Sharp 13L-M100 Does Not Power On

[QuazyPat]

Hey everyone, looking for some assistance.

I have a Sharp 13L-M100 that I RGB modded by following this guide: https://crtdatabase.com/crts/sharp/sharp-13k-m100b and getting some assistance in the RGB mod thread. Everything was successful and I've been enjoying some PS2 for the last couple of weeks. Unfortunately, the TV suddenly turned off the other day and I have not been able to get it to power on again. I took out the mainboard to inspect it and noticed one of my jumper wires on the underside had come loose (poor solder joint on my part). I'm guessing that when it came loose, it made contact with some other component/joint and shorted something out. I reconnected it and checked to make sure the fuse near the power supply was still intact (it is). After reconnecting everything, the TV still does not turn on. The power button does nothing at all.

Would you be able to help me diagnose what's wrong? Here's a link to the service manual: https://www.manualslib.com/manual/13649 ... tml#manual. I have a basic multimeter, I'm just not sure where I should start poking around.

Thanks

[MKL]

Check for 120VAC across the anodes of D703 and D704. If present, check for 150VDC at the cathode of D703 or D704 or the side of R707 connected to them (whichever is easier to reach). Use the heat sink of IC701 or Q602 for ground. If not present, check for 24VDC at the cathode of D756 or D760 or the side of R750 connected to them (whichever is easier to reach). If present, check for 12VDC on the other side of R750. Use the heat sink of IC501 for ground.

[QuazyPat]

Thank you very much, this is clear and provides a good starting point. I'll give it a shot when I get home from work tonight.

[QuazyPat]

Check for 120VAC across the anodes of D703 and D704. If present, check for 150VDC at the cathode of D703 or D704 or the side of R707 connected to them (whichever is easier to reach). Use the heat sink of IC701 or Q602 for ground. If not present, check for 24VDC at the cathode of D756 or D760 or the side of R750 connected to them (whichever is easier to reach). If present, check for 12VDC on the other side of R750. Use the heat sink of IC501 for ground.

OK, did some testing and assuming I did everything correctly (totally possible I didn't) it seems like there's something wrong in this area. Here are my results.

D703 and D704 anodes: 0.14VAC
D703 and D704 cathodes: 0VDC
D756 and D760 cathodes: 5VDC
One side of R750 was 5VDC and the other was a little over 3VDC.

To make sure my multimeter's auto-ranging was working properly, I probed the contact post for the power cord and got 112VAC. Not sure why I didn't read 120V. Might just be because my apartment building and the wiring in my unit are older? At least I know the multimeter is functioning properly.

I then checked F701 and R702 and got the same 112VAC on either side of those. Then I probed each of the four joints of coil L701. I got 112VAC on two of them, and then just .1 or .2VAC on the other two. I don't know if that's correct or not. Maybe the main filter cap C705 is bad?

[MKL]

The live/fused AC side can get to the main bridge rectifier (D701/702/703/704) only if the relay RY701 is closed (pins 1-2) and to have that, the coil side of the relay (pins 3-4) needs to be energized. This is made possible by a startup circuit. The circuit starts with stepdown transformer T701 receiving AC mains power on pins 1-3 from pins 2-3 of the line filter L701. Energy is transferred to the secondary side of T701 (pins 5-7) and it is then rectified to DC by the bridge rectifier consisting of D756/760/761/762 and smoothed out by C751. This DC voltage of about 24V is the input to the 7812 regulator IC761 (pin 1) which outputs 12V from pin 3 (mistake in the schematics: reference voltage value 21.8 is of course 12.8 ). This 12V eventually goes to pin 4 of RY701 whereas pin 3 is driven by transistor Q752 which gets a signal at the base from pin 1 of IC2001. IC2001 needs a voltage of about 4.4V on pin 11 and this comes from the abovementioned regulator IC761 and the voltage divider R2011/2012. IC2001 also needs a 5V supply on pin 39. This comes from the 7805 regulator IC101 (pin 1) which in turn works with a 12V supply on pin 3 coming from the same 12V line as above (through D753/C760). This chain of things can only happen if AC mains voltage is present on the primary side of T701 (= pins 2-3 of L701) where you said you have no voltage. Is it really so? How are you measuring that? I see no exposed points on parts side in the board layout in the manual (any actual pics?). Did you somehow put the chassis (fully connected to the tube) in vertical position so that you can reach the bottom with the probes or is it disconnected from the tube and just flipped over? The degaussing coil is better left disconnected for the time being. What's the purpose of the wire that came loose? 5V?

