Proscan 55LC55S240v69

[vinceroger69]

Re: Proscan 55LC55S240v69

by the capacitor you replaced with the burnt leg whats the device connected to it on the heat sink? could this be faulty?

[gsmy]

Re: Proscan 55LC55S240v69

selldoor- in the smallest red circle is a component board number VD804

[gsmy]

Re: Proscan 55LC55S240v69

Went over all the solder joints on the transformer but it didnt change anything. I happened to look at the high voltage cap I originally replaced and compared the value to the other board and noticed it is different. The original cap is a CBB881 1000v 68nj but the other board has a CB881 2000v 22nj cap. I dont know if it would make a big difference or it has something to do with the different transformer. The numbers on the original transformer are different than the borrowed one.

[budm]

Re: Proscan 55LC55S240v69

That C857 DC blocker/coupling capacitor for half-bridge circuits that drive the 12/24V transformer, it is VERY important to use the same value for replacement (you can go for higher voltage) otherwise it will mess up the feedback circuit and can cause the transformer to be over driven or under driven due the Xc of the cap. The j is the spec for 5% tolerance for the cap. When this cap has resistance leakage or shorted out, it can take out the two MOSFET's with it.
See example of typical setup.

[gsmy]

Re: Proscan 55LC55S240v69

The original cap in my board is a CBB81 68nJ 1000v. I replaced it with this one.

http://search.digikey.com/us/en/prod...1922-ND/502702

The borrowed board that appears to have been rebuilt has the CBB81 22nJ 2000v. Any explanation why it would be a different value?

[vinceroger69]

Re: Proscan 55LC55S240v69

can you just swop that capacitor over and try it in your bad psu to see if it works ok?

[gsmy]

Re: Proscan 55LC55S240v69

I can, but dont want to burn up anything on the borrowed board.

[budm]

Re: Proscan 55LC55S240v69

So you have already installed the new cap and it still does not work, the MOSFET's were not damaged when the cap went out?

[gsmy]

Re: Proscan 55LC55S240v69

I tested the MOSFETS on the board and they checked out ok for shorts. I guess I will need to remove them from the board to do better testing.

When I got the tv, I recapped the psu. I also noticed at that time that the dc cap was bad and replaced it. I went over and tested everything else on the board and everything else checks out good. I did not remove the mosfets to test them completly though.

[vinceroger69]

Re: Proscan 55LC55S240v69

you did also change the c857 capacitor on your bad board but its a different value to the one on your working board? as post 45 could this be the problem if it was me i would just borrow it from your working board and put it in your bad board just to test it.

[gsmy]

Re: Proscan 55LC55S240v69

Yes I replaced the cap with the same value, did not have the other board at the time to notice it was different. Just wondering if anybody had an explanation as to why it would have changed and if there is any risk to damaging the borrowed capacitor.

[vinceroger69]

Re: Proscan 55LC55S240v69

i would test the mosfets out of circuit first if there ok i would swop the capacitor then just to test. are the mosfets the same on both boards so you can compare readings.

[budm]

Re: Proscan 55LC55S240v69

They could have redesigned the circuits due to new parts such as transformer, or they had problem. You will have to look at the Revision number of both boards assembly (not blank PCB) and other components also to find out what else is changed.

[gsmy]

Re: Proscan 55LC55S240v69

My original board has a decal with the tv's model and serial number on it. The borrowed board does not, but it has a couple of other decals, one looks like it might be a revision or rebuilders decal. The transformer is a different number than the original and I think that may be why the cap is different as well. I am hesitant to swap the cap and dont see much point in swapping the transformer if I cant buy a replacement. I am going to check the mosfets again though. Can anybody recommend a good procedure for testing them?

[selldoor]

Re: Proscan 55LC55S240v69

Post 19 in here https://www.badcaps.net/forum/showthread.php?t=10419

[gsmy]

Re: Proscan 55LC55S240v69

I went back over the board and tested the mosfets again, comparing the readings to the borrowed board. The 2 mosfets V805 & V806 that are next to the dc cap have different readings than the borrowed board. On my board they measure 15.5 ohms across the two outside pins for both, on the borrowed board they measure 78.7 ohms on the outside pins. The other pins measure open. Could this be bad mosfets or the difference between the different rated capacitor and possibly different transformer?

[gsmy]

Re: Proscan 55LC55S240v69

I desoldered the source pin to isolate it and retested and it shows open now between the source and gate. There must be some difference between the 2 boards.

[budm]

Re: Proscan 55LC55S240v69

The resistance you are reading may be the Gate-Source resistor a shown in the diagram, you need to look at bothe boards to see if they are the same value. You may not be able to use the spare board as a reference since the design is not the same as your board.

[gsmy]

Re: Proscan 55LC55S240v69

I went ahead and removed the mosfet and bench tested it and its good. There is definately something different between the boards as I still measure the resistance difference. I suspect they changed something to deal with the failure problems of this board. I think I am going to have to look for a replacement board and call it quits. It drives me crazy that its 95% there.

[DBacksFan5205]

Re: Proscan 55LC55S240v69

I just came across this forum. Sounds very promising!
Been struggling with my 55LC55S240V69 for weeks, so it's a little comforting to know that I'm not the only one. I've been through most of the steps discussed so far. I have one silly question that dawned on me today. I know that it's not unusual for there to be connectors on boards that have no cable connected, but I was wondering if there should be a 4-wire cable connected between the main (tuner) board at XP3 or XP4 and the Pre-TCon board (TX9B261990D) at CN15? There was no cable between these points in my TV. The pin markings on the boards are identical between CN15 and XP3, so it sounds reasonable to me... I just don't have access to a working Proscan to check and can't seem to find a Service Manual so I can check the schematics.
Thanks!