Akai LCT3285TA Power Supply Issues Unstable 24V

[kevinm34232]

The power supply is a Megmeet MLT169A. I am unsure of the history of this TV, it was given to me.

Tv was dead. I replaced several bad/vented caps and 1 transistor.
Q9(2N4401), EC4(22uF 50V), a 10uF 50V next to EC2B, EC18,19,20(1000uF 16V), EC9,10(1000uF 35V)

All voltages now come up except the 24V is fluctuating 2-3v, then sometimes down to nothing and will come back back up on it's own randomly. I can hear clicking from the transformer when it starts working, and clicking slows down as voltage drops.

I've reflowed the transformer pins and everything else around.. I didn't repalce all the caps but did check them, including the 22uF 450V with the blue mark on top, which tests good.

I'm not sure what is going on!

[tom66]

Re: Akai LCT3285TA Power Supply Issues Unstable 24V

What's the 12V or other output do?

[kevinm34232]

Re: Akai LCT3285TA Power Supply Issues Unstable 24V

The other outputs are good and stable. The 24V jumps around. It drops quicker whith the load of the inverter board connected but does it without it also. The voltage around Q11 (which I just killed) seems to flucuate aswell.

Is Q11, that transformer, and the rest of the circuit on the top of the board just for the 24V?

[tom66]

Re: Akai LCT3285TA Power Supply Issues Unstable 24V

The main ("24V") transformer will have one or more diodes connected to it. How many does it have?

[kevinm34232]

Re: Akai LCT3285TA Power Supply Issues Unstable 24V

1 of those 3 legged diodes

[tom66]

Re: Akai LCT3285TA Power Supply Issues Unstable 24V

So it only does 24V then. In which case you need to check the diode has no shorts/opens, check the startup cap and check Vcc across startup cap.

[kevinm34232]

Re: Akai LCT3285TA Power Supply Issues Unstable 24V

Is the start up cap the medium size 450V cap with the blue mark on top?

[tom66]

Re: Akai LCT3285TA Power Supply Issues Unstable 24V

No. 16V to 50V or so on primary.

[imp]

Re: Akai LCT3285TA Power Supply Issues Unstable 24V

I see CapXon´s on the photos, have you checked these? They are equally likely to either dry up, leak or both since it´s a truly horrible brand. I wouldn´t put the TV back together before they are exchanged to a new and better brand....

[tom66]

Re: Akai LCT3285TA Power Supply Issues Unstable 24V

Good point, in particular CapXon startup caps are very prone to drying up without any external signs. I've also had one fail circuit open completely with no bulge. Truly among the worst!

[kevinm34232]

Re: Akai LCT3285TA Power Supply Issues Unstable 24V

Yes I ordered all new caps yesterday and a new Q11 Im hoping it's one of the caps causing the issue.

TV looks to have main board problems too. Ill have to get PS going first but TV goes from standby RED to on with green LED, but it won't shut back off with power button. Also no BL-ON signal.

There are AS1117 regulators, the fixed ones test good.
The ADJ ones have a 2V output, if I add 1.25V to pin one which is 0.7v the output should be around 1.3V right?

[tom66]

Re: Akai LCT3285TA Power Supply Issues Unstable 24V

It might not shut off if it uses the 24V supply for something like audio and is detecting a fault with that.

Pin #1 of a regulator should always be 1.25V below the output.

[kevinm34232]

Re: Akai LCT3285TA Power Supply Issues Unstable 24V

Replaced ALL caps except main filter cap which stays steady, still unstable 24V. Wondering if it's one of the small IC's on the bottom of board.

Anyone have schematic for Megmeet MLT169A?

[kevinm34232]

Re: Akai LCT3285TA Power Supply Issues Unstable 24V

I got back to this TV, and found IC1 on bottom of board seems to be burned but not shorted, could not read full # on IC but found another Megmeet schematic that showed it to be L6562D TRANSITION-MODE PFC CONTROLLER, I ordered it and will see happens.. Could this cause the unstable 24V?

[tom66]

Re: Akai LCT3285TA Power Supply Issues Unstable 24V

If the voltage across the 450V cap is low, then definitely a possibility.

[kevinm34232]

Re: Akai LCT3285TA Power Supply Issues Unstable 24V

I replaced that IC still same problem. I noticed the voltage does not go above 166V on the 450V cap when turned on. If anyone has any last guesses before I give up please let me know. Thanks!

[kc8adu]

Re: Akai LCT3285TA Power Supply Issues Unstable 24V

you have a pfc section problem.fix that and your +24 will be ok.
should be near 400v with the unit running.166 is fine in standby.

[budm]

Re: Akai LCT3285TA Power Supply Issues Unstable 24V

What DC voltage do you get at the VCC pin and the Ground pin of the PFC IC?

[imp]

Re: Akai LCT3285TA Power Supply Issues Unstable 24V

What brand and type is the primary cap? If CapXon, they dry up regardless and I never leave them in any case .... Repaired far too many ATX PSU´s with dried up CapXon primaries.

[kevinm34232]

Re: Akai LCT3285TA Power Supply Issues Unstable 24V

Sorry for the delay, looks like Im only getting 0.5V on VCC pin 8 of IC L6562D - TRANSITION-MODE PFC CONTROLLER using negative leg of 450V cap as ground.