View Full Version : Akai LCT2715 power supply repair?


miniman82
12-06-2015, 05:43 PM
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
12-06-2015, 06:34 PM
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
12-06-2015, 06:40 PM
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
12-06-2015, 08:37 PM
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
12-06-2015, 09:10 PM
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
12-06-2015, 09:46 PM
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
12-06-2015, 10:28 PM
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
12-06-2015, 11:18 PM
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
12-07-2015, 01:34 PM
since the buttons seem to be possibly broken, did you try a remote?

miniman82
12-07-2015, 04:24 PM
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
12-07-2015, 09:25 PM
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
12-07-2015, 10:46 PM
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
12-07-2015, 11:17 PM
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
12-08-2015, 09:41 AM
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
12-14-2015, 05:06 PM
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?

timmy
12-16-2015, 09:58 AM
Look into these very tiny SMD diodes on the underside of the power supply board if there are any could be one. To be sure you must remove it from the board to check it because most of the time the diode will be wired to a secondary part of a transformer and would show shorted even if good. Even around the inverter feed.

miniman82
12-21-2015, 11:39 PM
It's not the power supply anymore, I repaired that. I'm beginning to think the main board is toast, perhaps a casualty of the (formerly) bad PSU? No idea, won't have any time for it till after the holidays.

timmy
12-22-2015, 05:29 PM
What was the problem you have now ?

miniman82
12-23-2015, 06:53 PM
:sigh:


I got the new inverter board to light up the lamps today, turns out the main board wasn't putting out enough voltage to cause it to come on. The inverter wants to see around 1 vdc as its turn on logic signal, the main board was only putting out .78 vdc so it wasn't coming on. I put my meter on that same pin set to diode test mode (which outputs a few volts), and it lit right up. I ordered a new main board, cost was $20. Hopefully that will be the end of this troubleshooting session.

miniman82
12-27-2015, 12:17 AM
So it wasn't the main board at all, turns out it was more power supply problems...

Could have saved myself another 20 bucks, but what the heck I'm still less than $50 into this thing including buying it so I'll call it a win.

I swear this power supply is far more complicated than it needs to be: There's the 5vdc standby supply that's always running (+5vs/1A in the schematic), which powers only a few circuits like the LED on the front and the circuit which tells the rest of it to boot up once you hit the power on switch. Then there's the 12vdc/24vdc end of it that does all the heavy lifting for the inverter and audio sections, but there's another 5vdc leg I didn't see before that powers only the main board (labelled +5v/4A in the schematic) and it wasn't doing anything.

Turns out there's a MOSFET supplying the main board fed from the standby supply, with a pair of transistors telling it turn on with a signal coming from the standby portion of the board. STBY pin is supposed to get latched 1vdc from the power switch, which in turn tells the 12/24v sections to wake up along with the 5v one powering the main board. The 2 NPN transistors and a 4.7k SMD resistor that control the MOSFET were bad (4.7k had drifted down to around 700 ohms because of the shorted transistors), along with another random one in the standby circuit for the 12v leg. That caused all sorts of strange things to happen, like sometimes 12v wouldn't wake up or when I'd jumper out 5v/1A (stby power) to 5v/4A I'd get a bunch of lines on the screen.

The parts on the power supply that were bad are circled in red, and don't forget I had to recap the whole thing as well. I guess if I get another flat screen with a bad supply, I'll save myself the headache and just order a good one. Lesson learned...

http://videokarma.org/attachment.php?attachmentid=189373&d=1451192959

timmy
12-28-2015, 07:06 AM
So you give up repairing that power supply, well it must be a nice tv. So now you will be getting another power supply, well I would at the point you are try and fix what you have now and save your money. 3 transistors and a resistor may do it unless there is something down the line that caused this to happen. The transistors there by number look like general purpose type and the resistor don't have to be SMD to see if it's going to work changing those 3 components. I'm not sure but did you infact fix it?

miniman82
12-28-2015, 02:07 PM
I watched it last night, the parts I replaced fixed it. I think this all started with the bad capacitors in the board, those shorting out or just plain having high leakage could explain the shorted transistors. Hope it lasts, I want to use this as my new computer monitor.

timmy
12-28-2015, 03:12 PM
Every one of these flat screen sets are not made to be repaired but tossed out after 3-5 years or when the caps go bad. If the caps were rated the right way they would last but progress dictates all products have to keep going round and round and people keep getting richer. So throw it out buy another and another and so on. And yes no doubt the caps were the first problem that caused everything else. These tvs of today have between1&2 % tolerance which is really nothing compared to 50 year old televisions of yesteryear having 20% built in.

rca2000
12-28-2015, 03:39 PM
I do NOT---and have NEVER..subscribed to the "throwaway" mentality. Most of the electronic items I have ARE 'throwaways"--from someone ELSE..which I grabbed and FIXED..to use myself or to pass on to someone else. ALL of my tv sets fit this category. One exception is my COMPUTERS...both my main one and laptop I am using now were purchased NEW.

BTW...the likely reason for the two 5 volt lines to to reduce POWER DRAW. The stby 5 volts is used to keep the processor alive so it can be ready to receive an on command..from the IR or power switch.. The switched line powers other things NOT NEEDED during standby--which would WASTE POWER_-if in the "on" state..

timmy
12-28-2015, 03:56 PM
It's just how society is these days. It's to bad how much is waisted but then again we do our thing by lifting from the trash for ourselfs, I do it too,lol. If people knew how to fix things like tvs eBay has repair kits for flat screen tvs for the power supply or other areas and they are cheap too along with higher ratings on diodes and mosfets to ensure the same problem don't happen again. But if the design was correct the first time around these sets would last.

miniman82
12-29-2015, 12:25 PM
Not bashing the design of the power supply, just not the way I would have done it is all. The electronic portion of it seems pretty stable, I did mess up and put a 16v cap where a 32v one belongs and it started smelling funny late last night. Caught it before it blew though, now has a higher rated 50v one in its place. It works like it's supposed to now, but if I was designing the actual layout of all the parts I definitely would not have placed the caps above a hot rectifier heat sink. That just seems wrong, but what the hey it lasted close to 10 years with cheap caps in it. If I get another 10 out of it I'll be pleasantly surprised.