--- /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 <https://www.badcaps.net/forum/showthread.php?t=14882&page=1>`_
+(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 <https://www.onsemi.com/pub/Collateral/AND8069-D.PDF>`_.
+
+
+
+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 ;-)