[QuazyPat]

I didn't take note of which pins of L701 had mains voltage on them, but it does sound like it was likely the correct ones. As for how I accesed them, half of the mainboard hangs out of the chasis when you remove the rear cover. So I just let it hang off the edge of my desk (the front/weight of the tube securely on my desk), and probed L701 from underneath.

And yes, the wire that came loose was the +5V supply. In this picture from the mod guide, it's the orange wire. My wires were not as neatly bundled as this, but it was the end at point 305 that had come out.
https://crtdatabase.com/images/8/4/e/3/ ... -pcb-5.jpg

And thanks for the detailed explanation and sequence of events.

Edit: Home from work, I tested again. Here's a picture of the underside of the board indicating what I measured on each of L701, T701 and RY701.
https://imgur.com/ahF0WND

[MKL]

half of the mainboard hangs out of the chasis when you remove the rear cover. So I just let it hang off the edge of my desk (the front/weight of the tube securely on my desk), and probed L701 from underneath.

That sounds neither comfortable nor safe. If the TV is stable in "tate" position, it would be better.

And yes, the wire that came loose was the +5V supply. In this picture from the mod guide, it's the orange wire. My wires were not as neatly bundled as this, but it was the end at point 305 that had come out.

Who knows what parts were shorted by the 5V... Measure the resistance across 5V and ground (with TV off!).

Home from work, I tested again. Here's a picture of the underside of the board indicating what I measured on each of L701, T701 and RY701.
https://imgur.com/ahF0WND

Not very clear to me. Maybe this picture will be clearer to you:

https://postimg.cc/DmNTVQdB

AC voltage readings:

Across any A and B points: 120VAC (or whatever your mains voltage is)

Across any A and C points: 120VAC (or whatever your mains voltage is)

DC voltage readings:

Black meter probe on any red point (cold ground) and red meter probe on D: 24VDC

Black meter probe on any red point (cold ground) and red meter probe on E: 12VDC

[QuazyPat]

half of the mainboard hangs out of the chasis when you remove the rear cover. So I just let it hang off the edge of my desk (the front/weight of the tube securely on my desk), and probed L701 from underneath.

That sounds neither comfortable nor safe. If the TV is stable in "tate" position, it would be better.

And yes, the wire that came loose was the +5V supply. In this picture from the mod guide, it's the orange wire. My wires were not as neatly bundled as this, but it was the end at point 305 that had come out.

Who knows what parts were shorted by the 5V... Measure the resistance across 5V and ground (with TV off!).

Home from work, I tested again. Here's a picture of the underside of the board indicating what I measured on each of L701, T701 and RY701.
https://imgur.com/ahF0WND

Not very clear to me. Maybe this picture will be clearer to you:

https://postimg.cc/DmNTVQdB

AC voltage readings:

Across any A and B points: 120VAC (or whatever your mains voltage is)

Across any A and C points: 120VAC (or whatever your mains voltage is)

DC voltage readings:

Black meter probe on any red point (cold ground) and red meter probe on D: 24VDC

Black meter probe on any red point (cold ground) and red meter probe on E: 12VDC

First, thank you for your patience. It was definitely user error on my part. The labeled diagram helped a ton and I see what I had misunderstood before. So, here's what I found.

I checked the resistance between 5V and ground in multiple spots (TV unplugged). I checked each end of my orange jumper wire and each of these three points: https://crtdatabase.com/images/4/9/2/5/ ... -pcb-3.jpg. I read ~5.5 ohms at each of them.

As for the voltage tests:
AC voltage readings
Across any A and B points: 120VAC (or whatever your mains voltage is)
These were all good. Got mains voltage at each of them.

