JVC LT-48N770A Backlight OK, no picture or sound

[dkneyle]

I'm unsure if I have a failed power supply or failed mainboard.

The backlights come on, but no other signs of life. I have all power supply voltages EXCEPT 24V, which reads out at 12V. BUT, I've looked closely with a magnifying glass and it appears to me that the 12V rail is physically connected to the 24V rail. Makes no sense, but the meter confirms 0 ohms between the two rails. The power supply is part number ER993-B.

(These are a badge-engineered TVs out of China; this power supply turns up in other models of TVs.)

I wouldn't have thought the Backlight ON would appear if the mainboard failed its self-check? I stand to be corrected.

History of set? This was running from a small consumer generator used to power a number of appliances (as a result of loss of consumer mains power after devestating bushfires here in Australia last Summer). Owner tells me she had a number of those appliances fail whilt connected to the generator, including this TV. Hence why I was heading towards a power supply fault.

[dick_barton]

Re: JVC LT-48N770A Backlight OK, no picture or sound

I would check all the diodes in the scecondary supply since it's likely that the 12 & 24V supply are derived from the same transformer.
Are there any 24V or greater voltage rating electrolytic capacitors on the secondary side?


The solder joint looks a little odd in the red box but it may just be the photo.

[dkneyle]

Re: JVC LT-48N770A Backlight OK, no picture or sound

I would check all the diodes in the scecondary supply since it's likely that the 12 & 24V supply are derived from the same transformer.
Are there any 24V or greater voltage rating electrolytic capacitors on the secondary side?


The solder joint looks a little odd in the red box but it may just be the photo.

That solder joint is just a strange artifact of the light. I checked them out; they are fine.

Good tip about electros greater than 24V. There is only one: a 10uF 50V, and nearby is a zener diode. I'll start looking around that area. On the opposite side of the board are some 3 terminal devices that look like transistors or FETs. I'll see what they seem to be doing.

[dkneyle]

Re: JVC LT-48N770A Backlight OK, no picture or sound

Well that was a journey up a dry gultch, as we say!

The output wiring loom/cable doesn't actually connect to those pins marked 24V; there are no wires. In others words, the mainboard does not utilise that voltage line. But of course, I realised that after spending a few hours looking for a non-existent fault on the power supply board.

And of course, being Chinese, it looks like they have produced a PCB that can be tweaked for different circumstances. There is a jumper, J2, which appears to be utilised when you want those connector pins in question to output at 12v. But the PCB screen printing says 24V for the other situation where a few other components, which are missing off this board, are added and J2 is not utilised. All very frustrating, but I suppose they never expect any mug to try to have a go at repairing a faulty power supply.

So it appears I'm back to a failed mainboard, which will likely write the TV off. I'll actually put it into storage, and who knows, maybe one day I'll be junking a JVC with a compatible mainboard?

[dkneyle]

Re: JVC LT-48N770A Backlight OK, no picture or sound

Was quoted A$160 by JVC for a new mainboard. Person who donated the TV is more than OK for it to be canabilised.