content/HPZR24W.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
changeset 104
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parent 72
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permissions
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improve hgignore

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1 =====================
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2 HP ZR24W PSU Repair
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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: ZR24W, PSU, PWB-1336-02, TPS-10036
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8
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9 A friend of mine had a problem with his (a bit old) 24" HP monitor: the device
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10 was working properly, but only for a while (few minutes at most).
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11
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12 Looked like a good candidate for a quick repair, problem being most probably
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13 bad caps are so.
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15 The first step was to find out how to disassemble the down thing, since these
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16 "modern" things are not designed to be serviced and have no screw, only plastic
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17 clips which are so easy to break.
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18
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19 Whatever, I finally succeded in opening the box (with only one or 2 broken
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20 clips).
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21
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22 Overview
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23 ========
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24
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25 Inside the beast, the usual stuff under the shields:
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26
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27 - a PSU,
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28 - a main controller board,
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29 - an LCD driver board, and the monitor being a bit old,
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30 - a blacklight HV driver board.
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31
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32
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33 .. image:: {filename}images/ZR24W/back.jpg
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34 :alt: Back of the HP ZR24W
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35
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36 First thing, to make sure the problem comes from the PSU, I powered the monitor
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37 from my bench power supplies. I used one output of my HP E3648A to generate the
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38 19V rail that powers the backlight. It was a bit short (CC limited at 16V), but
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39 enough to power the backlight. The second output was used to produce the 12V,
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40 and the 5V was generated by my very old home made PSU.
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41
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42 .. image:: {filename}images/ZR24W/ext_power.jpg
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43 :alt: Testing the display with my bench PSU.
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44
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45 The monitor was working fine when powered from these PSUs, so the problem was,
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46 indeed, this Tatung PWB-1336-02 switching PSU:
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47
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48 .. image:: {filename}images/ZR24W/psu_top.jpg
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49 :alt: Top of the Tatung PWB-1336-02 PSU
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50
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51 .. image:: {filename}images/ZR24W/psu_bottom.jpg
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52 :alt: Bottom view of the Tatung PWB-1336-02 PSU
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53
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54 The design is nice and clean. Electrolytic caps however are not the best ones
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55 (mostly Lelong ones I think), which is not really a surprise in this kind of
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56 device.
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57
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58 At first sight, nothing strike the eye, no leaky cap, no burnt resistor or
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59 PCB...
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60
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61 Not a 10mn fix, in the end.
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62
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63 The PSU generates 3 voltages:
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64
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65 - 5V @2.7A
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66 - 12V @0.8A
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67 - 19V @3.2A
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68
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69
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70 Finding the problematic power rail
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71 ==================================
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72
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73 There is a hard switch on the PSU, next to the IEC socket, and When the power
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74 is on, the 5V is hot, wether the monitor is on or in standby.
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75
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76 When the display is powered on, the 2 other voltages are started.
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77
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78 The pin 10 on the cable between the PSU and the main CPU board is dedicated
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79 to the "power saving" state. It must be high (at 5V, which is always present)
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80 to enable the 2 other voltages.
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81
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82 Note that there are 2 other pins dedicated to power management (pin 11 and 12,
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83 marked as "On/Off" and "Vadj"). But these are directly routed to the backlight
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84 board and take no part in the PSU management.
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85
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86
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87 My first test has been to plug my cheap `electronic load
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88 <{filename}/ZPB30A1.rst>`_ on the 5V with the 2 other voltages stopped.
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89
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90 And I could reliably sink 3A from there. So the problem must be on one of the 2
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91 other rails.
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92
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93 But I also could sink the max amperage from the 2 other power rails (1A from
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94 the 12V and 3.5A from the 19V)...
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95
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96 Ok, so each power rail seems to work fine alone.
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97
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98 But when I sink current from the 5V rail while the 2 other voltages are up,
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99 then the PSU fails after a short while.
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100
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101 I've tried to probe a bit the switching curves using my Rigol DS1054, but the
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102 PSU being "hot" (around 400V), and having no isolation transformer, I could not
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103 probe the signals correctly (using 2 probe and displaying the A-B curve, which
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104 is some kind of a joke on the Rigol, since you cannot hide the A and B curves:
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105 the curve substraction is purely computed from displayed curve. When I found
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106 how useless this was on the Rigol, I had not enough energy to extract my old
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107 `Tek 2445A <{filename}/TeK2445.rst>`_ from under the pile of test equipment
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108 stowed in the closet...)
