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| 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 | |
| 33 <{filename}/hp34970a.rst>`_. | |
| 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 | |
| 41 .. image:: {filename}/images/hp3456a/psu.jpg | |
| 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 | |
| 48 .. image:: {filename}/images/hp3456a/psu_C7.jpg | |
| 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 | |
| 54 .. image:: {filename}/images/hp3456a/psu_schematic.svg | |
| 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 | |
| 72 .. image:: {filename}/images/hp3456a/1.0011v.jpg | |
| 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 | |
| 79 .. image:: {filename}/images/hp3456a/geller_2016_02.svg | |
| 80 :alt: 10V | |
| 81 | |
| 82 and now: | |
| 83 | |
| 84 .. image:: {filename}/images/hp3456a/geller_2016_11.svg | |
| 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. | |
| 91 | |
| 92 | |
| 93 |
