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

[retiredcaps]

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

5) What is the resistance measurement of R944 (10k) and R945 (1k)?

6) What is the resistance measurement of R947 (22k) and R941 (18k)?

[dood]

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

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.

5) What is the resistance measurement of R944 (10k) and R945 (1k)?

6) What is the resistance measurement of R947 (22k) and R941 (18k)?

2) I have no voltage on either side of R920 or R946

3) and 4) Only one pin on Q918 has any voltage, and that's the pin that connects to R947 in the diagram, with 2.46v. I'm having some difficulty understanding that part of the diagram, since it shows that pin being top left, but it's actually the single pin on the one side.

5) and 6) When I probe these out, the numbers immediately start to climb. Is that normal?

I'm really horrible at this type of troubleshooting because I can't understand circuit diagrams and apply them to real life to save my own life... With my current level of frustration, an eBay power supply is looking more and more likely

[retiredcaps]

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

2
I'm really horrible at this type of troubleshooting because I can't understand circuit diagrams and apply them to real life to save my own life...

1) No, don't give up yet. PlainBill and tom66 will probably have more input into this. PlainBill has the years of experience and tom66 is young up and comer. I have neither in this area of my life.

I would like a chance to help you through this because I want to learn this myself and there is no better way than to think this through. If this was networking, I would have solved this a long time ago.

I do have to head out for a few hours though in about 60 minutes.

2) Can we get a close photo of the area in question (both sides of the board)? The closeup will help and then I can markup the pins to test.

[dood]

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

Here's some closer pics. Lemme know if they're crap.

[PlainBill]

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

I'm really horrible at this type of troubleshooting because I can't understand circuit diagrams and apply them to real life to save my own life... With my current level of frustration, an eBay power supply is looking more and more likely

I use the 'arm waving' approach to analyzing this circuit. All components EXCEPT Q918, R920 and R947 are straight out of the TL431 application note. The simple explanation is until the voltage at pin 3 of IC913 gets up to 2.5 volts no current flows through the emitter of IC911, so the SMPS controller is driving to output full power. As soon as IC913, pin 3 reaches 2.5 volts current flows, and the SMPS controller throttles back. It's the electronic equivalent used in the first steam engine Edison used when he the Pearl Street generating plant.

Q918 and associated resistors is an unusual addition. When STANDBY is low Q918 is turned off, so R947 has no effect. When STANDBY is high, R947 results in a boosted output voltage.

PlainBill

[randtek]

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

I believe your problem is not on the power board. I work on a lot of these. It is normal for the 5v line to run a little low in standby mode. For Energy Star benefit, many newer TV designs drop the standby voltage down when in standby mode. Your problem is probably on the main board. If you need to order a board for this unit, try to find part numbers directly on the board. There are 2 versions of both the main and power boards in this model that ARE NOT interchangeable. The service manual only lists one of the possible part numbers for each board.

[retiredcaps]

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

I believe your problem is not on the power board. I work on a lot of these. It is normal for the 5v line to run a little low in standby mode.

Hi Randtek,

Obviously with your hands on experience, this is valuable insight.

There is nothing in the service manual to even suggest that 5V standby can run as low as 4.4V DC and still work?

I guess the interpretation of "a little low" varies. 4.4V is 12% lower than the usual 5V and wouldn't qualify in my books as a "little" low. If dood had reported 4.85V, I would have regarded that as "little" low (3%).

[randtek]

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

You are correct retiredcaps. 12% would be out of tolerance in most cases. This idea they are using of lowering the standby voltages is a pretty new concept. The logic on the main board usually runs at 3.3v, so the main can still regulate 4.4v down to a stable 3.3. But running the standby voltage down decreases overall current draw when the unit is off.

And about the "service manuals". Since the advent of flat panel TVs, The service manuals can no longer be considered adequate for troubleshooting. The schematics are considedrably simplified from what the circuitry in the units actually contain. The manufacturers do not anticipate/support component level repair, so they see no need to provide complete and accurate schematics. Also, Manufacturers often have numerous versions of the same model, with parts that may or may not be interchangeable. These differing versions NEVER show up in schematics, and rarely in service manuals.

[dood]

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

randtek - since you are pointing to the mainboard now, I will mention - the main processor does get warm when the TV is left plugged in.

I'll try recapping the mainboard, if I have the appropriate values.

Also, with this TV, is the power LED supposed to be lit red when the TV is in standby? Because as it is, the LED is not lit at all.

[randtek]

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

Power LED should be lit green when power is on for this model.

