of
its sort HP ever certified as a transfer standard.
There are a couple of failures you should watch for.
First, the more desirable is the meter without the fan,
if you have a fan, it is a much older model, and I don't
know much about those.
DO NOT TOUCH ANY OF THE BOARDS WITH YOUR BARE FINGERS!
DO NOT TRY TO CLEAN PCBs, UNLESS YOU WANT TO SPEND HOURS
SCRUBBING WITH PURE 99% IPA. and even then, you may fail.
The large axial leaded filter capacitors tend to dry out,
and may need replacing. Check them with an ESR meter.
When they go bad, the meter will count erratically.
These are very high impedance meters. They will drift
when open circuit, in all but the highest DCV range.
The big reed relay that routes the ohms current tends to
get its contacts coated with something that makes it
unreliable. I usually remove a coil wire, and put a
wetting current through the contacts, and buzz them by
applying 25Hz, or so to the coil. I put a volt meter
across the contacts (mechanical, not dvm) and when the
needle gets to 50% of the wetting voltage, you are done.
10ma, with a series resistor is a good wetting current.
The front panel push switch that switches between front and
rear panel input is known to never work again, if you ever
push it. It is bound to need cleaning with some deoxit.
The trim pots that are for calibration are under the
input connector front panel, and are wire wound, and get
dirty. If you find you need to adjust them, get some
deoxit slider lube into them first.
The AC is calibrated for 1KHz, not 50/60Hz. There is a
serious noise problem at or near 50/60 hz, so that if you
put, say, 61Hz input in 60Hz land, your lower digits will
count up and down with the beat note.
If you ever find a way to fix that, please let us know!
Enjoy your new meters!
-Chuck Harris
On Fri, 04 Apr 2025 10:57:54 -0700 "BobH via groups.io"
<wanderingmetalhead@...> wrote:
I picked up the meters today. They are model 3456A. Both work, but
need some cleanup. The power switch on on unit sticks on and the
front/rear terminal switch is dirty. A quick checkout shows both
reading within a few mV of my 34401A. Now I need to port my GPIB
tools to talk to them.
