RCA 32LB30RQD - Power on, No picture, sound, or backlight

[caphair]

Re: RCA 32LB30RQD - Power on, No picture, sound, or backlight

You need to check the E-B DC Voltage to see if it is biased on/off when the TV is turned on and off.

Voltage of the emitter and base or across the emitter and base?

Also of the two transistors in that cluster you mean?

[budm]

Re: RCA 32LB30RQD - Power on, No picture, sound, or backlight

Yes, Emitter and Base DC Voltage, it should toggle, but I cannot tell which one is for the BL_ON, but the main key that we are looking for the E-B Voltage to toggle and at the same time that E-C Voltage should also toggle (when E-B =~ 0.6V, then E-C should then be < 1V (Transistor is turned on).

[caphair]

Re: RCA 32LB30RQD - Power on, No picture, sound, or backlight

Ok I think I found the issue. It looks like a resistor that feeds into the B of one of the smd transistors. There's 5v on one side when power is applied and .6v on the other end which that end connects to the base of the transistor that's directly linked to the pin of bl_on.

I have no idea the value, can I just solder in the 100ohm I used earlier in its place?

[caphair]

Re: RCA 32LB30RQD - Power on, No picture, sound, or backlight

Fixed! Not the prettiest thing but it works lol. Thanks everyone for the help especially budm

[caphair]

Re: RCA 32LB30RQD - Power on, No picture, sound, or backlight

Update: Original problem occurs intermittently even with the fix. I measured bl_on and it seems to waver from 4.97v to 4.18 and back up.

Sometimes the set starts up no problem. Other times tries to power on but no pic/backlight and won't power off with the power button.

[budm]

Re: RCA 32LB30RQD - Power on, No picture, sound, or backlight

That center pin is the Collector. If you look at the transistor with center pin up and the two pins down, the left pin is Base, the center pin is Collector, the Right pin is Emitter.
Base-Emitter forward voltage is 0.6~0.7V.

[caphair]

Re: RCA 32LB30RQD - Power on, No picture, sound, or backlight

That center pin is the Collector. If you look at the transistor with center pin up and the two pins down, the left pin is Base, the center pin is Collector, the Right pin is Emitter.
Base-Emitter forward voltage is 0.6~0.7V.

Are you saying I hooked that resistor up wrong?

[budm]

Re: RCA 32LB30RQD - Power on, No picture, sound, or backlight

Based on this picture: https://www.badcaps.net/forum/attach...2&d=1416435893
the left side of the resistor is connected the Collector, the right side I cannot what it is hooked up to.

[caphair]

Re: RCA 32LB30RQD - Power on, No picture, sound, or backlight

Based on this picture: https://www.badcaps.net/forum/attach...2&d=1416435893
the left side of the resistor is connected the Collector, the right side I cannot what it is hooked up to.

The right side is connected to a smd resistor that went open preventing 5v reaching bl_on pin, traced other end going to the collector of that transistor (which I now noticed I mistakenly called base)

Do you think the size resistor I used caused problems? It was working fine for a day before returning to same symptoms

[budm]

Re: RCA 32LB30RQD - Power on, No picture, sound, or backlight

The 5V that is fed into one side of the resistor, and the other side shows 0.6V is correct because that is the Emitter- Base junction Voltage that the transistor is required to be turned on. If you check the resistance between the Emitter pin of that transistor and the chassis ground, it should be < 1 Ohm, can you verify that resistance for me?
http://www.electronics-tutorials.ws/...r-biasing.html

http://www.dummies.com/how-to/conten...s-a-switc.html

[budm]

Re: RCA 32LB30RQD - Power on, No picture, sound, or backlight

This is typical circuit using transistor as a switch to control the ON/OFF signal, in your case the Voltage is 5V instead of 3.3V.
0V between E-B = transistor off, Collector Voltage will go HIGH toward 5V.
0.6~0.7V between E-B = transistor on, Collector will go LO toward ground (<1V).

[caphair]

Re: RCA 32LB30RQD - Power on, No picture, sound, or backlight

The 5V that is fed into one side of the resistor, and the other side shows 0.6V is correct because that is the Emitter- Base junction Voltage that the transistor is required to be turned on. If you check the resistance between the Emitter pin of that transistor and the chassis ground, it should be < 1 Ohm, can you verify that resistance for me?
http://www.electronics-tutorials.ws/...r-biasing.html

http://www.dummies.com/how-to/conten...s-a-switc.html

The resistance is about 1 ohm. The resistor I soldered in is the 5v line that goes to the collector of that transistor.

