content/dell2407.rst

Wed, 13 Mar 2024 11:03:13 +0100

author
David Douard <david.douard@sdf3.org>
date
Wed, 13 Mar 2024 11:03:13 +0100
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Update URLs for pcb & firmware to sourcehut

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1 ======================
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2 Dell 2407WFP Monitor
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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: LCD, Display, PSU, Dell, 2407WFP
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8
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9 A had a non-working Dell 2407WFP LCD Monitor in my stash for quite a long time.
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10 I recently decided to see if I can fix it. It is a quite old but pretty nice
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11 24" LCD monitor, with a 1920x1200 resolution.
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13 The symptom was the nice kind: dead. No light, no LED, no power consumption.
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14
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15 Presentation
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16 ============
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17
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18 The enclosure is reasonably easy to disassemble: 4 screws on the back, then the
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19 usual plastic clips to unclipse all around the enclose. The result is a very
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20 standard view for these LCD monitors: a big metallic enclosure:
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22 .. image:: {static}/images/dell2407wfp/back.jpg
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23 :alt: Back of the Dell 2407WFP
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25
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26 When removed, the also very classic view on the boards:
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28 .. image:: {static}/images/dell2407wfp/back-pcbs.jpg
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29 :alt: PCBs of the Dell 2407WFP
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31 Since there is nothing at all, the first and most probable place to start
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32 looking is the PSU:
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34 .. image:: {static}/images/dell2407wfp/psu.jpg
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35 :alt: PSU of the Dell 2407WFP
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38 Diagnostic
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39 ==========
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41 Inspecting the PSU, the first culprit (and the probable origin of the failure)
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42 is pretty obvious:
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44 .. image:: {static}/images/dell2407wfp/badcap.jpg
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45 :alt: Bad cap on the Dell 2407WFP
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47 This one is dead, right, but the question is if it died alone (very unlikely)
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48 or if it took a series of other parts with it.
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49
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50 Using the multimeter in diode mode, I immediately found a dead fuse (good sign,
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51 might have done its job and protect some other parts on the PSU... or not), but
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52 also a dead MOSFET (Q651, the one on the PFC stage) and maybe more:
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54 .. image:: {static}/images/dell2407wfp/dead-parts.jpg
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55 :alt: First batch of dead parts on the Dell 2407WFP PSU
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57 At this point, finding datasheets for the controller chips on this board looked
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58 like a good idea. Most of the time, these PSU designs are very close to the
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59 application circuit described in the datasheets.
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62 The schematic
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63 -------------
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65 There 2 controller chips on this PSU, the classic pair with a primary PFC
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66 controller (L6561D) in charge of providing a nice B+ DC rail for the PWM
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67 controller (NCP1200A) driving the main transformer.
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69 I could easily find datasheets for these 2 chips. But the nice thing when
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70 fixing such a mass produced device is that obviously some other people already
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71 had a similar failure to fix.
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73 It seems that the problem my monitor suffered is a very common one. I found a
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74 `long thread <https://www.badcaps.net/forum/showthread.php?t=14882&page=1>`_
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75 (obviously on https://www.badcaps.net) where people discuss this flaw and their
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76 quest for fixes. Which led me to a PDF with the full schematic of the PSU
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77 (which is in fact a BenQ brand).
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78
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79 The first stage of the PSU, around the L6561D controller looks like:
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81 .. image:: {static}/images/dell2407wfp/psu-schematic-p1.svg
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82 :alt: Schematic of the first (PFC) stage of the PSU of the Dell 2407WFP
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83
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84 And the regulation stage:
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86 .. image:: {static}/images/dell2407wfp/psu-schematic-p2.svg
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87 :alt: Schematic of the second (PWM + secondary) stage of the PSU of the Dell 2407WFP
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90 For the record, here are the typical application schematics from datasheets:
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92 .. image:: {static}/images/dell2407wfp/L6561-AN.svg
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93 :alt: Typical application schematic of the L6561 PFC controller.
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95 As you can see, it is very similar to the first page of the schematic above.
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96 This later has the extra power button management circuit, and a few details
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97 and component values are different, but overall, a very close application of
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98 the typical application schematic (as expected).
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99
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100 And for the PMW stage, I could not find such a typical application circuit more
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101 advanced than this very basic one (from the datasheet):
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103 .. image:: {static}/images/dell2407wfp/NCP1200A.svg
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104 :alt: Typical application schematic of the NCP1200A PWM controller.
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105
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106 However some interesting details and circuit improvements can be found in this
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107 `application note <https://www.onsemi.com/pub/Collateral/AND8069-D.PDF>`_.
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111 Dead parts
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112 ----------
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113
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114 Having Q651 dead, I started looking for other victims of the deadly C605
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115 electrolytic capacitor. I quickly found a few more parts, especially R670
