content/hp8904a.rst

Tue, 15 Mar 2016 16:23:22 +0100

author
David Douard <david.douard@logilab.fr>
date
Tue, 15 Mar 2016 16:23:22 +0100
changeset 38
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[10MHz] typos

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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 :Date: 2016-02-20
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7 :Category: Electronics
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8 :Tags: HP8904A, repair, test equipment, signal generator
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9
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11 What Is This About
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12 ==================
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14 I have a couple of HP8904A multifunction synthetizers for years
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15 (bought them on ebay around 2007 IIRC). One of them is working
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16 properly, but the second one had very inaccurate output levels,
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17 generating a non-symmetric signal when output level was set above 5V
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18 or something like that.
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19
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20 .. image:: {filename}images/hp8904a/twin_HP8904A.jpg
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21 :alt: My 2 HP8904A synthetizers.
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22
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23 I knew for a fact that the PSU was the problem (thanks to the fact I
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24 have 2 units, I could easily swap elements between the units). Back
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25 then, I spent a bit of time trying to figure out what was going wrong,
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26 and detected the +16.3V output of the PSU was failing at around 9 or
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27 10V. I did not had a chance to investigate more then.
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29 For some strange reasons, I've recently decided to fix this PSU
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30 problem.
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31
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32
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33 The PSU Problem
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34 ===============
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35
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36 .. image:: {filename}images/hp8904a/hp8904a_top.jpg
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37 :alt: Top view of the HP8904A
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38
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39 The PSU board is the one at the rear of the unit, on the upper part of
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40 the case (the lower part hold the analogic boards). The PSU is not a
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41 custom HP board, but is a Computer Products switchmode supply (90
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42 Watts, Model XL51-5601).
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43
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44 It delivers 4 voltages on 2 separated ground planes:
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45
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46 - a +16.3V and -16.3V rail (with their own ground plane),
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47 - a 5.1V for logic devices,
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48 - a -15V (same ground as the +5.1V) which powers at least the fan.
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49
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50 There are not too much material on the web about the possible PSU
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51 problems of this equipment. A few threads on forums and
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52 `this very interesting blog by Simon Schrödle`_
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53
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54 .. _`this very interesting blog by Simon Schrödle`: http://www.simonsdialogs.com/category/repairs/hpak-8904a/
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55
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56 But his problem was different from mine.
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57
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58 On my device, the problem was with the +16.3V output falling at around
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59 9V *on load*. The level was at his proper 16.3V level when the main
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60 board as not plugged in.
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61
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62 I first gave a close look at the caps, looking for some evidence of a
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63 failing capacitor... which is always the culprit to think of in this
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64 kind of situation. But I could not find anything obvious.
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65
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66 So I started to reverse engineer the circuit to try to understand a
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67 bit the possible causes.
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68
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69 The PSU consists in 2 PCBs, the main one which is a double-layer PCB,
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70 and a small daughter board with several op amps and zener
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71 regulators. Obviously the regulation stuff.
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72
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73 In order to be able to follow the tracks, I had to remove the heat
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74 sink and split these two boards apart, since it's not easy to deal
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75 with this:
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76
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77 .. image:: {filename}images/hp8904a/hp8904_psu_followme.jpg
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78 :alt: part of the +16.3V PSU rail
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79
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80 So I removed the heatsink and disassembled this small board from the
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81 main one (which was quite painful, since I don't have a disoldering
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82 station. Disassembling the 3x 8 pins angle connectors from the double
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83 sided PCB was not a piece of cake...)
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84
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85 .. image:: {filename}images/hp8904a/HP8904A_daughter_board.jpg
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86 :alt: the small daughter board of the PSU
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87
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88 After that, I noticed a possible leak of a small cap, but this was on
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89 the 5.1V rail. Should not be related to my +16.3V problem.
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90
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91 But I decided to take advantage of having disassembled the PSU to
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92 replace a few of these caps, starting with this dubious one.
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93
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94 I also decided to replace the soldered assembly for the two PCBs by
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95 some connectors so I can easily install and remove the daughter board.
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96
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97 After having replaced the caps, the PSU was still failing. So I really
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98 had to understand the schematic to investigate more.
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99
