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| 1 ================================= | |
| 2 HP 34970A Data Acquisition Unit | |
| 3 ================================= | |
| 4 | |
| 5 :Author: David Douard | |
| 6 :Category: Electronics | |
| 7 :Tags: HP, 34970A, DMM, repair, test equipment | |
| 8 :series: HP 34970A repair | |
| 9 :series_index: 1 | |
| 10 | |
| 11 | |
| 12 I recently "scored" a HP 34970A with the DMM included and 2 plugin | |
| 13 cards (HP 34901A and HP 34907A) for less than 200€. The plugins should | |
| 14 be in working condition, but the main switch unit was almost "given" | |
| 15 with the plugin modules since it is non functional. | |
| 16 | |
| 17 .. image:: {filename}/images/hp34970a/front-view.jpg | |
| 18 :alt: front view of the HP 34970A data acquisition unit | |
| 19 | |
| 20 It's in very good shape, the front panel and the push buttons are | |
| 21 clean. The knob is nice but it has a rough step when turning it for a | |
| 22 complete revolution. | |
| 23 | |
| 24 When powered up, however, things are a bit nastier... The VFD is, as | |
| 25 often with these equipment, very dim and the digits tend to puke on | |
| 26 the others. In fact, it's mostly unreadable, and the unit beeps | |
| 27 several times, signs of errors (which I cannot really read them see on | |
| 28 the display). | |
| 29 | |
| 30 Internals | |
| 31 ========= | |
| 32 | |
| 33 The unit consists in 4 parts: | |
| 34 | |
| 35 - the main logic/cpu board on the bottom left side, | |
| 36 - the DMM module, juste above the main board, | |
| 37 - the frontpanel and | |
| 38 - the back plane where plugin modules are connected. | |
| 39 | |
| 40 .. image:: {filename}/images/hp34970a/bottom-view.jpg | |
| 41 :alt: bottom view of the HP 34970A | |
| 42 | |
| 43 .. image:: {filename}/images/hp34970a/top-view.jpg | |
| 44 :alt: top view of the HP 34970A with DMM removed | |
| 45 | |
| 46 .. image:: {filename}/images/hp34970a/dmm.jpg | |
| 47 :alt: DMM module of the HP 34970A | |
| 48 | |
| 49 The front panel hold the keyboard, the rotary encoder and the diplay system. | |
| 50 | |
| 51 .. image:: {filename}/images/hp34970a/front-panel.jpg | |
| 52 :alt: front panel of the HP 34970A | |
| 53 | |
| 54 The front panel is managed by a 80C51 (a 87C51 actually) | |
| 55 microcontroller. It's responsible for handling the keyboard and | |
| 56 feeding the shift registers of the VFD drivers (a pair of | |
| 57 HV518PJ). The communication with the main CPU is a serial line with | |
| 58 opamps to adjust the signal levels. | |
| 59 | |
| 60 The chips are PLCC44 located under the VFD are the drivers: | |
| 61 | |
| 62 .. image:: {filename}/images/hp34970a/front-panel-pcb.jpg | |
| 63 :alt: front panel PCB of the HP 34970A | |
| 64 | |
| 65 On my unit, the voltage levels for the VFD are fine. I suspected that | |
| 66 one of the 2 VFD drivers (the one responsible for driving the grids) | |
| 67 were partially failing, so I ordered a pair of new drivers, as well as | |
| 68 some PLCC sockets. It was my first real smd revork. I though that it | |
| 69 might be easier to cut the pins of the soldered chip, but it was not a | |
| 70 good idea, I peeled 2 pads on the PCB. Thanksfully, they were not | |
| 71 broken. So I've undoldered the other VFD with my new cheap hot air | |
| 72 soldering station, and it's been way easier and did not damage the PCB | |
| 73 at all... Lesson learned. | |
| 74 | |
| 75 After that first real SMD unsoldering experiment came the first SMD | |
| 76 soldering: install the plastic made PLCC sockets... I was not sure | |
| 77 wether to use the hot air gun or my Weller soldering iron, nor what a | |
| 78 correct temperature for the hot air station whould be adequate. The | |
| 79 informations on the socket datasheet were not very clear to me. | |
| 80 | |
| 81 So I gave a try (I think at around 280°C). The sockets seemed properly | |
| 82 soldered, but in fact, a few pins were not making contact. So I | |
| 83 decided to cut the bottom of the socket (which is essentially useless) | |
| 84 so I can more easily rework these "cold" pins with my soldering iron. | |
| 85 | |
| 86 The tricky part of installing these PLCC sockets is that decoupling | |
| 87 capacitors (C6, C7, C9 and C10) are a bit too close, so I had to move | |
| 88 them. | |
| 89 | |
| 90 .. image:: {filename}/images/hp34970a/front-panel-sockets.jpg | |
| 91 :alt: PLCC sockets for the VFD drivers of the HP 34970A | |
| 92 | |
| 93 | |
| 94 Now I had to try to rejuvenate the tungsten filaments. Like for old | |
| 95 CRT (and even for triodes), the filament, which is responsible for | |
| 96 emiting electrons toward the anode (coated with phosphor) tend to be | |
| 97 polluted and loose their emissive power. A common trick to "clean" | |
| 98 them is to apply a much higher voltage than rated so they shine for a | |
| 99 few seconds. One mst be careful not to burn them, otherwise the VFD is | |
| 100 definitively lost. | |
| 101 | |
| 102 .. image:: {filename}/images/hp34970a/vfd-back.jpg | |
| 103 :alt: back side of the VFD (probably dead) module | |
| 104 | |
| 105 So I tried such a rejuvenation on my VFD module. I wasn't sure what | |
| 106 color the filaments must be heated to (between a light orange to an | |
| 107 almost white yellow). I was doing this using my `Lambda PSU | |
| 108 <{filename}/LPD422FM.rst>`_ raising the voltage while looking at the | |
| 109 filaments. | |
| 110 | |
| 111 The results are very disappointing. The VFD is a little bit brighter, | |
| 112 but the spilling over between digits is now much worse... | |
| 113 | |
| 114 I guess this VFD module cannot be saved, after all... | |
| 115 | |
| 116 So I've been thinking of a plan B, and I think I'll try to replace the | |
| 117 VFD display with a blue 256x32 OLED module. It's smaller (I cannot | |
| 118 find a module near the size of the VFD display), but it should still | |
| 119 be quite readable. Indeed, that requires a bit of work: I plan to | |
| 120 build an arduino-based adapter (using an arduino micor) that will take | |
| 121 place of the VFD drivers (then be fed by the serial-to-shift registers | |
| 122 outputs of the 80C51). One other solution is to replace the 80C51 with | |
| 123 a newly programmed one, but I think reverse ingineering the serial | |
| 124 communication between the front-panel microcontroller and the main CPU | |
| 125 would require much more work. Since it's a 87C51, I doubt I can easily | |
| 126 dump it's ROM... | |
| 127 | |
| 128 | |
| 129 Meanwhile, next step is to take care of all these errors. |