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116 (0.22R 3W). From there, I bought a bunch of parts, obviously the one already
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117 mentioned plus a few more references that were listed by a few people on the
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118 thread list on badcaps (15v zener diodes, 4148 (SMD), and PMBT2907 (PNP)
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119 transistor for Q602). I also ordered a few replacement parts for IC601 and
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120 IC651 since they had also probably suffered from the failure.
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123 I first replaced the obviously failed components (Q651, C605, R670 and the
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124 fuse). I probed all the transistors and diodes I could. Even the controler
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125 chips did not seem obviously wrong (no short legs).
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126
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127 I applied gradually power (via an autotransformer) watching at power
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128 consumption. Nothing wrong, no magic smoke. And even 19v on the output rail!
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129
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130 More dead parts
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131 ---------------
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133 So I attached a bunch of power resistors to the 19V output rail, which made the
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134 PCB rapidly start to smell and smoke.
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135
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136 Something was wrong, not catastrophic (no big explosion nor massive smoke, but
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137 still).
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138
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139 I finally found the smoke was generated by... the earth wire which was touching
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140 the input NTC! This later was getting very hot.
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141
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142 So, it looks like it worked fine as long as I did not load the output.
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143 Interesting. From there, the MOSFET transistor driving the PFC stage (Q651)
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144 was dead again.
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145
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146 So I decided to replace IC651 (the PFC controller, L6561D). And since I had not
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147 that much spare STP10NK60ZFP transistor for Q651, I decided to try first with
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148 some junk parts from a PCB laying around, on which are many APT8075BN:
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149
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150 .. image:: {static}/images/dell2407wfp/junk_pcb.jpg
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151 :alt: The PCB from which I took an APT8075BN as test Q651 replacement.
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152
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153 The leads are not exactly the same spacing, but I managed to do something:
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154
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155 .. image:: {static}/images/dell2407wfp/apt8075bn.jpg
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156 :alt: The APT8075BN used as test Q651 replacement.
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157
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158
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159 Which resulted in a rather acrobatic setup: I did not want to fight to make it
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160 fit nicely enough that I can screw it on the heatsink (plus it is not
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161 insulated, like the original transistor, so I would have had to find an
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162 insulator. I have some, but finding them would have required a bit of digging
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163 in my overcrowded empire of dirt).
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164
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165 The resulting setup looks like:
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166
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167 .. image:: {static}/images/dell2407wfp/psu-repaired-1.jpg
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168 :alt: The APT8075BN used as test Q651 replacement.
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169
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170 However, before applying power again, I checked all the silicon components once
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171 more. And I discovered (what I missed the first time) that Q653 was also dead.
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172
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173 The problem was I had no proper SOT23 replacement part for it. The good news is
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174 I do have a few 2N7000 that can be used, but are TO92 packages.
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175 So I did a bit of surgery:
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176
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177 .. image:: {static}/images/dell2407wfp/hack.jpg
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178 :alt: The APT8075BN used as test Q651 replacement.
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179
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180
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181 Fixed
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182 -----
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183
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184 Applying power again, the 19V is back when no load is applied to the 19V rail.
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185 And stays alive when I draw current (1A).
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186
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187 I plugged it back in the monitor, the green LED lit as well as the display
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188 itself showing the 'No signal detected' message box!
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189
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190 .. image:: {static}/images/dell2407wfp/alive-1.jpg
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191 :alt: It's alive!
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192
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193 I replaced the temporary Q651 transistor with the proper reference, checked the
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194 result, then put everything back together.
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195
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196 .. image:: {static}/images/dell2407wfp/alive.jpg
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197 :alt: It's alive!
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198
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199
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200 However, before closing the box, I did a few tests and measurements, for
118
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201 fun. I found the reason for the properly working 19v rail when unloaded at the
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202 beginning: in fact, the L6561D PFC chip is only turned on when some load is
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203 detected by the NCP1200A controller. This allows the standby power consumption
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204 of the monitor to be very low. I believe this is done via the Vcc1 signal that
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205 goes from the second stage (PWM controller) to the power on circuitry on the
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206 PCF stage (Q654, R678, R656 and the transisor part of the IC652 optocoupler),
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207 which only applies power to IC651 (the L6561D PFC controller) when needed.
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208
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209 A nice monitor. Definitly not for my desk ;-)

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