content/hp3456a.rst

Wed, 16 Oct 2019 21:53:47 +0200

author
David Douard <david.douard@sdfa3.org>
date
Wed, 16 Oct 2019 21:53:47 +0200
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1 ============================
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2 HP 3456A Digital Voltmeter
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3 ============================
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4
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5 :Author: David Douard
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6 :Category: Electronics
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7 :Tags: HP, 3456A, DVM, repair, test equipment
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8 :series: HP 3454A
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9 :series_index: 1
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11
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12 A very long time ago, I bought a venerable HP 3456A Digital
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13 Voltmeter. Back then, I had no clear idea what kind of device it is;
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14 but it was cheap. It worked quite fine (it is indeed not calibrated),
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15 but I noticed for a while that it was quite unstable: the last 2 or
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16 even 3 digits (in 6 1/2 digits mode) were jumping a bit too much.
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17
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18 Back then, I did not took the time to investigate the reasons of this
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19 behavior.
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20
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21 However, a few days ago I plugged it to roughly test the stability of
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22 an old Time Electronic Limitd D.C. Voltage Calibrator I bought for
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23 almost nothing. After a few hours, the display showed an overload
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24 condition (display OL).
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25
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26 So I started to read the service manual and to follow the
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27 troubleshooting guide. At some point I reached the step where the
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28 procedure requires a Signature Anayzer (which I obviously don't have).
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29 I briefly tried to dig the signals with my cheap USB logic analyzer,
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30 but the readings were quite disappointing. I was considering
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31 implementing a signature analyzer using a Nucleo STM32 board, since I
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32 have several of them for my `HP 34970A display replacement project
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33 <{static}/hp34970a.rst>`_.
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34
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35 Suddenly, I realized I did not even check the power rails! And
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36 bingo, the +33V was dead (something like 5V).
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37
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38 Ok, much better, a power supply failure is a way better
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39 news. Especially since this kind of device uses a linear power supply:
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41 .. image:: {static}/images/hp3456a/psu.jpg
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42 :alt: HP 3456A's power supply board
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43
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44 Yes, it is very dusty.
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45 The culprit appeared in plain sight as soon as I removed the power
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46 supply board (A10) from the unit:
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48 .. image:: {static}/images/hp3456a/psu_C7.jpg
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49 :alt: failing caps on HP 3456A's power supply
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51 The schematic is very straighforward; few bridge rectifiers, some
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52 capatcitors, some regultators, nothing fancy:
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54 .. image:: {static}/images/hp3456a/psu_schematic.svg
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55 :alt: schematic of the power supply board
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57 The dead capacitor is C7 (47µF, 50V, Sprague). I also checked the
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58 other capacitors on the board, and noticed some of them were not very
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59 tight. A quick inspection of the solder layer showed a few "cold"
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60 solderings on C16 and C19. That would for sure not help the DVM to
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61 keep it's accuracy and stability!
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63 So I replaced these caps (and cleaned the dust everywhere I
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64 could). Unfortunately I did not have caps with the correct spacing, so
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65 the caps I soldered on are not plated on the PCB, and the only
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66 replacement cap I could find for C19 is a (probably) poor quality
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67 noname brand... I'll have to order some better replacement caps next
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68 time I have to purchase on DigiKey or Mouser.
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70 Whatever, after this quick recap, the 3456A is back alive:
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72 .. image:: {static}/images/hp3456a/1.0011v.jpg
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73 :alt: 1.0011V
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75 And even better, it look much more stable than it used to be!
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77 For example, here a plot of my Geller Voltage Reference a few month ago:
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79 .. image:: {static}/images/hp3456a/geller_2016_02.svg
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80 :alt: 10V
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82 and now:
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84 .. image:: {static}/images/hp3456a/geller_2016_11.svg
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85 :alt: 10V
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88 It's time for a nice enclosure for my Geller Labs reference now. I
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89 also still have in my todo list a DC reference built around a AD5791
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90 and a LT1100 I have.
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