Sat, 09 Oct 2021 16:52:19 +0200
HP34970A - part 6
70 | 1 | ============================ |
2 | HP 3456A Digital Voltmeter | |
3 | ============================ | |
4 | ||
5 | :Author: David Douard | |
6 | :Category: Electronics | |
7 | :Tags: HP, 3456A, DVM, repair, test equipment | |
8 | :series: HP 3454A | |
9 | :series_index: 1 | |
10 | ||
11 | ||
12 | A very long time ago, I bought a venerable HP 3456A Digital | |
13 | Voltmeter. Back then, I had no clear idea what kind of device it is; | |
14 | but it was cheap. It worked quite fine (it is indeed not calibrated), | |
15 | but I noticed for a while that it was quite unstable: the last 2 or | |
16 | even 3 digits (in 6 1/2 digits mode) were jumping a bit too much. | |
17 | ||
18 | Back then, I did not took the time to investigate the reasons of this | |
19 | behavior. | |
20 | ||
21 | However, a few days ago I plugged it to roughly test the stability of | |
22 | an old Time Electronic Limitd D.C. Voltage Calibrator I bought for | |
23 | almost nothing. After a few hours, the display showed an overload | |
24 | condition (display OL). | |
25 | ||
26 | So I started to read the service manual and to follow the | |
27 | troubleshooting guide. At some point I reached the step where the | |
28 | procedure requires a Signature Anayzer (which I obviously don't have). | |
29 | I briefly tried to dig the signals with my cheap USB logic analyzer, | |
30 | but the readings were quite disappointing. I was considering | |
31 | implementing a signature analyzer using a Nucleo STM32 board, since I | |
32 | have several of them for my `HP 34970A display replacement project | |
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33 | <{filename}/hp34970a.rst>`_. |
70 | 34 | |
35 | Suddenly, I realized I did not even check the power rails! And | |
36 | bingo, the +33V was dead (something like 5V). | |
37 | ||
38 | Ok, much better, a power supply failure is a way better | |
39 | news. Especially since this kind of device uses a linear power supply: | |
40 | ||
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41 | .. image:: {static}/images/hp3456a/psu.jpg |
70 | 42 | :alt: HP 3456A's power supply board |
43 | ||
44 | Yes, it is very dusty. | |
45 | The culprit appeared in plain sight as soon as I removed the power | |
46 | supply board (A10) from the unit: | |
47 | ||
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48 | .. image:: {static}/images/hp3456a/psu_C7.jpg |
70 | 49 | :alt: failing caps on HP 3456A's power supply |
50 | ||
51 | The schematic is very straighforward; few bridge rectifiers, some | |
52 | capatcitors, some regultators, nothing fancy: | |
53 | ||
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54 | .. image:: {static}/images/hp3456a/psu_schematic.svg |
70 | 55 | :alt: schematic of the power supply board |
56 | ||
57 | The dead capacitor is C7 (47µF, 50V, Sprague). I also checked the | |
58 | other capacitors on the board, and noticed some of them were not very | |
59 | tight. A quick inspection of the solder layer showed a few "cold" | |
60 | solderings on C16 and C19. That would for sure not help the DVM to | |
61 | keep it's accuracy and stability! | |
62 | ||
63 | So I replaced these caps (and cleaned the dust everywhere I | |
64 | could). Unfortunately I did not have caps with the correct spacing, so | |
65 | the caps I soldered on are not plated on the PCB, and the only | |
66 | replacement cap I could find for C19 is a (probably) poor quality | |
67 | noname brand... I'll have to order some better replacement caps next | |
68 | time I have to purchase on DigiKey or Mouser. | |
69 | ||
70 | Whatever, after this quick recap, the 3456A is back alive: | |
71 | ||
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72 | .. image:: {static}/images/hp3456a/1.0011v.jpg |
70 | 73 | :alt: 1.0011V |
74 | ||
75 | And even better, it look much more stable than it used to be! | |
76 | ||
77 | For example, here a plot of my Geller Voltage Reference a few month ago: | |
78 | ||
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79 | .. image:: {static}/images/hp3456a/geller_2016_02.svg |
70 | 80 | :alt: 10V |
81 | ||
82 | and now: | |
83 | ||
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84 | .. image:: {static}/images/hp3456a/geller_2016_11.svg |
70 | 85 | :alt: 10V |
86 | ||
87 | ||
88 | It's time for a nice enclosure for my Geller Labs reference now. I | |
89 | also still have in my todo list a DC reference built around a AD5791 | |
90 | and a LT1100 I have. |