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109
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110
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111 A glimpse at schematics
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112 -----------------------
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113
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114 At this point, I needed to try to understand a bit the schematic. I wasn't even
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115 sure the problem came from this side of the PSU, it could also be a problem in
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116 the "isolated" part (bad caps, bad optocoupler, bad voltage reference...). I
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117 had checked the main caps, and they seemed to be ok (not quality japanese
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118 brands, but still the correct value and low ESR).
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119
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120
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121 On the live part, before the transformers, there are 2 chips :
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122
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123 - a TNY279PN_ dedicated to the 5V rail
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124 - a CM6807_ for the 19V rail
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125
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126 The 12V is produced from the 19V rail by a small DC to DC converter (FP6185_).
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127
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128 In fact, this PSU design is almost just the 2 application circuits (found in
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129 their respective datasheets) merged together.
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130
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131 The noticable points are:
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132
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133 - The TNY279PN DC input can be come from either the input bridge rectifiers
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134 (via a diode and a thermistor), or, when powered, by the DC produced by the
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135 CM6807 via the PFC circuit. So when the CM6807 is down (when the monitor is
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136 in standby), the DC input for the TNY279 is around 318V, but when the monitor
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137 is on, the CM6807's PFC circuit rise this voltage to almost 400V.
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138
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139 - The bias winding, normally used to power the TNY279 (to allow a very low
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140 no-load power consumption, datasheet says <50mw) also powers the CM6807.
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141
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142 A funny side effect of this design is that the CM6807 cannot be started if
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143 there is no load on the 5V rail: in this case, the auxiliary voltage of the
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144 transformer managed by the TNY279 is not enough to start the CM6807.
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145
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146
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147 The application circuits are as follow:
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148
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149 .. image:: {filename}images/ZR24W/tny279_app_circuit.svg
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150 :alt: Example application circuit for the TNY279PN controller (from the `datasheet <TNY279PN>`_)
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151
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152 .. image:: {filename}images/ZR24W/CM6807_app_circuit.svg
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153 :alt: Example application circuit for the CM6807 controller (from the `datasheet <CM6807>`_)
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154
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155 In this PSU, the input DC of the TNY279 (the point just before R5 in the app
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156 circuit) in connected just behind the PFC circuit, i.e. just after the D2 diode
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157 of the CM6807 app circuit.
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158
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159 In order for the PSU to generate the 5V rails even when the CM6807 is off,
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160 there is derivated DC input path, from the bridge rectifier to the input DC
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161 rail, consisting in a diode (D922 on the PCB) followed by a thermistor (R915):
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162
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163 .. image:: {filename}images/ZR24W/input_DC.jpeg
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164 :alt: Input DC showing both paths (yellow: via the PFC, red: the alternate
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165 path when power saving is on).
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166
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167
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168 Finally, the culprit
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169 --------------------
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170
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171 While testing the PSU powering the CM6807 from an external source, I notices
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172 another strange behaviour: the TNY279 enters a failure as soon as I sink
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173 current from the 5V rail, but it remains faulty as long as I let the CM6807,
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174 thus the PFC running, and the input DC at 400V (instead of 318V when the PFC is
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175 not activated)..
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176
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177 At this point, I though there is not way the problem can come from the 5V
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178 regulation loop, neither from the CM6807 circuit, so the only culprit left
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179 would be either the TNY279PN or a component close to it, a cap (especially the
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180 BP/M one), a diode or a resistor. So I tested these parts (again, to be fair),
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181 and I replaced the cap connected to the BP/M pin. With no improvement, the only
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182 remaining faulty part must be the TNY279PN.
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183
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184 So I decided to buy a couple of them from RS, and 2 days later I replaced it,
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185 which did fix the PSU.
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186
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187 What a tricky half-failing part!
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188
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189
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190
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191
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192 .. _TNY279PN:
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193 https://www.power.com/sites/default/files/product-docs/tny274-280.pdf
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194 .. _CM6807: http://www.championmicro.com.tw/datasheet/Analog%20Device/CM6807.pdf
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195 .. _FP6185: http://www.feeling-tech.com.tw/km-master/ezcatfiles/cust/img/img/24/fp6185drv02-g1.pdf

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