content/hp3456a.rst

============================
 HP 3456A Digital Voltmeter
============================

:Author: David Douard
:Category: Electronics
:Tags: HP, 3456A, DVM, repair, test equipment
:series: HP 3454A
:series_index: 1


A very long time ago, I bought a venerable HP 3456A Digital
Voltmeter. Back then, I had no clear idea what kind of device it is;
but it was cheap. It worked quite fine (it is indeed not calibrated),
but I noticed for a while that it was quite unstable: the last 2 or
even 3 digits (in 6 1/2 digits mode) were jumping a bit too much.

Back then, I did not took the time to investigate the reasons of this
behavior.

However, a few days ago I plugged it to roughly test the stability of
an old Time Electronic Limitd D.C. Voltage Calibrator I bought for
almost nothing. After a few hours, the display showed an overload
condition (display OL).

So I started to read the service manual and to follow the
troubleshooting guide. At some point I reached the step where the
procedure requires a Signature Anayzer (which I obviously don't have).
I briefly tried to dig the signals with my cheap USB logic analyzer,
but the readings were quite disappointing. I was considering
implementing a signature analyzer using a Nucleo STM32 board, since I
have several of them for my `HP 34970A display replacement project
<{filename}/hp34970a.rst>`_.

Suddenly, I realized I did not even check the power rails! And
bingo, the +33V was dead (something like 5V).

Ok, much better, a power supply failure is a way better
news. Especially since this kind of device uses a linear power supply:

.. image:: {filename}/images/hp3456a/psu.jpg
   :alt: HP 3456A's power supply board

Yes, it is very dusty.
The culprit appeared in plain sight as soon as I removed the power
supply board (A10) from the unit:

.. image:: {filename}/images/hp3456a/psu_C7.jpg
   :alt: failing caps on HP 3456A's power supply

The schematic is very straighforward; few bridge rectifiers, some
capatcitors, some regultators, nothing fancy:

.. image:: {filename}/images/hp3456a/psu_schematic.svg
   :alt: schematic of the power supply board

The dead capacitor is C7 (47µF, 50V, Sprague). I also checked the
other capacitors on the board, and noticed some of them were not very
tight. A quick inspection of the solder layer showed a few "cold"
solderings on C16 and C19. That would for sure not help the DVM to
keep it's accuracy and stability!

So I replaced these caps (and cleaned the dust everywhere I
could). Unfortunately I did not have caps with the correct spacing, so
the caps I soldered on are not plated on the PCB, and the only
replacement cap I could find for C19 is a (probably) poor quality
noname brand... I'll have to order some better replacement caps next
time I have to purchase on DigiKey or Mouser.

Whatever, after this quick recap, the 3456A is back alive:

.. image:: {filename}/images/hp3456a/1.0011v.jpg
   :alt: 1.0011V

And even better, it look much more stable than it used to be!

For example, here a plot of my Geller Voltage Reference a few month ago:

.. image:: {filename}/images/hp3456a/geller_2016_02.svg
   :alt: 10V

and now:

.. image:: {filename}/images/hp3456a/geller_2016_11.svg
   :alt: 10V


It's time for a nice enclosure for my Geller Labs reference now. I
also still have in my todo list a DC reference built around a AD5791
and a LT1100 I have.