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| 1 ====================== | |
| 2 Dell 2407WFP Monitor | |
| 3 ====================== | |
| 4 | |
| 5 :author: David Douard | |
| 6 :Category: Electronics | |
| 7 :Tags: LCD, Display, PSU, Dell, 2407WFP | |
| 8 | |
| 9 A had a non-working Dell 2407WFP LCD Monitor in my stash for quite a long time. | |
| 10 I recently decided to see if I can fix it. It is a quite old but pretty nice | |
| 11 24" LCD monitor, with a 1920x1200 resolution. | |
| 12 | |
| 13 The symptom was the nice kind: dead. No light, no LED, no power consumption. | |
| 14 | |
| 15 Presentation | |
| 16 ============ | |
| 17 | |
| 18 The enclosure is reasonably easy to disassemble: 4 screws on the back, then the | |
| 19 usual plastic clips to unclipse all around the enclose. The result is a very | |
| 20 standard view for these LCD monitors: a big metallic enclosure: | |
| 21 | |
| 22 .. image:: {static}images/dell2407wfp/back.jpg | |
| 23 :alt: Back of the Dell 2407WFP | |
| 24 | |
| 25 | |
| 26 When removed, the also very classic view on the boards: | |
| 27 | |
| 28 .. image:: {static}images/dell2407wfp/back-pcbs.jpg | |
| 29 :alt: PCBs of the Dell 2407WFP | |
| 30 | |
| 31 Since there is nothing at all, the first and most probable place to start | |
| 32 looking is the PSU: | |
| 33 | |
| 34 .. image:: {static}images/dell2407wfp/psu.jpg | |
| 35 :alt: PSU of the Dell 2407WFP | |
| 36 | |
| 37 | |
| 38 Diagnostic | |
| 39 ========== | |
| 40 | |
| 41 Inspecting the PSU, the first culprit (and the probable origin of the failure) | |
| 42 is pretty obvious: | |
| 43 | |
| 44 .. image:: {static}images/dell2407wfp/badcap.jpg | |
| 45 :alt: Bad cap on the Dell 2407WFP | |
| 46 | |
| 47 This one is dead, right, but the question is if it died alone (very unlikely) | |
| 48 or if it took a series of other parts with it. | |
| 49 | |
| 50 Using the multimeter in diode mode, I immediately found a dead fuse (good sign, | |
| 51 might have done its job and protect some other parts on the PSU... or not), but | |
| 52 also a dead MOSFET (Q651, the one on the PFC stage) and maybe more: | |
| 53 | |
| 54 .. image:: {static}images/dell2407wfp/dead-parts.jpg | |
| 55 :alt: First batch of dead parts on the Dell 2407WFP PSU | |
| 56 | |
| 57 At this point, finding datasheets for the controller chips on this board looked | |
| 58 like a good idea. Most of the time, these PSU designs are very close to the | |
| 59 application circuit described in the datasheets. | |
| 60 | |
| 61 | |
| 62 The schematic | |
| 63 ------------- | |
| 64 | |
| 65 There 2 controller chips on this PSU, the classic pair with a primary PFC | |
| 66 controller (L6561D) in charge of providing a nice B+ DC rail for the PWM | |
| 67 controller (NCP1200A) driving the main transformer. | |
| 68 | |
| 69 I could easily find datasheets for these 2 chips. But the noce thing when | |
| 70 fixing such a mass produced device is that obviously some other people already | |
| 71 had a similar failure to fix. | |
| 72 | |
| 73 It seems that the problem my monitor suffered is a very common one. I found a | |
| 74 `long thread <https://www.badcaps.net/forum/showthread.php?t=14882&page=1>`_ | |
| 75 (obviously on https://www,badcaos.net) where people discuss this flaw and their | |
| 76 quest for fixes. Which led me to a PDF with the full schematic of the PSU | |
| 77 (which is in fact a BenQ brand). | |
| 78 | |
| 79 The first stage of the PSU, around the L6561D controller looks like: | |
| 80 | |
| 81 .. image:: {static}images/dell2407wfp/psu-schematic-p1.svg | |
| 82 :alt: Schematic of the first (PFC) stage of the PSU of the Dell 2407WFP | |
| 83 | |
| 84 And the regulation stage: | |
| 85 | |
| 86 .. image:: {static}images/dell2407wfp/psu-schematic-p2.svg | |
| 87 :alt: Schematic of the second (PWM + secondary) stage of the PSU of the Dell 2407WFP | |
| 88 | |
| 89 | |
| 90 For the record, here are the typical application schematics from datasheets: | |
| 91 | |
| 92 .. image:: {static}images/dell2407wfp/L6561-AN.svg | |
| 93 :alt: Typical application schematic of the L6561 PFC controller. | |
| 94 | |
| 95 As tou can see, it is very similar to the first page of the schematic above. | |
| 96 This later have the extra power button management circuit, and a few details | |
| 97 and compenent values are different, but overall, a very close application of | |
| 98 the typical application schematic (as expected). | |
| 99 | |
| 100 And for the PMW stage, I could not find such a typical application circuit more | |
| 101 advanced than this very basic one (from the datasheet): | |
| 102 | |
| 103 .. image:: {static}images/dell2407wfp/NCP1200A.svg | |
| 104 :alt: Typical application schematic of the NCP1200A PWM controller. | |
| 105 | |
| 106 However some interesting details and circuit improvements can be found in this | |
| 107 `application note <https://www.onsemi.com/pub/Collateral/AND8069-D.PDF>`_. | |
| 108 | |
| 109 | |
| 110 | |
| 111 Dead parts | |