[Toasty]

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

Dood- to answer your question(s)...


and, has this board been fully recapped now? (Except for mains cap)

[Toasty]

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

Although the energy saving may be a factor, as randtek eludes to, I rather doubt it. The manual is way too specific.

If you check the troubleshooting guide on page 33 of the SM, it specifically asks about the presence of the +5 volt line and what to check if not present.

IMO, recapping the mainboard is a waste of time, until you get the PSU fixed.

My suspects here are the zeners ZD910 or ZD911, the switcher IC903, or the cap at C969. Also check any chip resistors of <500Ω value for burns or discoloration, like R991(47Ω) or R976 (22Ω).

What is the voltage across the mains cap C907? (strangely, no one asked this question...?)
Should be present at the anode of ZD911 (junction T904 - pin1/C945/ZD911).

Toast

[randtek]

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

I misread your Power LED question. I'm pretty sure it does light red when unit is in standby, it turns on green when power is turned on.

[dood]

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

Although the energy saving may be a factor, as randtek eludes to, I rather doubt it. The manual is way too specific.

If you check the troubleshooting guide on page 33 of the SM, it specifically asks about the presence of the +5 volt line and what to check if not present.

IMO, recapping the mainboard is a waste of time, until you get the PSU fixed.

My suspects here are the zeners ZD910 or ZD911, the switcher IC903, or the cap at C969. Also check any chip resistors of <500Ω value for burns or discoloration, like R991(47Ω) or R976 (22Ω).

What is the voltage across the mains cap C907? (strangely, no one asked this question...?)
Should be present at the anode of ZD911 (junction T904 - pin1/C945/ZD911).

Toast

I did not replace the two 50v caps. They're on the +24v line, so for testing purposes I could replace them with 25v caps and they should be okay. Permanent replacements should be 50v, though. I'll replace them when I get home and check the other things that you mentioned.

[Toasty]

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

Follow the SM tips first, before you go replacing those caps and verify the mains supply voltage. More concerned about the 5v line than the 24v. Need to have the 5v present before the 24v.

Anyone -
Is there some way to fool the supply into thinking the mainboard is good?
Just to narrow the search for where to devote the efforts here...?


>>the main processor does get warm when the TV is left plugged in.<<

Is that unusual?

Is recapping that mainboard really an issue? Or is it more the processor that we need to look at?

Toast

[randtek]

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

Is there some way to fool the supply into thinking the mainboard is good?
Just to narrow the search for where to devote the efforts here...?

Yes, we probably can do that. Start by hooking the power connector back up to the main board. Measure the voltage on the "Power On/Off" pin of CN902 on the power board. I believe it is pin 10, but go with what is marked on the board next to the connector.

Let me know what that voltage is with the connector connected. I believe this pin to be active high (3~5v in standby) but it could be active low. Does it change when the power button is pressed? I know you have probably already stated what this voltage is, but I'm getting old and forgetful, so humor me!

>>the main processor does get warm when the TV is left plugged in.<<

Is that unusual?

Is recapping that mainboard really an issue? Or is it more the processor that we need to look at?

Toast

The main processor will probably be a little warm in standby mode, if it feels hot to the touch, it may be of some concern. I am rather skeptical that a general recapping of the main board will be a productive plan. an ESR meter would be a big help here.

[dood]

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

Yes, we probably can do that. Start by hooking the power connector back up to the main board. Measure the voltage on the "Power On/Off" pin of CN902 on the power board. I believe it is pin 10, but go with what is marked on the board next to the connector.

Let me know what that voltage is with the connector connected. I believe this pin to be active high (3~5v in standby) but it could be active low. Does it change when the power button is pressed? I know you have probably already stated what this voltage is, but I'm getting old and forgetful, so humor me!



The main processor will probably be a little warm in standby mode, if it feels hot to the touch, it may be of some concern. I am rather skeptical that a general recapping of the main board will be a productive plan. an ESR meter would be a big help here.

With everything hooked back up, I have 0.02v on Power On/Off. Pressing the button shows no change. On the mainboard, CN401, (To key pad board), pin 4 measures 0.00v, pin 3 measures 3.11v, pin 2 measures 3.11v, and pin 1 measures 0.60v. When the power button is pressed, Pin 1 drops to 0.00v, but no other voltages change.

[dood]

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

I left the TV plugged in for the last 30 minutes or so. I don't have a temp probe, but I would say that the processor heatsink is "quite hot". Failed processor, maybe?

[dood]

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

Anyone have anything else for me to try? If not, which should I buy? Mainboard, or power?

[tom66]

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

If it's burn-your-finger-hot, it's probably bad.

If it's quite warm, that's unlikely.

Have you checked that pulling up or down the STANDBY control line will cause voltages to rise or fall?