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+++ b/content/hp3456a.rst Tue Nov 22 22:50:31 2016 +0100
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+============================
+ 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.
+
+
+