content/hp8904a.rst

Sat, 19 Mar 2016 00:55:18 +0100

author
David Douard <david.douard@logilab.fr>
date
Sat, 19 Mar 2016 00:55:18 +0100
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1 ===============================================
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2 HP8904A Multifunction Synthetizer Restoration
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3 ===============================================
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5 :Author: David Douard
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6 :Category: Electronics
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7 :Tags: HP8904A, repair, test equipment, signal generator
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10 What Is This About
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11 ==================
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13 I have a couple of HP8904A multifunction synthetizers for years
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14 (bought them on ebay around 2007 IIRC). One of them is working
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15 properly, but the second one had very inaccurate output levels,
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16 generating a non-symmetric signal when output level was set above 5V
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17 or something like that.
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18
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19 .. image:: {filename}images/hp8904a/twin_HP8904A.jpg
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20 :alt: My 2 HP8904A synthetizers.
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21
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22 I knew for a fact that the PSU was the problem (thanks to the fact I
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23 have 2 units, I could easily swap elements between the units). Back
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24 then, I spent a bit of time trying to figure out what was going wrong,
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25 and detected the +16.3V output of the PSU was failing at around 9 or
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26 10V. I did not had a chance to investigate more then.
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28 For some strange reasons, I've recently decided to fix this PSU
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29 problem.
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32 The PSU Problem
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33 ===============
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34
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35 .. image:: {filename}images/hp8904a/hp8904a_top.jpg
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36 :alt: Top view of the HP8904A
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37
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38 The PSU board is the one at the rear of the unit, on the upper part of
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39 the case (the lower part hold the analogic boards). The PSU is not a
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40 custom HP board, but is a Computer Products switchmode supply (90
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41 Watts, Model XL51-5601).
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42
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43 It delivers 4 voltages on 2 separated ground planes:
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44
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45 - a +16.3V and -16.3V rail (with their own ground plane),
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46 - a 5.1V for logic devices,
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47 - a -15V (same ground as the +5.1V) which powers at least the fan.
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48
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49 There are not too much material on the web about the possible PSU
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50 problems of this equipment. A few threads on forums and
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51 `this very interesting blog by Simon Schrödle`_
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52
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53 .. _`this very interesting blog by Simon Schrödle`: http://www.simonsdialogs.com/category/repairs/hpak-8904a/
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54
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55 But his problem was different from mine.
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57 On my device, the problem was with the +16.3V output falling at around
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58 9V *on load*. The level was at his proper 16.3V level when the main
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59 board as not plugged in.
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60
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61 I first gave a close look at the caps, looking for some evidence of a
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62 failing capacitor... which is always the culprit to think of in this
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63 kind of situation. But I could not find anything obvious.
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64
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65 So I started to reverse engineer the circuit to try to understand a
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66 bit the possible causes.
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67
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68 The PSU consists in 2 PCBs, the main one which is a double-layer PCB,
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69 and a small daughter board with several op amps and zener
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70 regulators. Obviously the regulation stuff.
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71
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72 In order to be able to follow the tracks, I had to remove the heat
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73 sink and split these two boards apart, since it's not easy to deal
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74 with this:
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75
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76 .. image:: {filename}images/hp8904a/hp8904_psu_followme.jpg
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77 :alt: part of the +16.3V PSU rail
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78
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79 So I removed the heatsink and disassembled this small board from the
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80 main one (which was quite painful, since I don't have a disoldering
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81 station. Disassembling the 3x 8 pins angle connectors from the double
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82 sided PCB was not a piece of cake...)
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83
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84 .. image:: {filename}images/hp8904a/HP8904A_daughter_board.jpg
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85 :alt: the small daughter board of the PSU
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86
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87 After that, I noticed a possible leak of a small cap, but this was on
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88 the 5.1V rail. Should not be related to my +16.3V problem.
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89
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90 But I decided to take advantage of having disassembled the PSU to
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91 replace a few of these caps, starting with this dubious one.
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92
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93 I also decided to replace the soldered assembly for the two PCBs by
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94 some connectors so I can easily install and remove the daughter board.
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95
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96 After having replaced the caps, the PSU was still failing. So I really
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97 had to understand the schematic to investigate more.
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98
