content/hp34970a.rst

Tue, 28 Aug 2018 15:05:01 +0200

author
David Douard <david.douard@logilab.fr>
date
Tue, 28 Aug 2018 15:05:01 +0200
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improve hgignore

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1 =================================
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2 HP 34970A Data Acquisition Unit
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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, 34970A, DMM, repair, test equipment
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8 :series: HP 34970A repair
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9 :series_index: 1
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12 I recently "scored" a HP 34970A with the DMM included and 2 plugin
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13 cards (HP 34901A and HP 34907A) for less than 200€. The plugins should
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14 be in working condition, but the main switch unit was almost "given"
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15 with the plugin modules since it is non functional.
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17 .. image:: {filename}/images/hp34970a/front-view.jpg
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18 :alt: front view of the HP 34970A data acquisition unit
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20 It's in very good shape, the front panel and the push buttons are
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21 clean. The knob is nice but it has a rough step when turning it for a
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22 complete revolution.
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23
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24 When powered up, however, things are a bit nastier... The VFD is, as
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25 often with these equipment, very dim and the digits tend to puke on
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26 the others. In fact, it's mostly unreadable, and the unit beeps
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27 several times, signs of errors (which I cannot really read them see on
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28 the display).
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29
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30 Internals
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31 =========
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32
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33 The unit consists in 4 parts:
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34
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35 - the main logic/cpu board on the bottom left side,
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36 - the DMM module, juste above the main board,
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37 - the frontpanel and
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38 - the back plane where plugin modules are connected.
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39
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40 .. image:: {filename}/images/hp34970a/bottom-view.jpg
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41 :alt: bottom view of the HP 34970A
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42
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43 .. image:: {filename}/images/hp34970a/top-view.jpg
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44 :alt: top view of the HP 34970A with DMM removed
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45
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46 .. image:: {filename}/images/hp34970a/dmm.jpg
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47 :alt: DMM module of the HP 34970A
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48
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49 The front panel hold the keyboard, the rotary encoder and the diplay system.
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50
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51 .. image:: {filename}/images/hp34970a/front-panel.jpg
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52 :alt: front panel of the HP 34970A
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53
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54 The front panel is managed by a 80C51 (a 87C51 actually)
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55 microcontroller. It's responsible for handling the keyboard and
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56 feeding the shift registers of the VFD drivers (a pair of
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57 HV518PJ). The communication with the main CPU is a serial line with
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58 opamps to adjust the signal levels.
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59
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60 The chips are PLCC44 located under the VFD are the drivers:
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61
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62 .. image:: {filename}/images/hp34970a/front-panel-pcb.jpg
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63 :alt: front panel PCB of the HP 34970A
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64
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65 On my unit, the voltage levels for the VFD are fine. I suspected that
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66 one of the 2 VFD drivers (the one responsible for driving the grids)
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67 were partially failing, so I ordered a pair of new drivers, as well as
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68 some PLCC sockets. It was my first real smd revork. I though that it
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69 might be easier to cut the pins of the soldered chip, but it was not a
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70 good idea, I peeled 2 pads on the PCB. Thanksfully, they were not
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71 broken. So I've undoldered the other VFD with my new cheap hot air
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72 soldering station, and it's been way easier and did not damage the PCB
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73 at all... Lesson learned.
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74
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75 After that first real SMD unsoldering experiment came the first SMD
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76 soldering: install the plastic made PLCC sockets... I was not sure
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77 wether to use the hot air gun or my Weller soldering iron, nor what a
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78 correct temperature for the hot air station whould be adequate. The
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79 informations on the socket datasheet were not very clear to me.
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80
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81 So I gave a try (I think at around 280°C). The sockets seemed properly
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82 soldered, but in fact, a few pins were not making contact. So I
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83 decided to cut the bottom of the socket (which is essentially useless)
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84 so I can more easily rework these "cold" pins with my soldering iron.
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85
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86 The tricky part of installing these PLCC sockets is that decoupling
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87 capacitors (C6, C7, C9 and C10) are a bit too close, so I had to move
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88 them.
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89
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90 .. image:: {filename}/images/hp34970a/front-panel-sockets.jpg
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91 :alt: PLCC sockets for the VFD drivers of the HP 34970A
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92
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93
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94 Now I had to try to rejuvenate the tungsten filaments. Like for old
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95 CRT (and even for triodes), the filament, which is responsible for
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96 emiting electrons toward the anode (coated with phosphor) tend to be
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97 polluted and loose their emissive power. A common trick to "clean"
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98 them is to apply a much higher voltage than rated so they shine for a
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99 few seconds. One mst be careful not to burn them, otherwise the VFD is
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100 definitively lost.
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101
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102 .. image:: {filename}/images/hp34970a/vfd-back.jpg
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103 :alt: back side of the VFD (probably dead) module
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104
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105 So I tried such a rejuvenation on my VFD module. I wasn't sure what
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106 color the filaments must be heated to (between a light orange to an
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107 almost white yellow). I was doing this using my `Lambda PSU
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108 <{filename}/LPD422FM.rst>`_ raising the voltage while looking at the
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109 filaments.
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110
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111 The results are very disappointing. The VFD is a little bit brighter,
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112 but the spilling over between digits is now much worse...
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113
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114 I guess this VFD module cannot be saved, after all...
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115
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116 So I've been thinking of a plan B, and I think I'll try to replace the
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117 VFD display with a blue 256x32 OLED module. It's smaller (I cannot
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118 find a module near the size of the VFD display), but it should still
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119 be quite readable. Indeed, that requires a bit of work: I plan to
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120 build an arduino-based adapter (using an arduino micor) that will take
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121 place of the VFD drivers (then be fed by the serial-to-shift registers
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122 outputs of the 80C51). One other solution is to replace the 80C51 with
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123 a newly programmed one, but I think reverse ingineering the serial
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124 communication between the front-panel microcontroller and the main CPU
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125 would require much more work. Since it's a 87C51, I doubt I can easily
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126 dump it's ROM...
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58e79464cd09 [hp34970a] new blog post about the HP 34970A
David Douard <david.douard@logilab.fr>
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58e79464cd09 [hp34970a] new blog post about the HP 34970A
David Douard <david.douard@logilab.fr>
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129 Meanwhile, next step is to take care of all these errors.

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