| 112 ---------- | |
| 113 | |
| 114 Having Q651 dead, I started looking for other victims of the deadly C605 | |
| 115 electrolytic capacitor. I quickly found a few more parts, especially R670 | |
| 116 (0.22R 3W). From there, I bought a bunch of parts, obviously the one already | |
| 117 mentioned plus a few more references that were listed by a few people on the | |
| 118 thread list on badcaps (15v zener diodes, 4148 (SMD), and PMBT2907 (PNP) | |
| 119 transistor for Q602). I also ordered a few replacement parts for IC601 and | |
| 120 IC651 since they had also probably suffered from the failure. | |
| 121 | |
| 122 | |
| 123 I first replaced the obviously failed components (Q651, C605, R670 and the | |
| 124 fuse). I probed all the transistors and diodes I could. Even the controler | |
| 125 chips did not seem obviously wrong (no short legs). | |
| 126 | |
| 127 I applied gradually power (via an autotransformer) watching at power | |
| 128 consumption. Nothing wrong, no magic smoke. And even 19v on the output rail! | |
| 129 | |
| 130 More dead parts | |
| 131 --------------- | |
| 132 | |
| 133 So I attached a bunch of power resistors to the 19V output rail, which made the | |
| 134 PCB rapidly start to smell and smoke. | |
| 135 | |
| 136 Something was wrong, not catastrophic (no big explosion nor massive smoke, but | |
| 137 still). | |
| 138 | |
| 139 I finally found the smoke was generated by... the earth wire which was touching | |
| 140 the input NTC! This later was getting very hot. | |
| 141 | |
| 142 So, it looks like it worked fine as long as I did not load the output. | |
| 143 Interesting. From there, the MOSFET transistor driving the PFC stage (Q651) | |
| 144 was dead again. | |
| 145 | |
| 146 So I decided to replace IC651 (the PFC controller, L6561D). And since I had not | |
| 147 that much spare STP10NK60ZFP transistor for Q651, I decided to try first with | |
| 148 some junk parts from a PCB laying around, on which are many APT8075BN: | |
| 149 | |
| 150 .. image:: {static}images/dell2407wfp/junk_pcb.jpg | |
| 151 :alt: The PCB from which I took an APT8075BN as test Q651 replacement. | |
| 152 | |
| 153 The leads qre not exactly the same spacing, but I managed to do something: | |
| 154 | |
| 155 .. image:: {static}images/dell2407wfp/apt8075bn.jpg | |
| 156 :alt: The APT8075BN used as test Q651 replacement. | |
| 157 | |
| 158 | |
| 159 Which resulted in a rather acrobatic setup: I did not want to fight to make it | |
| 160 fit nicely enough that I can screw it on the heatsink (plus it is not | |
| 161 insulated, like the original transistor, so I would have had to find an | |
| 162 insulator. I have some, but finding them would have required a bit fo digging | |
| 163 in my overcrowded empire of dirt). | |
| 164 | |
| 165 The resulting setup looks like: | |
| 166 | |
| 167 .. image:: {static}images/dell2407wfp/psu-repaired-1.jpg | |
| 168 :alt: The APT8075BN used as test Q651 replacement. | |
| 169 | |
| 170 However, before applying power again, I checked all the silicon components once | |
| 171 more. And I discovered (what I missed the first time) that Q653 was also dead. | |
| 172 | |
| 173 The problem was I had no proper SOT23 replacement part for it. THe good news is | |
| 174 I do have a few 2N7000 that can be used, but are TO92 packages. | |
| 175 So I did a bit of surgery: | |
| 176 | |
| 177 .. image:: {static}images/dell2407wfp/hack.jpg | |
| 178 :alt: The APT8075BN used as test Q651 replacement. | |
| 179 | |
| 180 | |
| 181 Fixed | |
| 182 ----- | |
| 183 | |
| 184 Applying power again, the 19V is back when no load is applied to the 19V rail. | |
| 185 And stays alive when I draw current (1A). | |
| 186 | |
| 187 I plugged it back in the monitor, the green LED lit as well as the display | |
| 188 itself showing the 'No signal detected' message box! | |
| 189 | |
| 190 .. image:: {static}images/dell2407wfp/alive-1.jpg | |
| 191 :alt: It's alive! | |
| 192 | |
| 193 I replaced the temporary Q651 transistor with the proper reference, checked the | |
| 194 result, then put everything back together. | |
| 195 | |
| 196 .. image:: {static}images/dell2407wfp/alive.jpg | |
| 197 :alt: It's alive! | |
| 198 | |
| 199 | |
| 200 However, before closing the box, I did a few tests and measurements, for the | |
| 201 fun. I found the reason for the properly working 19v rail when unloaded at the | |
| 202 beginning: in fact, the L6561D PFC chip is only turned on when some load is | |
| 203 detected by the NCP1200A controller. This allows the standby power consumption | |
| 204 of the monitor to be very low. I believe this is done via the Vcc1 signal that | |
| 205 goes from the second stage (PWM controller) to the power on circuitry on the | |
| 206 PCF stage (Q654, R678, R656 and the transisor part of the IC652 optocoupler), | |
| 207 which only applies power to IC651 (the L6561D PFC controller) when needed. | |
| 208 | |
| 209 A nice monitor. Definitly not for my desk ;-) |