Across any A and C points: 120VAC (or whatever your mains voltage is)
I did not get mains voltage at any of these. I got very low voltages, less than 2VAC.

DC voltage readings
Black meter probe on any red point (cold ground) and red meter probe on D: 24VDC
I did not get 24VDC at either of these. I read a little over 5VDC at each one.

Black meter probe on any red point (cold ground) and red meter probe on E: 12VDC
I did not get 12VDC here. I read a little over 3VDC instead.

[MKL]

It's possible that IC101 (7805 regulator) got shorted when the 5V wire came loose. Desolder it and check the resistance across its legs and see if the resistance across 5V and ground on the chassis is now higher than the 5.5 ohm you were previously measuring.

[QuazyPat]

It's possible that IC101 (7805 regulator) got shorted when the 5V wire came loose. Desolder it and check the resistance across its legs and see if the resistance across 5V and ground on the chassis is now higher than the 5.5 ohm you were previously measuring.

After removing IC101, I got the same 5.5 ohms from 5V to chassis ground (I used the metal grounding strap on the back of the tube) that I had before.

As for measuring the resistance across IC101's legs, I wasn't sure if it mattered which probe was on which pin. I see in the schematic that pin 2 goes to ground so I started with that, but did every combination just in case. Here's what I got.

Black on pin 2
2-1: 5.16k ohms
2-3: 11.3M ohms

Black on 1
1-2: 5.15k ohms
1-3: 11.8M ohms

Black on pin 3
3-1: 0.498M ohms
3-2: 0.490M ohms

[MKL]

IC101 is probably fine. The low resistance of the 5V line to ground may or may not be an issue. If it is, another part connected to it will be at fault (hopefully not IC2001). Let's take a step back and see if now that IC101 is out of circuit 24V is back on the cathodes of D756/D760.

[QuazyPat]

The low resistance of the 5V line to ground may or may not be an issue. If it is, another part connected to it will be at fault (hopefully not IC2001).

Ugh, I've been hoping that's not the case, but it's not looking too promising at this point haha. Can't exactly just order a new one from Mouser.

Let's take a step back and see if now that IC101 is out of circuit 24V is back on the cathodes of D756/D760.

Yep I get ~24VDC there now (measured 22.7).

[MKL]

Now check for 12VDC on point E (other end of R750/anode of D753) as per picture posted earlier. If present, see if you have about the same voltage on the cathode of D753.

[QuazyPat]

Now check for 12VDC on point E (other end of R750/anode of D753) as per picture posted earlier. If present, see if you have about the same voltage on the cathode of D753.

All good. Got 12.65VDC on the anode of D753 and that side of R750, then got 12.4VDC on the cathode of D753.

[MKL]

Desolder/lift either side of jumper 713 and then see on what side you have the 5.5 ohm resistance to ground.

https://postimg.cc/yDDxncwq

[QuazyPat]

https://imgur.com/PX0vCCi

First I lifted the end closer to the edge of the board (A in the photo), leaving the inside end (B) attached. I measured the same 5.5 ohms to ground there. Then I reattached A and lifted B, measured 138 ohms.

If I'm following this, the low resistance on that side of the jumper suggests that pin 39 of IC2001 (5V in) is potentially shorted?

[MKL]

Put IC101 back in its place and with jumper 713 lifted (side doesn't matter), check for 5V on side A of the jumper.

[QuazyPat]

Yep, got 4.99V at side A.

[MKL]

As you see, all the voltages are back once the short on the 5V line has been isolated. Desolder C2062 (the electrolytic cap just below jumper 713) and see if the resistance of side B to ground gets higher. If it remains the same, the short is in IC2001 and it would need to be replaced which I don't think is feasible.

[QuazyPat]

*sigh* It's dead, Jim. Got 5.5 ohms again.

Well, that's a bummer. It wasn't a $1,000 PVM or anything, but still fully functional and likely would have worked for a long time considering it was new old stock. If anything it's a reminder to always double (or triple) check your work and be more careful handling components. Maybe someone else will want it to salvage all the other components that are (presumably) still good.

Anyway, thank you so much for your help, I really appreciate it. At least I learned more about the actual circuitry so there's that.