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100 The part of the schematic involved in the +16.3V rail is the following:
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101
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102 .. image:: {filename}images/hp8904a/psu_schematic.png
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103 :alt: part of the schemtic of the PSU
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104
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105 At first glance (the schematic was then a bunch of poorly made
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106 "Dave-CAD-like" sketches on several sheets), the exact role of the
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107 LM392 opamp was not clear to me, but when I redrew the schematic a bit
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108 more clearly, it became obvious it was some sort of current limitation
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109 circuit. And, sure, my problem could very well be a current limitation
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110 system being a bit zealous.
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111
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112 So I started to probe the resistors around the LM392 to check their
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113 values. And guess what? R7 was dead. It's a 20k 1% resistor, and it
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114 was not letting any current pass through.
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115
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116 What are the odd of such a failure on a PSU like that? Unbelievable! I
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117 was expecting the failure of a cap, of course, maybe an opamp or a
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118 diode, but not a resistor.
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119
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120 I have not yet bought a proper 20k stable 1% resistor (which I don't
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121 have laying around), but I've installed a pair of 1% resistors in parallel
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122 (can't remember the exact values I've used) to get a close 20k. And bingo!
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123 the HP8904A can output a nice 10Vrms again!
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124
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125 Of course, while I was digging around my PSU problem, the Schaffner
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126 power line filter died in a violent smelly smoky way (took a while to
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127 get rid of the smell).
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128
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129 Battery, RAM and software options
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130 =================================
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131
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132 One of my goals when I looked into my two HP8904A was to check the on
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133 board batteries responsible for keeping the content of the memory.
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134
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135 This is a critical thing, since the content of the RAM must NOT be
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136 lost otherwise the device won't work anymore (the official procedure
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137 to change the battery involeve sodering a power source in parallel to
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138 ensure the RAM chip is always under power).
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139
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140 The HP8904A seems to be the first HP test equipment to have
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141 software-enabled options. The option activation depends on the serial
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142 number of the machine, which is also stored in this "non-volatile"
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143 RAM.
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144
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145 There is mainly one option one does not want to loose, it's the
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146 Option 1. It enables the Channel Configuration Mode Enhancement. It's
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147 the one that make the unit so handy (allowing to mix up to 4
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148 waveforms).
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149
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150 I already cheated a bit back then. In fact, one of my 2 HP8904 came
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151 with no option, the other with option 1 and 2. Option 2 means it have
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152 a second analogic board allowing two separated outputs. So I used the
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153 serial number of the unit coming with option 1 on the other one, and
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154 bingo, this latest had then the option 1 activated also.
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155
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156 But I recently found on the EEVblog a
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157 `thread pointing to an HP service document`_ describing how to modify
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158 the correct byte in the RAM to activate all the options!
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159
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160 And the magic trick is::
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161
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162 Power-Cycle/Power-On
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163 SHIFT, SERVICE, SHIFT, f4, 0, 9, 2, 4, 4, 8, SHIFT, SERVICE
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164 NEXT, NEXT, NEXT, f1, 3, F, F, C
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165 f2, F, F, f4, f4
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166 Power-Cycle
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167
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168 That is: write the value 0xFF at address 0x3FFC.
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169
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170 The procedure to restore the serial number can be found in the service
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171 manual of the HP8904.
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172
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173 And in case of failed battery, the memory can be restored following
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174 `this document <{filename}pdfs/HP_8904A_Service_Notes_2.pdf>`_
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175
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176 .. _`thread pointing to an HP service document`: http://www.eevblog.com/forum/testgear/hp-8904a-options/
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177
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178
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179 Next steps
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180 ==========
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181
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182 The first thing I have to do is to replace my temporary fix with a new
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183 stable resistor.
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184
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185 I'll probably replace the power line filter of the other signal
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186 generator before it dies in a smoky way.
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187
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188 Then, I need to recalibrate these puppies, but I need to build a bit
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189 of confidence in my HP3456A DVM for this. But that's an other story.
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190
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191 I also want to replace the fans with some more recent and quiet models.

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