content/hp8662a.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 HP8662A Synthetized Signal Generator - Part 1
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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: HP8662A, repair, test equipment, HP, Generator, RF
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8 :series: HP 8662A
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9 :series_index: 1
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11 I recently pulled an HP 8662A I bought a while ago from my cellar. I don't
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12 remember exactly in what condition it was advertized when I purchased it, but
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13 it was most probably advertized as non working (otherwise it would have been
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14 cheap enough, more, what the fun in buying properly working test equipment?)
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15
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16 I wanted to make a break in my current other repair project, the `EIP 545B
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17 Microwave Fequency Meter <{filename}eip545b.rst>`_, also having a properly
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18 working unit such as this incredible signal synthetizer would be quite useful
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19 to perform several adjustement tasks on the EIP 545B.
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21 Carrying the unit from my cellar to my appartement was some kind of a job: it's
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22 a robust 30kg device I had to climb the 7 levels: no lift in the old
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23 parisian building I live in...
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24
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25 Overall state and disassembly
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26 =============================
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27
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28 The unit was in reasonably good condition, but is missing a few parts: the
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29 feets, 2 of the plastic corners on the back, and the nice quick sliding guide
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30 normally located on the bottom. Buttons however were very hard for most of
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31 them but 2, which were very soft and sloppy.
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32
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33 I think I tried to power it up when I received it a couple of years ago, and
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34 that it did start, but the keybord did not respond smoothly.
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35
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36 So when I started to look at it two weeks ago, I opened the upper cover and the
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37 protection plate above the PSU, and tried to plug it and power it up.
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38 Obviously, a small magic smoke escaped.
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39
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40 So I gave a more detailed look inside the unit, not only the PSU. I found the
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41 unit so dirty inside I decided to take it apart as far as I can to clean it
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42 before even attempting to fix the PSU.
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45 .. image:: {filename}images/hp8662a/dirty_hp.jpg
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46 :class: image-process-large-photo
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49 And I must say that if most of the unit is very easy to service due to the
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50 modular design of all RF modules, making is very easy to remove all the boards
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51 and modules, there are also many parts that are connected together with wires
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52 soldered point to point, which makes disassembling the case, motherboards and
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53 so very tedious.
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55 .. image:: {filename}images/hp8662a/top_before_2.jpg
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56 :class: image-process-large-photo
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58 .. image:: {filename}images/hp8662a/bottom_before.jpg
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59 :class: image-process-large-photo
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61 .. image:: {filename}images/hp8662a/front_after.jpg
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62 :class: image-process-large-photo
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64 Also, there are hundreds of screws in this unit! it's insane.
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66 .. image:: {filename}images/hp8662a/top_after.jpg
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67 :class: image-process-large-photo
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69 Whatever, I finally got to a point I could clean most of the parts of the unit,
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70 so I started to reassemble it. Unfortunately, I did not took enough pictures
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71 while the disassembling, so it took me some effort to have something that looks
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72 like an HP 8662A...
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74 .. image:: {filename}images/hp8662a/reassembling_2.jpg
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75 :class: image-process-large-photo
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77 Once I had the structure of the generator back togother, with the back plane in
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78 place but no other board nor module plugged in, I started to take care of the
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79 PSU.
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81 .. image:: {filename}images/hp8662a/reassembling_1.jpg
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82 :class: image-process-large-photo
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83
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84 The PSU
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85 =======
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86
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87 As I said, the first attempt to plug (and not even power on) the unit ended
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88 with a small magic smoke. The PSU consists in 2 main parts:
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89
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90 - A small linear power supply that is always powered to keep the OCXO
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91 tempearure regulation on all the time (so it's not necesary to wait an hour
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92 before being able to actually use the device when turned on), This also power
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93 a small part of the control circuit of the main PSU.
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94 - The main switching power supply. The high voltage section is allways on, and
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95 the mani switch on the front panel only activate the switching.
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96
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97 .. image:: {filename}images/hp8662a/PSU_simplified_schematic.jpeg
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98 :class: image-process-large-photo
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100 The 4 boards the PSU is made of are:
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101
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102 - a main board (A7A4) on which are the first stages of the AC rectification and
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103 filtering,
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104
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105 - a daugther board (A7A3) on which is the inverter, generating several
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106 unregulated voltages: +5.2V (used as control rail for the feedback loop),
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107 +23V, -13V and -43V; note that this board doaes not have the switching
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108 control circuitry,
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109
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110 - a daughter board (A7A1) with the linear regulators to produce very clean and
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111 quiet power rails for the RF modules; its output voltages are +20V
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112 (+/-100mV), -10V (/- 40mV) and -40V (+/-200mV),
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113
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114 - a daughter board (A7A2) with the switching control circuit. The switching
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115 regulation being made on the 5.2V rail, which is not regulated by A7A1.
