LG 37LC2D No picture, believe bad power supply

[rlega]

Re: LG 37LC2D No picture, believe bad power supply

@minikoopa - To my understanding a small capacitor will discharge itself within seconds once the power has been disconnected, but large capacitors can hold a charge for weeks.

On this power supply I only worked on some caps and did not have any issue.

I created one myself to discharge the big capacitors, similar to this:
https://www.ifixit.com/Guide/Constru...arge+Tool/2177

[minikoopa]

Re: LG 37LC2D No picture, believe bad power supply

@rlega Thanks for the info. What counts as a large cap?

[Alastair E]

Re: LG 37LC2D No picture, believe bad power supply

@minikoopa - To my understanding a small capacitor will discharge itself within seconds once the power has been disconnected, but large capacitors can hold a charge for weeks.

]

Not strictly true.....

A 'Perfect' Capacitor--Such a thing doesn't exist, as far as I know-- but if it did, would retain a charge--Until the end of time, or unless it was discharged by some external resistance.

As an example-- 0.01uF is considered a pretty Small cap in comparison to the ones found in a PSU, which have caps of 10uF to 4700uF, of various voltage-ratings.

Mercedes-Benz years ago however, used the Charge of 3-11V across a 0.01uF Capacitor as a reference for the Cruise-Control, A Specialised MOSFET with extremely low leakage used as a buffer in the design, to connect this 'set-speed' circuit to the VCO.
According to the voltage across that cap gives the reference for the 'set-speed' To Increase set speed, you apply a voltage through a high value resistor to Leak more volts into the cap from the supply, lower speed, leak volts Out of it in a similar way. They used reed-relays to connect the charge/discharge control on steering-column to minimise any external leaks.
I have personal experience that this tiny film cap in an 1985 M.B. Cruise Control can hold the charge for weeks on end, --That cap is the original nearly 30 year old part--Reed relay went leaky, but the Cap and MOSFET are still fine......

A similar system was used to good effect on the preset Tuning of some German TV sets, when 'touch-tune' became all the rage instead of mechanical clunker-tuners where a voltage was stored directly in a cap, (1-34V depending on where in the band you're tuned to) buffered by the MOSFET from a stable TAA550 regulated supply and used as the Varicap Tuning voltage for the Tuner.

Each channel selector had its own cap, reed-relay and MOSFET. These sets were not ever known to lose the pre-set tuning, so guess those caps retained their charge indefinitely!

PSU's however are normally designed with some form of leak-off resistor to discharge within around 5 minutes--But Not Always, and surprises Still happen!
If in doubt, there's two methods of discharge for ya!-

Jeweller's method--Use a 470 ohm 10W resistor and use this to discharge the cap, alternatively, use a 100W 240V bulb,-- I prefer the bulb--as you know IF the cap had a charge, And When its discharged and safe--You often dont even need to attach wires to the bulb, just line up the two pips of the bulb with the cap pins on the underside of the board, Little flicker of light, its all over......

Brave-Man Method--Short the bugger with a screwdriver-- Big bang and a flash, all over in a second, Dangerous possibly--But MUCH more fun!

[minikoopa]

Re: LG 37LC2D No picture, believe bad power supply

Thanks for the info. I read about the bleed resistor. Can you tell if there is one in this PSU? I only plan on changing the 2 1000uf 25v and 2 1000uf 35v as they are bulging. I know I should do the whole lot, but im lazy.

[Alastair E]

Re: LG 37LC2D No picture, believe bad power supply

The main one that would cause you electric-shock issues if charged, would be the big bugger in the middle of the board.
--Check across its pins with a meter set to 400-1000v range, Use a bulb to discharge if more than say 20V measured.

Other caps will be low voltage and present less danger, just shove a screwdriver across Only The Cap Pins to discharge, but most of the low-volts supplies and their caps generally have leakers to discharge 'em but the main reservoir cap--The Big Bugger--generally will not.

[minikoopa]

Re: LG 37LC2D No picture, believe bad power supply

The main one that would cause you electric-shock issues if charged, would be the big bugger in the middle of the board.
--Check across its pins with a meter set to 400-1000v range, Use a bulb to discharge if more than say 20V measured.

Cheers for that. I think I might need to invest in a multimeter. Could you solder some probes to a bulb to make it easy to touch the contacts on the cap to discharge.

