Sharp LC-80UQ17U Power Supply Board problem

[bscottjohnston]

I just picked up a Sharp LC-80UQ17U here in Denver on Craig's. The previous owner said he was watching it and "pop" it stopped. Opened it up to find a blown fuse on the high Voltage side of the PS. I bypassed it with a 7.5 Ampere automotive fuse and saw a flash in component NTC7001 (as the fuse popped). I am not going to try to apply any more power to the board until I can locate a schematic. Does anyone have the schematics to RUNTKB217WJQZ?

[francoservo]

hello pin AC_DET what condition is maybe is to hi

[bscottjohnston]

hello pin AC_DET what condition is maybe is to hi

I am afraid I don't understand what you mean. I perceive you mean "check the AC_DET test point and see if its lo or hi" but I am not sure where to do that...

[bscottjohnston]

It would appear as though some sort of schematic is on the back of the board. Is it safe to assume that these boards are single layer? Sorry for the DQs - I did avionics in the Navy but never at a board level.

[bscottjohnston]

Okay so here's a question for the experts. RL7001 on the back of the board (HF12FF marked on the relay package) is showing 300 Ohms or so on the relay coil but short across the switch contact. The datasheet for HF12FF doesn't show if its normally open or normally closed. The icon on the backside of the board shows open. It just so happens its adjacent to NTC7001 that gave off a small flash when I did my "fuse testing". Anyone have specific knowledge or experience with this part?

[прямо]

"The datasheet for HF12FF doesn't show if its normally open or normally closed."

Sure it does. It shows a normally open diagram, and the switch contacts will be closed when the coil is energized.
If you measured almost no resistance (0 ohm or exactly the resistance of your probes being touched together) then yes, it is shorted.

Most likely because the contacts have fused together, or just simply stuck. If stuck, you can try applying 12V DC to the coil and check if that will make them unstuck, if not then replace the relay.

Also, it is recommended when working with power supplies to use a DBT (dim bulb tester) so you won't end up with components blown to bits when there is still a failure in the circuit.

When the main fuse is blown, what you need to check first is always the bridge rectifier in diode mode, then the semiconductors in the primary side (more diodes, transistors or mosfets)

After the primary side is confirmed to be ok, then you can move on to the secondary side where the low voltages are.

[nomoresonys]

I second the dim bulb tester, easy to make for a few dollars, can check youtube for many how tos.

[bscottjohnston]

Holy Cow - this dim bulb tester is brilliant. What a great way to work on power supplies without roaching them. Heading to my Home Despot today to grab parts and build. Thanks all.

[bscottjohnston]

This MOV reads .86 MegaOhms in circuit. I have other PS boards that work with MOVs that are wide open this close to the fuse. Is close to 1 MegaOhm a defect? I have to think it is without any Voltage applied.

[прямо]

If you can read the fine print on the varistor you can find out the spec, but you should measure its resistance out of circuit to get a better reading.

[francoservo]

hello check the voltage on d7950 (BU5V) or conect in this point extermla power supply an check if the main borad tour on the relay or the stnd by led is blinking

[bscottjohnston]

This TV is too huge and too expensive. I have decided to go with a repair service on eBay while I continue to teach myself SMPS circuits. Thanks for all the opinions here. I did indeed learn something even though its been 30 years since I left Navy electronics school.

[прямо]

The power supply board is very easy to remove, it is usually mounted by screws on a galvanized steel chassis inside the TV, and repairing it doesn't really need the TV at all.

Don't understand why the TV size and value would be a hindrance.

[bscottjohnston]

It was indeed easy to remove and was fairly easy to diagnose to a degree. However, I cannot find the relay here in the US and didn't want to mess with it. I don't want to recycle this one as its huge, looks great and is extremely heavy.

[lotas]

On the relay contacts there is a low resistance thermistor in parallel for a soft start and it seems to you that the contacts are stuck together.

[bscottjohnston]

I think that is true too. But I cannot source the relay so I sent the entire board out for repair