Akai LCT2715 power supply repair?

[timmy]

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]

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]

What was the problem you have now ?

[miniman82]

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]

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...

[timmy]

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]

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]

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]

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]

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]

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.