[culmesm]

Re: LG 37LC2D No picture, believe bad power supply

I think it will be better to make a photo on the other side so we can tell you which resistor is

[diif]

Re: LG 37LC2D No picture, believe bad power supply

Cheers for that. I think I might need to invest in a multimeter. Could you solder some probes to a bulb to make it easy to touch the contacts on the cap to discharge.

Yes, or use a bulb holder, 75p from your local Wilkinsons !

[ceedy]

Re: LG 37LC2D No picture, believe bad power supply

Hi all,

Not been on for a while as had no problems

Hope you don't mind me resurrecting this thread ?

But now I have returned with another duff LCD PSU , not mine ,
just had it dumped on me by Number 2 Son ?, ( mend it dad)
( muggins)

Looks identical to the one in this thread , but from a 42 inch screen .

I'm now a bit limited in equipment now , only iron and Fluke meter redface:

So grovelling for some help

Tv reported as working some of the time ? but mostly just a Whistling sound from the psu .

I found 5 bulging caps so replaced them ..

Powered back up and no whistle .. yeah!!! short lived tho' .. after about
1 min the 450V 65Ohm Cap popped and smoked ..

replaced that and Ditto , so something a bit more serious then ..

but now obviously can't do any voltage checks without a mask

hmm wot now!! Maybe just try and source another PSU ?

ta Chris

[budm]

Re: LG 37LC2D No picture, believe bad power supply

Cap installed properly? Or the PFC Voltage booster lost its regulation and cause the Voltage on the cap to go way higher that 450V?
Pictures of the your board?

[ceedy]

Re: LG 37LC2D No picture, believe bad power supply

All caps in correctly ..

PC as requested.. Those with small Red dots were replaced.



Cheers

Chris

[budm]

Re: LG 37LC2D No picture, believe bad power supply

So that big cap C500 with bulging top is the original cap that was OK before and now it just blew up?
http://www.shopjimmy.com/lg-6709900016d-smps.htm

[ceedy]

Re: LG 37LC2D No picture, believe bad power supply

So that big cap C500 with bulging top is the original cap that was OK before and now it just blew up?
http://www.shopjimmy.com/lg-6709900016d-smps.htm

yes, the 450v 68Uf just popped . nice jet of white smoke

found uk based seller of this PCB,

so will investigate this as an easier option


C

[ceedy]

Re: LG 37LC2D No picture, believe bad power supply

Damm.... they had one in stock yesterday . but now sold out ..

[ceedy]

Re: LG 37LC2D No picture, believe bad power supply

Reason for no stock is My Son bought the last one!!

So probably sorted ...

will report back later

Cheers

Chris

[baddawg1991]

Re: LG 37LC2D No picture, believe bad power supply

I have a similar tv, the lg 32lc2d-ue. Power supply board looks the same as the 37 in this thread. Anyway, the tv used to take a while to turn on, making a high pitch whining sound, and eventually the tv stopped working. Will attempt to turn itself on with led blinking green as soon as i plug the power cable in. Power button is inop at this time. It will then after a few seconds go solid green, but no display, no sound, and power button will not work. After a few minutes it shuts itself off, led goes red. Power button will turn it back on and process repeats. Took it apart to find one capacitor swelled, so I replaced it with good soldering (a friend who works aerospace building circuit boards for 747's soldered it in for me.) Still does the same thing. I noticed the whining sound to be coming from a yellow component next to the large rubber coated capacitor in the center of the board. Checking power, i had 11.81-11.9 volts on all 12v pins, 6.02 to 6.06 on the 6v pins nd 4.98v on the 5v pins. All consistent, however I only show 1.3v on the 3.4v pins. Im no tv expert, but i work on volkswagen electrical for a dealership, so i know my way mostly around electrical systems. Not sure what my problem is and im hoping someone can help me out. Thanks in advance! Picture attatched shows the component whining i am speaking of, which is above the large capacitor near center of the board. Again, im no expert but hey thats what this discussion is for right? Thanks again!

[vinceroger69]

Re: LG 37LC2D No picture, believe bad power supply

It may be worth you starting your own thread with the above information in.Can you also include the power supply number and we will need some good clear pictures of the front and back of the power supply so that when we zoom in on it we can see/read the parts etc.

[bestjm]

Re: LG 37LC2D No picture, believe bad power supply

Is there anyone still monitoring this thread?

[dskall]

Re: LG 37LC2D No picture, believe bad power supply

Threads are monitored when someone posts to them