eax55357701 repair

[ajshoe]

Have a LG 37lh40 with a bad power supply EAX55357701/32 board. tv tries to light back lights but can't - cycles forever. took power board out and bench tested. has 5V, jumper pwr on to 5v and 12V good, 20V good. Have a second power board - it turns on tv - works perfectly, I have checked all caps (ESR), looked for bad solder, checked all xsistors, and no luck, anybody have any experience with these power supply boards ? thanks

[ajshoe]

Re: eax55357701 repair

also observed main board green led flashes fast with bad power supply bd. with good power board in - main board green led is on solid.

[dick_barton]

Re: eax55357701 repair

Have you checked the dc voltage across the two legs of the 450V 68uF capacitors on both power supplies to see if there is a difference.

[Andrew F. Ali]

Re: eax55357701 repair

have you checked the dc voltage across the two legs of the 450v 68uf capacitors on both power supplies to see if there is a difference.

+10

[ajshoe]

Re: eax55357701 repair

Out of town til weds.i will test that & Report back next week. Thanks

[ajshoe]

Re: eax55357701 repair

tested both boards - broken board has 162V across C611 (68UF 450V cap) with plug in. with PWR_ON HI = 385V.
good board has 162V across C611 (68UF 450V cap) with plug in. with PWR_ON HI = 377V.
all fuses good. seems like a bad solder joint but don't see any

[dick_barton]

Re: eax55357701 repair

So one board powers everything OK. The original has all the voltages when tested but doesn't light up the lamps.
Check the On/Off voltage at pin 14 of IC301 (BD9215F) which should be 3V to enable the lamp driver circuits.

[budm]

Re: eax55357701 repair

So does the backlights try to come on at all when using the bad board?
Did you check inverter transformers winding resistances and compared them to the working board?

[ajshoe]

Re: eax55357701 repair

put small wire on ic301 pin 14 - never moves off 0 volts. I had a good ground (used GRD on main connector - P201). assume the IC301 turns on inv lights (ON/OFF) ? have bad IC301 ?

windings resistance across output of inv xformer were: good board - 22.9 ohms each output - total across both 46.8 ohms
bad board - 26.7 ohms across both - 50.9 ohms

symptoms of set were that it would gradually take longer and longer to turn on (red led indicator on front bottom of set would turn solid blue led). It finally stopped turning on (led front bottom indicator never turns blue - stays red).

seems like main board is telling it to shut down. going to check voltages again on bad board in set.

[ajshoe]

Re: eax55357701 repair

bad power supply board in set: turn power switch on - 12V varies between 8v and 12v, 20V varies between 9v and 14v. hitting the on/off button on side of set seems to do nothing - same voltage swings.

[dick_barton]

Re: eax55357701 repair

In your first post you say

took power board out and bench tested. has 5V, jumper pwr on to 5v and 12V good, 20V good."

Now you say the voltages on the bad power supply are varying?

On the bad board what voltage do you have at Fuse F501 when you power up out of standby.

[ajshoe]

Re: eax55357701 repair

earlier posts I was checking and reading voltages on bench. I tested them again on bench - they are correct and constant. The last posts were tested with bad power board in tv. I then plugged in set and then turned power switch on. took readings that I posted - voltages are varying. I thinking now that the power supply can't supply enough current on 12V. Can I load test the 12V and 20V line ?

also - I ohmed the pin 14 to inv on/off - it is good - 1K ohms on both boards

[budm]

Re: eax55357701 repair

The board only has one power transformer (LLM-001) to provide always on 5V Standby and the switched 12V, 20V, so you are indicating that the 12V and the 20V are not stable, so are you sure that the 5V Standby is stable since is generated by the same power transformer LLM-001 when connected back in the TV.

[ajshoe]

Re: eax55357701 repair

checked 5V - when power switch is turned on - 5V varies between 4.94V and 5.12V about every 3 seconds. hitting the on/off button on side of set just makes 5V cycle again between 4.94V and 5.12V

[budm]

Re: eax55357701 repair

Is the variation of the 5V in sync with the variation of the 12V/20V?
May be try heating the power supply board with hair dryer to see all the Voltages will become steady or not.

[ajshoe]

Re: eax55357701 repair

heated power board while monitoring 5V - got the cycling to 5.02V to 5.12V. saw the backlight come on for second or two. only happened once. think variation is in sync with 5V, 12V, 20V supplies

[ajshoe]

Re: eax55357701 repair

believe I've found problem. IC201 and IC202 (AS431AN - shunt diode) seem bad - going to get new ones.

[ajshoe]

Re: eax55357701 repair

local electronic store gave me lm431 as a sub. checked the diode out - don't think its compatible with as431an. looks like its configured backwards. anybody know of a acceptable sub ?

[ajshoe]

Re: eax55357701 repair

installed new lm431 - to92 - made lm431 work. installed board and now have solid 5 volts. 12V and 20V come up but cycles to 0 about every 5 secs.
DC voltage at F501 is 377V but cycles to 250V every 5 seconds. observing IC301 pin14 inv on/off - never goes to 3V

[budm]

Re: eax55357701 repair

It sounds like the PS-ON is going on and off.