Akai LCT2715 power supply repair?

[miniman82]

Just inherited this set, only the 5v section of the power supply shows any life after replacing all the burst caps in the secondary side. You guys have any pointers for getting the 24 and 12v sections working? The PWM chip for that part of the power supply has no VCC going to it, but I'm not sure if that's intentional as a sort of stby mode or what. Looks to me like the thing should be making voltage all the time, waiting for the set to pick it up when needed.

I do have 21v sitting waiting on a transistor that looks as if it switches power to the PWM chip, but just to test I jumpered it to VCC and nothing happened. Suppose it's possible the PWM chip gave out, but I don't have a replacement and can't work with SMD stuff at home anyway. Schematic sure would help.

[rca2000]

Look for a "ps on" pin or such..it needs to either go to 5volts or be GROUNDED...to turn on the RUN voltages. Take a resistor of at least 100 ohms...and short from the 5v. to the on pin...THEN see if the run voltages come on. If there is ALREADY voltage on that pin..the short the resistor to GROUND.

[Findm-Keepm]

The manual is on ManualsLib.com - I just downloaded it, and it includes a schematic of the SMPS, page 30. Give it a try...

[miniman82]

Duh, should have looked at the board closer. There's a pin marked "on/off", putting 5vdc on that made all the other voltages appear like they're supposed to. So the power supply is fine again. Still no light on the screen, the green light in front comes on for like 30 seconds, then it turns orange and stays that way till I cycle power. I think the main board is probably shot, considering the power supply is now fine. The little PC mount fuse for 12v is still good, and there doesn't seem to be any shorts anywhere. Anything else I should check?

[rca2000]

YOU need to see if your backlight is working. It likely is powered by that 24 volt source. If you have that and the BL is not working...you need to know is it is being "turned on" by the signal board. There should be a "bl on" or such pin on the board. It must change when the power is turned on, like the power pin. There is likely ALSO a "dim" pin. it ALSO has to change--or the BL will NOT come on. If BOTH of those seem to be there....and your 24 volt source is on to the BL inverter (IF this set uses one)...you might have a bad inverter.

ALSO...take a light of some sort--and shin is RIGHT at the screen...after you turn it on. See it there seems to be ANY sort of activity there, like a menu or source indicator..

[miniman82]

Haven't checked the backlight yet, I'll do that next. These aren't very intuitive, if this was a tube set I'd be watching it by now. lol

[miniman82]

Backlight no workie. 24v is definitely getting there on both plugs and good grounds, is there an enable pin or something? Oh, and all the buttons for the edge controls are shot. This thing must have had a tough life or something, probably a lost cause at this point.

[rca2000]

On CN 2 on the BL inverter...pin 12 is the BL on pin...and pin 10 is the PWM pin--that gets the dim info from the signal board. If you are gettng a change in those 2 pins when the set is turned on, and the 24 volts on CN -1, your inverter is not working --common failure on LCD sets.

[mstaton]

since the buttons seem to be possibly broken, did you try a remote?

[miniman82]

No remote, I've been shorting the switch terminals with my screwdriver to troubleshoot. lol

Some of the button do still work, but you have to press them really hard to make a connection. Guess that's what 10 years of being pressed will do, won't be tough to find replacements if I get the inverter working.

Thanks RCA2K, I'll poke around tonight and see what I can see.

[Findm-Keepm]

Originally Posted by miniman82

No remote, I've been shorting the switch terminals with my screwdriver to troubleshoot. lol

Some of the button do still work, but you have to press them really hard to make a connection. Guess that's what 10 years of being pressed will do, won't be tough to find replacements if I get the inverter working.

Thanks RCA2K, I'll poke around tonight and see what I can see.

Post a pic of the switches - I have my Dad's tactile switch assortment and may have a replacement or two. Easy to swap in new ones, usually 4 pins to desolder/solder.

[miniman82]

They are right angle switches with 4 posts, pretty common in TV's.

Update: I ordered the inverter board. I was able to determine that the onboard 5vdc supply powering all the IC's went dead, and couldn't trace it back to find the fault. $23 on Amazon won't be too objectionable on this set if it comes back to life, if it doesn't come back to life I'm pretty stumped.

[rca2000]

before you ordered the inverter...

DId you CHECK and see if it was getting its "must haves" from the signal board? The turn on command and the PWM signal for dimming ? you DID check and saw that the 24 volts was there I know...and did you see ANYTHING on the screen--if you shined a bright flashlight into the center of it--when the set is turned on?

You PROBABLY have a bad inverter--but You DO need to check these things first...because if the signal baord is not providing it with what it needs--it will NOT come on...even a new one will not .

[miniman82]

Yes, the signal board was putting out a 1.0vdc command on power up, which when you look up the data sheet for the CCFL control chip, is exactly what it's looking for to start up. But the reason it never did anything is it didn't have the +5vdc VCC input it needed to operate, that went missing somewhere else on the board. I jumperd 5v from elsewhere and it instantly blew every SMD fuse, so I'm pretty sure it's shorted. The board is unrepairable anyway, and for $23 I can't really complain.

PWM was there as well. I don't think this is a main board issue, I'm sure the inverter is toast. Question is, was that the only board that let go?

[miniman82]

Just installed the inverter, same symptoms. I think the main board may be shot, but I have to take the screen apart further to make sure it's not just the CCFL's being physically busted or something. If they are good, where should I look next?