Visio VW32L HDTV40A - no picture

[ahickman]

I have a Visio 32" LCD (VW32L HDTV40A) that was pulled from the trash. I have spent several hours looking at the obvious (bad caps, burned pcb boards, etc.) and found nothing obvious. Did my searches for helpful hints, but info on this model or similar is limited. Only specific thread was as follows:

https://www.badcaps.net/forum/showth...hlight=hdtv40a

So far I have done the following in terms of troubleshooting:

Using an isolated transformer, I removed the power supply board and tested the various points (filtering cap, +5/12/24 volt diodes). I do have +5 volts on power up. When I connect +5V through a 3.3K ohm resistor to PWR_ON pin, I get a brief +12/24 volts but it quickly decays.

Can anyone suggest a next step in troubleshooting? It seems as though my power supply is OK, but it is shutting all but standby off shortly after power up.

Aaron

[ahickman]

Re: Visio VW32L HDTV40A - no picture

One last bit of information I forgot to mention regarding the symptoms....with all boards plugged in and the set powered up by AC, the VISIO symbol on front lights (standby voltage), but when the power button is pressed, the VISIO symbol goes orange, but looks unstable (it kinda wavers) like there is a short or intermittent connection. No picture. No sound and no +12/24 volts.

[tom66]

Re: Visio VW32L HDTV40A - no picture

Someone else on these forums (Tetranitrate) has a very similar problem. In his case it seems to be a shorted T-con, so try disconnecting the connector with the thick black lead. You should get no picture or a white screen but it might then power up and give sound.

[budm]

Re: Visio VW32L HDTV40A - no picture

You should bench test the power supply again, using two 12v bulbs in series and attach them to the 24vdc output, then turn it on as before sunig the 3.3K resistor, the lamps should stay on, if not, you still have problem in the 24/12vdc power supply circuits.
See how I test the power supply here:
http://s807.photobucket.com/albums/y...%20SV470XVT1A/

[ahickman]

Re: Visio VW32L HDTV40A - no picture

OK, tried the dual 12v lights in series and get 1 second of lamp life.

[tom66]

Re: Visio VW32L HDTV40A - no picture

OK, tried the dual 12v lights in series and get 1 second of lamp life.

With the 3.3k resistor? Any connections going to main board? If so, disconnect them all.

[ahickman]

Re: Visio VW32L HDTV40A - no picture

With only the controller board connected the power supply, I noticed the processor heat sink was a bit warm. Inverting a can of compressed air, I sprayed the board down and then blew moisture from a breath of air across the board to look for hot spots. Found that U7 (marking on chip is K401, G117 which Digikey shows as a 5V LDO regulator) was extremely hot. So hot that I burned the pad of my finger! This looks like the power to the processor. I flipped the board over to test the caps ESR and noticed that somewhere along the line I had dislodged three VERY tiny SMD capacitors (CB82, CB83 and CB109). So small and little chance of finding them. Ugggh!

[ahickman]

Re: Visio VW32L HDTV40A - no picture

Tom66, when testing the PS, I had no connections to any boards. Just the lonesome power supply.

[tom66]

Re: Visio VW32L HDTV40A - no picture

Tom66, when testing the PS, I had no connections to any boards. Just the lonesome power supply.

So this seems to indicate a fault with the power supply.

What experience do you have with debugging power supplies?

[ahickman]

Re: Visio VW32L HDTV40A - no picture

Don't mind admitting that I am some what of a newbie to SMPS troubleshooting. I understand the major components/functions of the SMPS, but don't have the necesary experience.

[tom66]

Re: Visio VW32L HDTV40A - no picture

Don't mind admitting that I am some what of a newbie to SMPS troubleshooting. I understand the major components/functions of the SMPS, but don't have the necesary experience.

It definitely helps to have a background. I am still a newbie.

I just saw you post about the LDO. The LDO is probably hot because it is unstable without the caps. If you find the datasheet for the LDO, you can find the manufacturer's recommended capacitors and replace them. I wouldn't recommend running it with the hot LDO. It could damage the processor.