Samsung bn44-00623b help

[Dannyx]

Re: Samsung bn44-00623b help

if you want i have a brand new power supply bn44-00623b!

That won't be necessary. I believe I'll be able to fix this one for one thing and secondly, I WAS able to find one locally if it really comes to that, plus it's not mine - it's for work.

[Dannyx]

Re: Samsung bn44-00623b help

Some more research: there's definitely a feedback issue going on, because with no main board and no LEDs connected, the voltage DOES stabilize around 12.3v, which is a bit too low, but passable. I also get around 2.4v on the REF pin of ICM851.

If I connect JUST the LEDs, the bottom half flickers a lot and the output drops to around 8v. The LED supply is also low, with only around 40v after the rectifier diodes, whereas the schematic shows 67v for +Vdrv2 2. ICM851 now only reads about 1.7v on the REF pin, so it IS trying to compensate, but can't pump the output any higher for some reason. I also tried removing the two diodes DM858 and DM857 cutting power to the LED driver entirely and like before, this lessens the load enough for the main board to fire up.

If I connect JUST the main board, it power cycles like 5-6 times before eventually stayin on, but the voltage is at its lower limit, only 10v on the output, and around 2.1 on the REF pin, so again, the secondary appears to be doing its thing and asking for more, but the primary doesn't "respond". Must admit I haven't tried a new optoisolator: had a few lying in my bin of salvaged parts and threw one in there....

[Dannyx]

Re: Samsung bn44-00623b help

UPDATE: TV is now working. The big mystery ? RM814 had a cold solder joint on one side from when I originally removed it to check it. I never gave it any second thought because the space between the resistor and the pad was absolutely minuscule, so when I pressed my meter probes to double and quadruple check it, it would close the circuit enough to make contact and give me the correct reading since the resistor itself was fine. I noticed it when instead of putting the probes right on the resistor, I touched them to two adjacent pads and I got a larger value. I wiggled them around, probably flexing the board just enough to close that hairline crack and the right reading came back. Blasted it with some hot air and it's good as new !

TOP TIP: check your solder joints Cheers

-Danny

[budm]

Re: Samsung bn44-00623b help

"I also get around 2.4v on the REF pin of ICM851."
So are you still getting the same reading on that pin? I believe it should a lot less for working circuit.

[Dannyx]

Re: Samsung bn44-00623b help

"I also get around 2.4v on the REF pin of ICM851."
So are you still getting the same reading on that pin? I believe it should a lot less for working circuit.

On the contrary: I believe it should be 2.5v to indicate the PSU has reached "equilibrium" and the REF pin is equal to the internal reference of the TL431, essentially making the difference between the + and - inputs of the internal op-amp zero.

[budm]

Re: Samsung bn44-00623b help

If that 431 is turned on when the ref is at 2.5 Volt then the Anode to Cathode Voltage will drops which will pull down the Base Voltage of the linear regulator Transistor, that is why when you remove the jumper the output of the Transistor goes to higher Voltage.