Sharp LC-50P66U Power Cycling

[packrat79]

This set was brought into my shop a few weeks ago, the power supply was totally dead. Customer said the set had hardly ever been used. I ordered a power supply from an eBay seller, after installing the set would power up just long enough to display the Sharp logo, then shut off, and keep repeating. The voltages to the mainboard (12 and 16VDC) remained steady, as did the P-ON signal to the power supply, but the BL-ON and DIM signals cycle along with the power. I was sure the mainboard must be faulty too (lightning strike maybe?, so I ordered one, installed it, and got the exact same result. The seller thought the board might be defective so I'm sending it back, but before I order anything else I thought I'd see if anyone has any ideas. I tried to access the service menu, but the typical procedures for Sharp TV's don't apply because this is a rebranded Hisense. And not surprisingly, there isn't any service info available for this set.
Any ideas?

[nomoresonys]

Re: Sharp LC-50P66U Power Cycling

Test the voltages at the connector on the powerboard with EVERYTHING hooked up, it's the connector that the cable from powerboard to mainboard hooks into, most times there's a legend/map nearby that gives the expected voltages, you measure them with your mutimeter, set meter to 20 vdc then report them, put neg probe on chassis metal ground, pos probe on pin of connector, measure in standby take note, leave probes where they are then power on tv and again take note of voltages, report them here, example pin 1-ps-on-0 volts standby-power on-3.3 volts.

[packrat79]

Re: Sharp LC-50P66U Power Cycling

Yes that is exactly what I did:

Pin ID / V in standby / V powered
----------------------------------------
12VDC 12 12 steady
Vcc 16 16 steady
P-ON 0 ~3.1 steady
BL-ON 0 2.2 cycling
DIM 0 2.2 cycling

That's basically all there is on that connector, plus ground connections of course. The BL inverter (part of the power supply) does not send any feedback signal to the mainboard for detecting faults, something I have seen in older sets.

[dick_barton]

Re: Sharp LC-50P66U Power Cycling

What voltage (maximum) do you have at L925 and L926. Should be 200V

[nomoresonys]

Re: Sharp LC-50P66U Power Cycling

Possible mainboard issue, the bl-on should be steady, see if the bl-on is steady on the mainboard side of the connector.

[packrat79]

Re: Sharp LC-50P66U Power Cycling

The BL-ON signal cycles on both sides of the connector. As unlikely as it seems, everything points to the mainboard. I guess I'll have to order another one and see what happens.

[packrat79]

Re: Sharp LC-50P66U Power Cycling

Got the set working. Another eBay seller advised me to check the backlights. They were not defective, but it occurred to me that this symptom was very similar to a backlight fault. It didn't make sense though, because it didn't look like there was any way for the mainboard to sense a backlight fault and shut down. However, I started comparing the two power supplies and discovered that some components (mostly resistors) had different values. I also noticed that although the part numbers on the boards matched, the sub-numbers on little stickers were slightly different. So I tried swapping all the components of different value from the old board, including the switching transformer, and the set powered and stayed on. Both boards had the exact same power output specs printed on them (which is what fooled me) but apparently they were not the same.
Anyway, problem solved and lesson learned. Thanks anyway for your help.

[nomoresonys]

Re: Sharp LC-50P66U Power Cycling

Right, thats why you match up the sticker numbers, NOT just the board numbers, many use the same board number but each can be populated in a different manner for their application.