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116
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117 .. image:: {filename}images/hp8662a/PSU_block_diagram.jpeg
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118 :class: image-process-large-photo
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119
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120 So I gave a close look at thes boards, beginning by the main board, the I found
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121 the culprit for the smoke: the input protection thermistor. Strangely, it still
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122 measures a decent resistance value, but hey.
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123
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124 The thermistor is described the old fashion way: it's a 5Ω @25°C with a
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125 temperature coefficient of -3.8%/°C. I did not find an exact replacement in my
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126 junk parts; only a slighty highter value one (a XXX, which is more like
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127 20Ω @25°C). Not ideal, but for now, it seems to work fine.
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128
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129 .. image:: {filename}images/hp8662a/smoking_thermistor.jpg
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130 :class: image-process-large-photo
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131
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132 After this first step forward, I was not confident enought to plug the boards
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133 back in place and try again (I am always worried when I repair switching
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134 PSUs... they can fail in flame and glory sometimes). So I checked all the
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135 diodes and transistors of the PSU, but none looked short.
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136
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137 I first applied power to the main board: nothing exploded, I was now having
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138 a nice + and -160VDC and the oven power was on spec.
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139
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140 Next step, I checked transformers of the inverter board before plugging it
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141 back, still no catastrophic failure. The inverter alone cannot do anything
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142 without the control board.
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143
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144 So I quickly checked the 2 other board before plugging them back. So far so good.
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145
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146 But when I tried to turn power on, the voltege OK green LEDs flickered once but
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147 did not stay on, and the error LED did turn on. All the voltages produced by
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148 the inverter board were very low. Somethin was wrong.
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149
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150 .. image:: {filename}images/hp8662a/PSU_inverter_schematic.jpeg
96
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151 :class: image-process-large-photo
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152
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153 Looking at the schematics and the boards, I did find several failure points:
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154
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155 - one of the 2 150µH inductors on the 160VDC rails (L6 anf L7 on A7A3) ) looked
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156 a bit odd to me: it was not sitting flat on the PCB, which it should for this
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157 kind of inductor; and when I gently tried to wiggle it, it appeared a leg was
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158 broken, unfortunately too short to be able to solder a piece of wire, so I
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159 replaced it with a brand new inductor I recently bought when I repaired an
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160 old analog scope (Metrix OX734) for a friend of mine (I allways buy several
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161 more parts than I really need, when these are cheap, just in case; seems to
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162 be a good idea),
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163
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164 .. image:: {filename}images/hp8662a/broken_choke.jpg
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165 :class: image-process-large-photo
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166
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167 .. image:: {filename}images/hp8662a/replacement_choke.jpg
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168 :class: image-process-large-photo
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169
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170 - there are 2 small 400mA fuses on the inverter board to protect the base of
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171 the 2 switching (bipolar!) transistors (first time I see the base of an NPN
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172 transistor protected by a fuse, but it's also the first time I repair a
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173 switching PSU which the main switching transistors are bipolar ones and not
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174 MOSFET or so). These are small BUSS GMW 4/10 (400mA) fuses, they look nice
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175 and so but are very expensive (and a bit hard to find): something like 15€
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176 each. So for now, I've replaced it with a simple 5x20 glass fuse: not as
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177 skookum but does the job for a few cents.
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178
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179 .. image:: {filename}images/hp8662a/fuse_replacement.jpeg
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180 :class: image-process-large-photo
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181
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182
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183 After these fixes, I reassembled the PSU and tried again: the result was quite
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184 promising, but not a complete success: the PSU seemed to start, but very
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185 quickly stops itself with an error LED on: the overvoltage protection was
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186 shuting down the power.
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187
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188 I tried to check several things on the error detection circuits with no succes,
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189 and when (at last) I began to follow the service manual instructions, I found
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190 this paragraph about this overvoltage error:
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191
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192 If red LED in the upper left hand corner of the board is lit indicating the
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193 input voltagr from the A7A3 Inverter assembly was too high and the supply was
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194 shut down, it is most likely a problem with the regulator not drawing enough
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195 current.
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196
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197 Just had to plug back enough boards and modules for the PSU to be happy again!

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