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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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10 |
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11 |
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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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16 |
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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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19 |
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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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127 |
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128 |
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129 Meanwhile, next step is to take care of all these errors. |