I noticed if I move the resistor I soldered a bit the tv works. Maybe poor soldering on my part? I also took voltage measurements at the B of both transistors, when tv turns on the B voltage .67v drops and the other B voltage of transistor behind it goes high to .67v then drops when switched off.

I just can't tell if my soldering is bad or if there's another issue that's making it work intermittently

[caphair]

Re: RCA 32LB30RQD - Power on, No picture, sound, or backlight

Should I try soldering in a bigger value resistor? I know the smd resistor is open because with power on there's only .6v available at the collector of the transistor it's linked to and .6v at the bl_on pin.

With the 100ohm resistor I added it puts 5v at the collector and bl_on pin

[budm]

Re: RCA 32LB30RQD - Power on, No picture, sound, or backlight

Should I try soldering in a bigger value resistor? I know the smd resistor is open because with power on there's only .6v available at the collector of the transistor it's linked to and .6v at the bl_on pin.

With the 100ohm resistor I added it puts 5v at the collector and bl_on pin

OK, what you need to find is that you have 0.6V at the Collector which means that the transistor may be on and that is why the BL_ON line is low, to find out if this transistor is being bias on or not is to measure the Voltage between the Emitter and the Base of that transistor, if it shows 0.6V between E and B then means the transistor is being commanded to turn on, if it is on you do NOT want to put lower value resistor on the Collector due too much current will flow through the transistor and will damage it.

[caphair]

Re: RCA 32LB30RQD - Power on, No picture, sound, or backlight

OK, what you need to find is that you have 0.6V at the Collector which means that the transistor may be on and that is why the BL_ON line is low, to find out if this transistor is being bias on or not is to measure the Voltage between the Emitter and the Base of that transistor, if it shows 0.6V between E and B then means the transistor is being commanded to turn on, if it is on you do NOT want to put lower value resistor on the Collector due too much current will flow through the transistor and will damage it.

Ok when I measured between emitter and base I do get .6v but without the addition of that resistor I added I never see 5v at bl_on

[budm]

Re: RCA 32LB30RQD - Power on, No picture, sound, or backlight

"Ok when I measured between emitter and base I do get .6v but without the addition of that resistor I added I never see 5v at bl_on"
OK, the transistor is on so the BL_ON will be low (around 0.6V) and is normal. If you short the B-E pin together you should then see the Collector Voltage goes high (>2.5VDC).
That 0.6V E-B should not be there when TV is ON to allow the E-C Voltage to go HIGH.

[caphair]

Re: RCA 32LB30RQD - Power on, No picture, sound, or backlight

"Ok when I measured between emitter and base I do get .6v but without the addition of that resistor I added I never see 5v at bl_on"
OK, the transistor is on so the BL_ON will be low (around 0.6V) and is normal. If you short the B-E pin together you should then see the Collector Voltage goes high (>2.5VDC).
That 0.6V E-B should not be there when TV is ON to allow the E-C Voltage to go HIGH.

Ok so what do you think is the issue? And why does the tv turn on with the addition of that resistor I added but never without it

Also that .6v I measured is only there with set off, but on the other transistor next to it there's .6v on E-B when the set is on

[budm]

Re: RCA 32LB30RQD - Power on, No picture, sound, or backlight

"Also that .6v between emitter and base is only there when the set is turned on"
OK, check the Collector Voltage of that transistor when TV is OFF and when TV is ON.

[caphair]

Re: RCA 32LB30RQD - Power on, No picture, sound, or backlight

Ok I think I understand how it's supposed to work now.

The transistor responsible for power on (let's call T1) has .7v at the base when the set is OFF. And the transistor responsible for power off (let's call T2) has .0v at the base with the set OFF.

When I try powering on, T1 base does not go low but remains at .7v

In the few times the set actually powers on T1 base goes low to 0v and T2 base goes high to .7v

Also with set on, T1 only has 1v at the collector. So we know only 1v is needed to trigger inverter on.

With all this info, is it a processor issue or something else? It's intermittent

[budm]

Re: RCA 32LB30RQD - Power on, No picture, sound, or backlight

"When I try powering on, T1 base does not go low but remains at .7v

In the few times the set actually powers on T1 base goes low to 0v and T2 base goes high to .7v"
Since when it works, the T1 receives the correct signal from the processor, when it does not work the processor is not changing the state of the Voltage feeding T1. So the processor gets stuck some time. I do not know what else you can do when it comes down the bad signal from the processor.