diff -r 6d0820cef446 -r fb878b773377 content/dell2407.rst --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/content/dell2407.rst Sat Oct 19 00:32:18 2019 +0200 @@ -0,0 +1,209 @@ +====================== + Dell 2407WFP Monitor +====================== + +:author: David Douard +:Category: Electronics +:Tags: LCD, Display, PSU, Dell, 2407WFP + +A had a non-working Dell 2407WFP LCD Monitor in my stash for quite a long time. +I recently decided to see if I can fix it. It is a quite old but pretty nice +24" LCD monitor, with a 1920x1200 resolution. + +The symptom was the nice kind: dead. No light, no LED, no power consumption. + +Presentation +============ + +The enclosure is reasonably easy to disassemble: 4 screws on the back, then the +usual plastic clips to unclipse all around the enclose. The result is a very +standard view for these LCD monitors: a big metallic enclosure: + +.. image:: {static}images/dell2407wfp/back.jpg + :alt: Back of the Dell 2407WFP + + +When removed, the also very classic view on the boards: + +.. image:: {static}images/dell2407wfp/back-pcbs.jpg + :alt: PCBs of the Dell 2407WFP + +Since there is nothing at all, the first and most probable place to start +looking is the PSU: + +.. image:: {static}images/dell2407wfp/psu.jpg + :alt: PSU of the Dell 2407WFP + + +Diagnostic +========== + +Inspecting the PSU, the first culprit (and the probable origin of the failure) +is pretty obvious: + +.. image:: {static}images/dell2407wfp/badcap.jpg + :alt: Bad cap on the Dell 2407WFP + +This one is dead, right, but the question is if it died alone (very unlikely) +or if it took a series of other parts with it. + +Using the multimeter in diode mode, I immediately found a dead fuse (good sign, +might have done its job and protect some other parts on the PSU... or not), but +also a dead MOSFET (Q651, the one on the PFC stage) and maybe more: + +.. image:: {static}images/dell2407wfp/dead-parts.jpg + :alt: First batch of dead parts on the Dell 2407WFP PSU + +At this point, finding datasheets for the controller chips on this board looked +like a good idea. Most of the time, these PSU designs are very close to the +application circuit described in the datasheets. + + +The schematic +------------- + +There 2 controller chips on this PSU, the classic pair with a primary PFC +controller (L6561D) in charge of providing a nice B+ DC rail for the PWM +controller (NCP1200A) driving the main transformer. + +I could easily find datasheets for these 2 chips. But the noce thing when +fixing such a mass produced device is that obviously some other people already +had a similar failure to fix. + +It seems that the problem my monitor suffered is a very common one. I found a +`long thread `_ +(obviously on https://www,badcaos.net) where people discuss this flaw and their +quest for fixes. Which led me to a PDF with the full schematic of the PSU +(which is in fact a BenQ brand). + +The first stage of the PSU, around the L6561D controller looks like: + +.. image:: {static}images/dell2407wfp/psu-schematic-p1.svg + :alt: Schematic of the first (PFC) stage of the PSU of the Dell 2407WFP + +And the regulation stage: + +.. image:: {static}images/dell2407wfp/psu-schematic-p2.svg + :alt: Schematic of the second (PWM + secondary) stage of the PSU of the Dell 2407WFP + + +For the record, here are the typical application schematics from datasheets: + +.. image:: {static}images/dell2407wfp/L6561-AN.svg + :alt: Typical application schematic of the L6561 PFC controller. + +As tou can see, it is very similar to the first page of the schematic above. +This later have the extra power button management circuit, and a few details +and compenent values are different, but overall, a very close application of +the typical application schematic (as expected). + +And for the PMW stage, I could not find such a typical application circuit more +advanced than this very basic one (from the datasheet): + +.. image:: {static}images/dell2407wfp/NCP1200A.svg + :alt: Typical application schematic of the NCP1200A PWM controller. + +However some interesting details and circuit improvements can be found in this +`application note `_. + + + +Dead parts +---------- + +Having Q651 dead, I started looking for other victims of the deadly C605 +electrolytic capacitor. I quickly found a few more parts, especially R670 +(0.22R 3W). From there, I bought a bunch of parts, obviously the one already +mentioned plus a few more references that were listed by a few people on the +thread list on badcaps (15v zener diodes, 4148 (SMD), and PMBT2907 (PNP) +transistor for Q602). I also ordered a few replacement parts for IC601 and +IC651 since they had also probably suffered from the failure. + + +I first replaced the obviously failed components (Q651, C605, R670 and the +fuse). I probed all the transistors and diodes I could. Even the controler +chips did not seem obviously wrong (no short legs). + +I applied gradually power (via an autotransformer) watching at power +consumption. Nothing wrong, no magic smoke. And even 19v on the output rail! + +More dead parts +--------------- + +So I attached a bunch of power resistors to the 19V output rail, which made the +PCB rapidly start to smell and smoke. + +Something was wrong, not catastrophic (no big explosion nor massive smoke, but +still). + +I finally found the smoke was generated by... the earth wire which was touching +the input NTC! This later was getting very hot. + +So, it looks like it worked fine as long as I did not load the output. +Interesting. From there, the MOSFET transistor driving the PFC stage (Q651) +was dead again. + +So I decided to replace IC651 (the PFC controller, L6561D). And since I had not +that much spare STP10NK60ZFP transistor for Q651, I decided to try first with +some junk parts from a PCB laying around, on which are many APT8075BN: + +.. image:: {static}images/dell2407wfp/junk_pcb.jpg + :alt: The PCB from which I took an APT8075BN as test Q651 replacement. + +The leads qre not exactly the same spacing, but I managed to do something: + +.. image:: {static}images/dell2407wfp/apt8075bn.jpg + :alt: The APT8075BN used as test Q651 replacement. + + +Which resulted in a rather acrobatic setup: I did not want to fight to make it +fit nicely enough that I can screw it on the heatsink (plus it is not +insulated, like the original transistor, so I would have had to find an +insulator. I have some, but finding them would have required a bit fo digging +in my overcrowded empire of dirt). + +The resulting setup looks like: + +.. image:: {static}images/dell2407wfp/psu-repaired-1.jpg + :alt: The APT8075BN used as test Q651 replacement. + +However, before applying power again, I checked all the silicon components once +more. And I discovered (what I missed the first time) that Q653 was also dead. + +The problem was I had no proper SOT23 replacement part for it. THe good news is +I do have a few 2N7000 that can be used, but are TO92 packages. +So I did a bit of surgery: + +.. image:: {static}images/dell2407wfp/hack.jpg + :alt: The APT8075BN used as test Q651 replacement. + + +Fixed +----- + +Applying power again, the 19V is back when no load is applied to the 19V rail. +And stays alive when I draw current (1A). + +I plugged it back in the monitor, the green LED lit as well as the display +itself showing the 'No signal detected' message box! + +.. image:: {static}images/dell2407wfp/alive-1.jpg + :alt: It's alive! + +I replaced the temporary Q651 transistor with the proper reference, checked the +result, then put everything back together. + +.. image:: {static}images/dell2407wfp/alive.jpg + :alt: It's alive! + + +However, before closing the box, I did a few tests and measurements, for the +fun. I found the reason for the properly working 19v rail when unloaded at the +beginning: in fact, the L6561D PFC chip is only turned on when some load is +detected by the NCP1200A controller. This allows the standby power consumption +of the monitor to be very low. I believe this is done via the Vcc1 signal that +goes from the second stage (PWM controller) to the power on circuitry on the +PCF stage (Q654, R678, R656 and the transisor part of the IC652 optocoupler), +which only applies power to IC651 (the L6561D PFC controller) when needed. + +A nice monitor. Definitly not for my desk ;-)