Sharp lc-42sb45u no response... at all.

[dood]

Just got this TV in as dead... it's mine now, but I need to fix it for it to be worth anything. Caps are CapXon, and I replaced 2 of the 4 on the PSU board, but I didn't have any 680uf 50v caps for the other two.

The +5v is sitting at 4.4v coming off the PSU board. At the power switch itself, I have 2.7v.

There are 4 wires going to the board that has all the switches on it. I'm measuring them as such:

Red - 0.00 v
Grey - 2.7v
White - 2.7v
Black - 0.65v

When I push the power switch, black drops to 0.00v.

Is the low +5v the problem here? If so, would I look to the other two capxon caps as the culprit? I'm uploading pics now and will post them shortly.

[dood]

Re: Sharp lc-42sb45u no response... at all.

Here's the power board -

[dood]

Re: Sharp lc-42sb45u no response... at all.

I've got high-res shots if anyone thinks looking at them would help. I'm not the best at diagnosing circuits... I generally just replace caps and hope for the best.

[retiredcaps]

Re: Sharp lc-42sb45u no response... at all.

The +5v is sitting at 4.4v coming off the PSU board. At the power switch itself, I have 2.7v.

Is the low +5v the problem here?

That would be my guess. Almost all TVs require a stable 5V standby in order for the main board to work properly.

[retiredcaps]

Re: Sharp lc-42sb45u no response... at all.

I've got high-res shots if anyone thinks looking at them would help.

It will help. Please use manage attachments for my sake.

[dood]

Re: Sharp lc-42sb45u no response... at all.

For now, here's cell phone pics on smugmug - http://nadams.smugmug.com/Other/Sharp-TV/

I'll grab my digi now and get some better shots. I'll upload those here.

[dood]

Re: Sharp lc-42sb45u no response... at all.

Main power board -



The output of the power board is labeled as such. Both Power On/Off and +5v measure 4.4v



The 4-pin connector goes to the switch board-

[dood]

Re: Sharp lc-42sb45u no response... at all.

Sorry, Power on/off doesn't have any voltage on it. The two lower grey lines do. Is the switch supposed to pass voltage, or short to ground?

[retiredcaps]

Re: Sharp lc-42sb45u no response... at all.

Okay, let's break this down.

1) The most important voltage right now is 5V standby just below the power on/off. If it is not steady 5V, the main board will likely not work. 4.4V DC is way too low.

2) Power on/off is a pin that is toggled high (> 3V DC) by the main board when it is working properly. It tells the rest of the power supply to turn on.

3) Disconnect the main board from the power board. See if 5V standby is a solid steady 5V. If yes, then something on the main board is dragging down the power supply. If no, then something is wrong on the power board.

4) I see a lot of Capxon caps. You know what that means.

5) The service manual and power board schematic is on page 79. (22MB).



6) I snipped out the portion that relates to 5V. Refer to diagram.

7) With the main board disconnected, I would check D927 (middle pin or pin 2) and see what the DC voltage is. This should be 5V or more.

8) Check the DC voltage across C955 (the new Panasonic cap).

9) Check the DC voltage across C934 (47uF 25V) cap.

10) Remeasure 5V connector pin.

11) Report readings for 7, 8, 9, and 10.

[PlainBill]

Re: Sharp lc-42sb45u no response... at all.

Something strange here. R941, R944, R945 form a voltage divider that sets the output voltage of the supply. Q918 and R947 'tweek' that setting. With Q918 off the outpit voltage should be 2.50 * (1+ 11K/18K) With Q918 on the output voltage should be 2.50 * (1+ 11K/9.9K). You do the math.

PlainBill

[dood]

Re: Sharp lc-42sb45u no response... at all.

I'll be checking out everything when I get home from work later... I can tell you now that 4.4v is with the power supply board disconnected from everything, so the problem lies in the power supply itself. Also, the reading across C955 is also 4.4v

Like I said, I'll get all the info to you when I get home. If need be, the option is always there to just buy a replacement board off eBay and recap it. They're only $40... but I'd rather give fixing this one a shot.

[tom66]

Re: Sharp lc-42sb45u no response... at all.

What does the STANDBY logic line measure at?

[dood]

Re: Sharp lc-42sb45u no response... at all.

Retiredcaps - 7, 8, 9, and 10 all measure 4.44v

[retiredcaps]

Re: Sharp lc-42sb45u no response... at all.

Retiredcaps - 7, 8, 9, and 10 all measure 4.44v

I'm exhausted. I will take a look again when I get some rest and a clear head. I have the feeling the answer is front of me, but right now I can't see it.

And yes, I did the math that PlainBill wrote earlier.

[retiredcaps]

Re: Sharp lc-42sb45u no response... at all.

One last thought before I retire for the night.

If Q918 (a NPN) isn't turning on, then there are two possibilites.

One, R920, a 10K ohm resistor is open. Voltage never gets to the base.

Two, Q918, is non functional. It could be shorted? Test all combinations for shorts. That is, base-collector, base-emitter, emitter-collector. Any reading less than 30 ohms suggests it might be shorted and should be desoldered and verified out of circuit.

tiredcaps

[dood]

Re: Sharp lc-42sb45u no response... at all.

One last thought before I retire for the night.

If Q918 (a NPN) isn't turning on, then there are two possibilites.

One, R920, a 10K ohm resistor is open. Voltage never gets to the base.

Two, Q918, is non functional. It could be shorted? Test all combinations for shorts. That is, base-collector, base-emitter, emitter-collector. Any reading less than 30 ohms suggests it might be shorted and should be desoldered and verified out of circuit.

tiredcaps

R920 measures at 9991ohms

Q918 has 2.47v at what I'm assuming is base (the single pin on one side)

IC911 has 4.16v at pin 1 and 3.14v at pin 2.

IC909 appears to be where the 4.44v is coming from... I get 4.44v on pin 1 and 3.78v on pin 2.

What setting should my DMM be on to test the resistance of Q918? Also, if I'm looking at Q918 with two pins at the top and one pin down, Which is base/collector/emitter?

[retiredcaps]

Re: Sharp lc-42sb45u no response... at all.

What setting should my DMM be on to test the resistance of Q918? Also, if I'm looking at Q918 with two pins at the top and one pin down, Which is base/collector/emitter?

If you have a manual meter, set it to 200 ohms scale.

Emitter is the down arrow, collector is top, base is middle. It really doesn't matter if you just number the pins 1, 2, 3. Measure between 1-2, 1-3, and 2-3. That covers all the bases.

[retiredcaps]

Re: Sharp lc-42sb45u no response... at all.

What does the STANDBY logic line measure at?

Don't forget to check this as tom66 requested.

[dood]

Re: Sharp lc-42sb45u no response... at all.

Don't forget to check this as tom66 requested.

Would that be the pin for Power On/Off?

In-circuit, on the 200 ohm scale, Q918 measures open.

[retiredcaps]

Re: Sharp lc-42sb45u no response... at all.

Would that be the pin for Power On/Off?

In-circuit, on the 200 ohm scale, Q918 measures open.

1) Okay, let's assume Q918 is working fine. I neglected to see/spot C942 last night just before the base of Q918.

2) For the standby line, I can't see a test point on the connector so I would suggest measuring on the right side of the R920 or R946 resistor (as per schematic). I suspect that will be 4.4V DC as well.

3) I'm curious, what is the voltage reading on the collector (top) of Q918?

4) What is the voltage reading on the emitter (bottom) of Q918? I suspect I know the answer, but would like confirmation because my schematic interpretation skills aren't the best.