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99 The part of the schematic involved in the +16.3V rail is the following:
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100
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101 .. image:: {filename}images/hp8904a/psu_schematic.png
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102 :alt: part of the schemtic of the PSU
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103
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104 At first glance (the schematic was then a bunch of poorly made
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105 "Dave-CAD-like" sketches on several sheets), the exact role of the
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106 LM392 opamp was not clear to me, but when I redrew the schematic a bit
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107 more clearly, it became obvious it was some sort of current limitation
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108 circuit. And, sure, my problem could very well be a current limitation
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109 system being a bit zealous.
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110
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111 So I started to probe the resistors around the LM392 to check their
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112 values. And guess what? R7 was dead. It's a 20k 1% resistor, and it
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113 was not letting any current pass through.
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114
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115 What are the odd of such a failure on a PSU like that? Unbelievable! I
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116 was expecting the failure of a cap, of course, maybe an opamp or a
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117 diode, but not a resistor.
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118
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119 I have not yet bought a proper 20k stable 1% resistor (which I don't
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120 have laying around), but I've installed a pair of 1% resistors in parallel
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121 (can't remember the exact values I've used) to get a close 20k. And bingo!
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122 the HP8904A can output a nice 10Vrms again!
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123
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124 Of course, while I was digging around my PSU problem, the Schaffner
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125 power line filter died in a violent smelly smoky way (took a while to
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126 get rid of the smell).
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127
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128 Battery, RAM and software options
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129 =================================
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130
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131 One of my goals when I looked into my two HP8904A was to check the on
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132 board batteries responsible for keeping the content of the memory.
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133
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134 This is a critical thing, since the content of the RAM must NOT be
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135 lost otherwise the device won't work anymore (the official procedure
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136 to change the battery involeve sodering a power source in parallel to
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137 ensure the RAM chip is always under power).
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138
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139 The HP8904A seems to be the first HP test equipment to have
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140 software-enabled options. The option activation depends on the serial
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141 number of the machine, which is also stored in this "non-volatile"
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142 RAM.
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143
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144 There is mainly one option one does not want to loose, it's the
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145 Option 1. It enables the Channel Configuration Mode Enhancement. It's
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146 the one that make the unit so handy (allowing to mix up to 4
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147 waveforms).
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148
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149 I already cheated a bit back then. In fact, one of my 2 HP8904 came
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150 with no option, the other with option 1 and 2. Option 2 means it have
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151 a second analogic board allowing two separated outputs. So I used the
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152 serial number of the unit coming with option 1 on the other one, and
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153 bingo, this latest had then the option 1 activated also.
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154
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155 But I recently found on the EEVblog a
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156 `thread pointing to an HP service document`_ describing how to modify
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157 the correct byte in the RAM to activate all the options!
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158
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159 And the magic trick is::
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160
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161 Power-Cycle/Power-On
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162 SHIFT, SERVICE, SHIFT, f4, 0, 9, 2, 4, 4, 8, SHIFT, SERVICE
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163 NEXT, NEXT, NEXT, f1, 3, F, F, C
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164 f2, F, F, f4, f4
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165 Power-Cycle
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166
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167 That is: write the value 0xFF at address 0x3FFC.
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168
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169 The procedure to restore the serial number can be found in the service
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170 manual of the HP8904.
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171
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172 And in case of failed battery, the memory can be restored following
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173 `this document <{filename}pdfs/HP_8904A_Service_Notes_2.pdf>`_
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174
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175 .. _`thread pointing to an HP service document`: http://www.eevblog.com/forum/testgear/hp-8904a-options/
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176
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177
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178 Next steps
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179 ==========
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180
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181 The first thing I have to do is to replace my temporary fix with a new
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182 stable resistor.
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183
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184 I'll probably replace the power line filter of the other signal
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185 generator before it dies in a smoky way.
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186
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187 Then, I need to recalibrate these puppies, but I need to build a bit
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188 of confidence in my HP3456A DVM for this. But that's an other story.
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189
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190 I also want to replace the fans with some more recent and quiet models.

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