Need help troubleshooting a sylvania LC420SS8 power supply

[Op4500]

Picked up this TV for $20, it was manufactured in 08' serial #J33855376. It shows no signs of life, no sounds, blinking lights, nothing. With the inverter disconnected the main power supply is outputting 169.2v, it has 345v stamped on the board by that connection. Transistors tested good, diodes tested good, fuse was good, No signs of bulging/leaking caps or burn marks anywhere. The power supply for this TV is out of stock everywhere and it's pretty expensive even if I can find one, so I'm hoping I can find out which specific component has failed. I don't have an ESR meter or anything, just a fluke 179 and a soldering iron, and my probes are too fat to fit in the test holes, I will have to pick some up that fit.

Edit: I found the service manual on this forum, that should help a ton, but if anyone has experience working on this TV it would be helpful to know which components I should check first.

[Op4500]

Re: Need help troubleshooting a sylvania LC420SS8 power supply

So I used the schematic here to check voltages. I tested all of the connections to digital main, virtually nothing passed. Power button has no effect.

CN201-

1. .12v
2. .4v
3. 0
4. 1.1v
5. 0v
6. .7v
7. 0v
8. 0v
9. 0v
10. .15v
11. 0
12. 0
13. 0
14. 0
15. 0
16. .9v
17. 0
18. 0
19. 0

CN202

1. 0
2. 0
3. 0
4. .9v
5. .9v
6. 0
7. .9v
8. 0
9. 0
10. .8v
11. .9v
12. 0
13. 0
14. 0
15. .1v
16. 0
17. 0
18. .5v

CN203
1-22=0v
23-.8v

[philliesfan30m]

Re: Need help troubleshooting a sylvania LC420SS8 power supply

first thing is to take pics of the back of the tv with the cover off and individual clear pics of all the boards

[Op4500]

Re: Need help troubleshooting a sylvania LC420SS8 power supply

I will charge up my camera and see if it even still works. Tried taking some pics with my phone but they are too blurry to be of any use.

[Op4500]

Re: Need help troubleshooting a sylvania LC420SS8 power supply

Here is all I could get before my camera died, I can upload pics of other boards later if you need them, got a good shot of the main power supply.

http://imgur.com/a/81Q9J#0

The power supply board has this number on the back: BA81H0F01012-1

I don't suspect it to be anything other than the MPS, because there is no change whatsoever when I disconnect the digital main or the inverter board from it. It seems like the only thing it is even outputting power to is the inverter board, 169v connected or not, should be 345v.

[philliesfan30m]

Re: Need help troubleshooting a sylvania LC420SS8 power supply

i am not an expert but my guess is it might be something with the mainboard not sending the power supply on signal. someone will be able to help narrow it down when you put up pics of all of the boards and they will probably want close up pics of the area where the connectors connect to the power supply board. because they won't know what values are suppose to go to each pin that you have labeled above unless they know what that pin is for

[philliesfan30m]

Re: Need help troubleshooting a sylvania LC420SS8 power supply

i don't even get any search results from google when putting in the mainboard number. what are the numbers on the white sticker on the mainboard in the upper left of the picture?

[Op4500]

Re: Need help troubleshooting a sylvania LC420SS8 power supply

The thread I linked to has a service manual with schematics and voltages. the connectors that I tested CN 201, CN 202, and CN 203 run across the top of the MPS and connect to the digital main. Here is an album with a close up of those connectors and a couple other pics. http://imgur.com/a/bgIeh#0

[Op4500]

Re: Need help troubleshooting a sylvania LC420SS8 power supply

i don't even get any search results from google when putting in the mainboard number. what are the numbers on the white sticker on the mainboard in the upper left of the picture?

A81H1UH
7176000BA6
08081403276

I think this is it.
http://www.shopjimmy.com/sylvania-a8...al-board-1.htm

[Op4500]

Re: Need help troubleshooting a sylvania LC420SS8 power supply

So I decided to just randomly probe it, I noticed d604 is showing 17v, that seems like kind of a high voltage drop for a diode. When the TV is unplugged it tests good on diode test though. Might swap it out and see, that particular diode fr104-b seems to be sold out everywhere, need a suitable replacement if that is the problem.

[philliesfan30m]

Re: Need help troubleshooting a sylvania LC420SS8 power supply

try fr104g-b they have them at mouser.com. but i would wait and see if someone helps troubleshoot it before throwing parts into it.

[philliesfan30m]

Re: Need help troubleshooting a sylvania LC420SS8 power supply

just out of curiousity did the person you bought it off of have it apart? did you check to make sure everything was plugged in properly when you opened it up? just asking cause you never know what people do to them before you receive them.

[Op4500]

Re: Need help troubleshooting a sylvania LC420SS8 power supply

Yeah, it was taken apart, the guy I bought it from doesn't know anything about TV repair, he just took the back off, lost half the screws and put it back on, I checked all connectors and disconnected and reconnected them. I don't see anything that looks physically damaged.

[philliesfan30m]

Re: Need help troubleshooting a sylvania LC420SS8 power supply

so all the boards were grounded then and you removed all the screws on the boards? just making sure it isn't something simple first.

[budm]

Re: Need help troubleshooting a sylvania LC420SS8 power supply

The standby power supply is not working, you can run the power supply board by itself and you should have those voltages I marked in the diagram.

[Op4500]

Re: Need help troubleshooting a sylvania LC420SS8 power supply

Yeah, when I first took those voltage readings everything was connected and screwed in, I did a few checks with each board disconnected from the MPS with no change. The only sign of life I get from this set is a click of a relay when I plug it in and there is a slight high pitched whine from the transformer.

[Op4500]

Re: Need help troubleshooting a sylvania LC420SS8 power supply

Can't seem to find the problem, I've checked almost everything on that board, any ideas? The only places I get power are the 169VDC at the inverter board and 62VAC at each secondary pin of the power transformer.

[tom66]

Re: Need help troubleshooting a sylvania LC420SS8 power supply

You need to use the metal chassis when testing anything on the secondary, not the -ve lead of the capacitor.

[Op4500]

Re: Need help troubleshooting a sylvania LC420SS8 power supply

I took all my secondary voltage readings with the negative probe on one of the power supply screws, while it was screwed into the chassis. For primary readings I had it on the neutral wire where the AC power comes in.

[budm]

Re: Need help troubleshooting a sylvania LC420SS8 power supply

When take the DC voltage measurement in the primary side, the negative leg of the large filter cap is the circuit ground ref point for the primary side, not the neutral or the safety ground.