SHARP LC-80LE633U has open resistor r7133 on PSU

[ki4mqz]

Also, any alternatives on the diode? Mouser and Digikey are backordered. Thoughts on running without it?

At Budm suggestion, I found a 6.2 volt that will do the job. Should be good.

[R_J]

I'm sure digikey has 8.2 volt zener diodes in stock, any through hole 8.2 volt 500mw to 1 watt will work
1N5237BTR

[ki4mqz]

I have some findings on this noise. When I power the TV on, randomly, as the backlight comes on, this noise begins. I did some probing and found the following. See the photo attached.

20201107_135903.jpg

What seems to be at least a symptom is the higher voltage when the noise is being made. As the noise subsides, the voltage falls to 110v at the locations in the photo. Does this shed any light on this for anyone?

Another interesting find is at the leg of the capacitor I have marked with a red arrow. If I touch either of my multimeter probes to this leg with the other probe not touching anything and my meter set to read voltages, the noise changes and almost comes to a stop. This is very strange.

Also, the high side voltage is at 390v with the backlight up. It does not fluctuate with the noise at all.

[ki4mqz]

Just an update. Installed the diode when it arrived. TV is now functioning perfectly. Noise has not returned.

Thank you all again for all the help!

[budm]

Thanks for the follow-up on t he repair.

[sigmeyer]

As suspected , the diode is burned aswell , diodes and mosfets are like the main thing that burns in PSU's , I would trace to see if there are any diodes close to that circuit and you can replace them with similar, dont need to be the same , just make sure they have similar rating. The IC is toast have you checked the IC out of circuit for shorts , since you have the datasheet ... if there are no shorts i think its kinda safe to try it , but its most likely burnt as everyone suggests

[ki4mqz]

Reopening this thread.....again. TV worked for a while but has begin to exhibit a problem again. After probing around on the bench with everything connected, something is shutting off the 5V standby circuit causing the tv to power cycle randomly. Further probing shows that the PFC voltage is there, and when the TV does power on, PFC goes to proper ranges and TV function. For grins, I changed the 5VSB IC7102 as I ordered spares previously, but no change. New Zener diode I installed is also still ok. So, I am beginning to suspect something around the optocoupler train.

Tracing the circuit, I see that the Enable Pin (1) on IC7102 is connected to PC7103 collector and the emitter appears to go to ground. Reading the datasheet, I assume this opto is used to disable 5VSB by drawing current on pin 1 to ground. Thoughts on if this could possibly be a failing PC that is causing pin 1 to shut off the IC? There is also a capacitor (C7111) that could be failing as well. But something is shutting off my 5VSB.

All help is greatly appreciated folks. I love seeing outstanding troubleshooting at work.

[ki4mqz]

Also see that the BP/M pin of IC7102 is connected to one leg of PC7104 with the other leg tracing back to the bias winding of the transformer. Looking in the datasheet for IC7102, this appears to be an overvoltage protection circuit.

With this knowledge, am I wrong to assume either I have a problem with voltages on the secondary side or I am dealing with an optocoupler issue?

[R_J]

Did you ever replace the 2 electrolytics C7114 & C7107 for ic7102

[ki4mqz]

I have not. Visually they appear ok though I know thats not a definitive answer.

[ki4mqz]

Will be troubleshooting shortly. Stay tuned.

[ki4mqz]

So I plugged it in and cannot simulate the same problem. Currently everything comes up and all looks well. So until it begins again, I cannot see what the problem is.

[ki4mqz]

It began the problem again. Whatever it is, it is isolated to the primary side. When the TV shuts off, the BP pin goes to 0v and the EN pin goes to 0.35v. The IC also gets very hot while off. Intermittently it will come back on, but immediately go back off.

There are 3 like optocouplers in the area, so I moved them around and it seems to have no effect. Ideas?

[ki4mqz]

After troubleshooting more last night, I got the feeling that something could be going to ground around the BP/M pin circuit between the IC and the bias winding of T7101. See the attached photo below, but after doing some reading, I realized there is a diode (D7111) connected to the BP/M pin trace. According to the writeup, this is for overvoltage feedback protection for the IC. I am going to pull it tonight and test it. Being I found one diode already in this circuit bad from when IC7102 failed, I do not find it far fetched there is another.

R_J, I have highlighted the elec caps you mentioned in the photo as well. I will order them when I order diodes. Any idea of the specs on them?

[R_J]

The specs are not critical, just use quality 105 degree caps with the same rating.

[ki4mqz]

Parts ordered. I believe I may have caught it. I moved around some of the optos and after letting it sit a while, i got a new error blink light for AC_DET. Moved that opto (PC7101) back to the BP/M spot (PC7104), and viola, the power board came back up. So I ordered new optos and got the caps while I'm at it.

[ki4mqz]

Alright R_J and Budm, I need help. I have found the culprit for the 5vsb shutting off, but do not know what the circuit I have isolated does. See the attached tracing picture I borrowed and edited from this thread.

I got the PSU to act up pretty consistently today. I began probing voltages and found the BP/M pin was going to ground on shutdown and the IC was getting hot. So, I looked at the tracings and checked all zeners between that tracing and ground. All test good. So I decided to unsolder and lift J7109 at the red x I marked in the photo. This isolated an entire circuit from the BP/M solder pad. Bang! PSU powers up and never shuts off and output voltages look great.

So, that circuit is driving the pin to ground. Why? What is its purpose. I have traced it out and I do not understand what that circuitry is for. R_J, one of the elec caps you suggested I change is in it. C7101. I have checked the zeners marked in the photo and they are good. Failing transistor? Failing cap? Help a hobbiest out. Thanks in advance!

Edit: The voltage on the positive pin of C7101 randomly swings between 6.85v and 0v. Looking at the tracing, it is hard for me to see what would cause that. PFC voltage goes thru the 4 SMD resistors, across the top of a transistor? and to that pin. Bad resistor? Stumped here.