Power Supply PWB-0561-04 repair (for monitor).

[JonathanAnon]

Fixing another power supply in a monitor after my previous success..

Problem: You press the on button and nothing at all happens, no backlight and no light on the power LED.

So far: Tested the Primary section of the power supply. The main filter cap is reading a stable 336V, BUT the input to the PWM IC reads a wildly varying voltage between 11 and 14V... The capacitor beside the PWM IC also varies wildly between 12 and 16V... Same for the voltages across the Zener diodes, for example zener diode D802 voltage reads between 0 and 6V (sometimes even going negative).

The passive components seem to check out okay.. Still waiting on my new multimeter to be able to check out the MOSFET. Am I right in thinking that the varying voltage is more lightly to be caused by one of the active components malfunctioning?

http://omg.wthax.org/PWB_0560.jpg

(I'll link to the image cos it's very big).

[Mad_Professor]

Re: Power Supply PWB-0561-04 repair (for monitor).

What was the voltage output when connected to signal board or inverter and again disconnected from signal board and inverter?

[budm]

Re: Power Supply PWB-0561-04 repair (for monitor).

It can be the startup/running cap C807 that is used for running the SMPS IC in the primay section, it will be something like 22~56uF 50V cap connected to the VCC pin of the SMPS, or it can be in startup/shutdown/startup loop due to short or overloading of the secondary side circuits. I would try the new startup/running cap first.
What is the make and model of this monitor? Can we also get the whole picture of the board, sections of the board are cutout.
The file is not too big to post.

[JonathanAnon]

Re: Power Supply PWB-0561-04 repair (for monitor).

The monitor is a HP P9621D 17 inch monitor. I've found a link to the schema of the power supply but they want me to pay for it.

What was the voltage output when connected to signal board or inverter and again disconnected from signal board and inverter?

I havent been able to work out the pin outs from the power board to the VGA board, however the two Schottky diodes that rectify in the secondary part of the SMPS measure 7.5 varying (11.58V on working same model) and 3.62 stable (5.23V on working same model).

[budm]

Re: Power Supply PWB-0561-04 repair (for monitor).

Please post full pictures of both sides of the board, main board, inverter board, whole picture of the monitor so we can see how all the boards are connected.
By the way, pin 7 of the SMPS IC SG6841 is the VCC pin. Pin 3 is the startup voltage from the main B+, once the circuits start running it will get the voltage form the aux winding that supplies the B+ to pin 7 of the SMPS IC.

[selldoor]

Re: Power Supply PWB-0561-04 repair (for monitor).

I would think about swapping out all the caps they are going to be well cooked
one looks like it is leaning on the heatsink and the small (probably startup) cap is in a hot spot as well.
PWM controller datasheet attached

[JonathanAnon]

Re: Power Supply PWB-0561-04 repair (for monitor).

The pictures of the backs of the boards came out with camera flash spoiling the image.. I'll retake the images tomorrow..

Here is the from of the power board.

http://omg.wthax.org/powerboard_front.jpg

Video Board

http://omg.wthax.org/videoboard_front.jpg

I've replaced capacitor C807 with a similar cap, but the cap and board behave exactly the same way.

[budm]

Re: Power Supply PWB-0561-04 repair (for monitor).

Please edit the posting images and use attachment instead of posting then in-line, It takes a lot of time to load up the page.
It is a combo power supply/inverter board.

[selldoor]

Re: Power Supply PWB-0561-04 repair (for monitor).

Have you tested fuse or fusible resistor at F851

and fuse F101 on the video board

[JonathanAnon]

Re: Power Supply PWB-0561-04 repair (for monitor).

Both those fuses are good, Selldoor..

By the way, pin 7 of the SMPS IC SG6841 is the VCC pin. Pin 3 is the startup voltage from the main B+, once the circuits start running it will get the voltage form the aux winding that supplies the B+ to pin 7 of the SMPS IC.

Good point Bud, I measured the voltage at pin 7 of this IC and it matches the voltage on capacitor C807 which is 12-16V varying. I would have thought that the start pin (pin 3) would get an initial burst to turn it on and then gone to zero?

I'm just trying to think through the problem here.. The supply is fine up to the filter cap... This has a stable 336V.. The next device connected to the positive of the filter cap is a zener diode D802, which has a varying voltage that seems to go 0v-2v-4v-6v-0v-2v-4v-6v and keeps repeating this cycle. On the working monitor this D802 zener diode has 46v stable voltage across it. This varying voltage behaviour seems to happen across the components in the rest of the primary side and the secondary side. It seems to be consistent with the startup/shutdown that bud mentioned earlier so I'm gonna test the components on the secondary side. (I'm just spitballing here, feel free to contradict me here).

[budm]

Re: Power Supply PWB-0561-04 repair (for monitor).

One way to find out if it is the load problem (short circuits in the inverter board, you can see two 8-pin MOSFET's small sub board in the inverter section, chek them). You may be able to disconnect the load from the power supply tracing out which trace is used to feed the B+ to the inverter circuits.
The kick start votltage will always be there on pin 3, page 2 of the spec sheeto f the IC:
"This pin is pulled high to the rectified line input through a resistor for start-up. Since the
start-up current requirement for SG6841 is very small, a large start-up resistance can be
used to minimize power loss. Under normal operation, this pin is also used to detect line
voltage to compensate for constant output power limit for universal AC input."

[JonathanAnon]

Re: Power Supply PWB-0561-04 repair (for monitor).

Hi Ben, I was gonna suggest this earlier, but how about I disconnect the pico fuse connecting the inverter circuit. I thought that this would be an obvious thing to do, but assumed it must be a stupid idea if it hadnt been suggested already. But, I was reading a book last nite and Jestine Yong did that in one of his LCD repairs.. seemed to work for him.

I know this is similar to what you suggested above, but simply disconnecting the fuse would be easier for me than having to trace back the paths of those tiny MOSFETS.

[JonathanAnon]

Re: Power Supply PWB-0561-04 repair (for monitor).

Picked this power supply up again today and had another look... Quick recap (no pun intended):

The main filter cap was holding a charge of 330V, but not discharging at all when unplugged from the A/C... The voltages across the PWM Vcc pin and startup cap was varying greatly... The voltages across the two secondary Schottky diodes were under spec but stable .. 3V (should be 5V) and 7.8V (should be 12V). I tested every passive component on the primary side and on the secondary side but all seemed fine.

So today, I pulled the PWM IC from another identical working PSU and put it into this PSU, and BAM!!! ... everything magically came right... secondary voltages all moved to spec. The monitor is worth nothing, but the fix is significant progress for me.. Thanks to Bud and Selldoor for helping..