Need help identifying smd

[caphair]

Hi guys I've been working on a dual din radio for a car.

Problem is sometimes turns on sometimes down. I traced the tft_on line to a smd transistor? Has markings "halg"

What I noticed is there's 0v on top lead then when powered on it momentarily goes high to 5v then drops to 0v (this is when the display powers on as it should)

When the display doesn't turn on the top lead goes high to 5v and doesn't drop stays high at 5v

Would this part be the problem or something else? I've included a pic

[budm]

Re: Need help identifying smd

Check the Bottom PIN to see if it change state and the reading is about 0.6VDC.

[caphair]

Re: Need help identifying smd

When it powers up as it should yes the bottom pin is .6v

When it doesn't power up the top pin is at 5v and bottom as 0v

What does this mean?

[budm]

Re: Need help identifying smd

That means that Transistor (NPN) is working, when it has 0.6V to bias on the Transistor, the Collector (Top pin) will swing LOW, when there is no Bias (0V) the transistor will be off so the Collector Voltage will swing high (5V).
So it gets the command signal from some where.

[caphair]

Re: Need help identifying smd

Ok so the bottom pin is the base and I should trace what's connected to the base to find the problem?

[caphair]

Re: Need help identifying smd

Q33 seems to be connected to the base of Q12 and when powered on there's .6v on base of Q33 but nothing on its collector remains at 0v. Would Q33 be the culprit?

[budm]

Re: Need help identifying smd

Q33 seems to be connected to the base of Q12 and when powered on there's .6v on base of Q33 but nothing on its collector remains at 0v. Would Q33 be the culprit?

If Q33 has steady 0.6V between B and E then it will be staying on so the Collector will stay LOW all the time, so Q33 is being told to turn ON, are you sure that the Q33 B-E Voltage is not changing state?

[caphair]

Re: Need help identifying smd

Yes there's 0v at base of Q33 before pressing power button. When pressed base of Q33 gets .6v

[budm]

Re: Need help identifying smd

Yes there's 0v at base of Q33 before pressing power button. When pressed base of Q33 gets .6v

So now you you need to trace out where the Base pin of Q33 goes to next.

[caphair]

Re: Need help identifying smd

But when base of Q33 gets .6v with power on command, there's no voltage to Q12 when the display doesn't turn on.

When it does turn on Q12 gets .6v from Q33

So Q33 won't be the fault? Base of it connects to a resistor

[budm]

Re: Need help identifying smd

'But when base of Q33 gets .6v with power on command, there's no voltage to Q12 when the display doesn't turn on.'
So Q33 is turned on because it has 0.6V to Bias it on, so the Collector of Q33 (which is connected to the Base of Q12) will be LOW which turns OFF Q12 so the Collector of Q12 will be HIGH (5V), so that is what will happen.

'When it doesn't turn on Q12 gets .6v from Q33'
So when Q33 does not get the 0.6V to turn on, then the Collector of Q33 will go high to provide the 0.6V to turn on Q12 which will make the Collector of Q12 to be LOW (0V).

So the circuit is doing what it is being told to do, so for you to get 0V at the Collector of Q12, the Q33 must be stopped from getting the 0.6V command signal to turn on.
Q33 is getting command from processor or what ever that generates the command signal to do what it suppose to do.

[caphair]

Re: Need help identifying smd

Yeah sorry that was a typo I edited it to say "When it does turn on Q12 gets .6v from Q33"

So it seems like Q33 signals Q12 to turn on but it's not doing that every single time. Still would be something else? Could I bypass Q33 so .6v goes directly to Q12?

[fzabkar]

Re: Need help identifying smd

I suspect that even if you forced the display to turn on, it would remain blank or indeterminate. That's because the micro that switches the transistor also generates the information for the display (unless there is a slave micro).

[budm]

Re: Need help identifying smd

You can short B to E of Q33 which will turn off Q33 so the Collector will rise to bias on Q12, this will force Q12 Collector to be LOW which is the condition you want as indicated in post 1 (drops to 0v (this is when the display powers on as it should)).
You have control signal problem some where on the board.

[caphair]

Re: Need help identifying smd

Ok so here's a quick rundown of operation. Just remeasured to simplify

Power off/standby
Q33 base - .22v collector 0.22v
Q12 base - .22v collector .22v

Power on (display on)
Q33 base - 0v collector .62v
Q12 base - .62v collector 0v

Power on (display stays off)
Q33 base - .68v collector .0v
Q12 base - 0v collector 5v

[caphair]

Re: Need help identifying smd

You can short B to E of Q33 which will turn off Q33 so the Collector will rise to bias on Q12, this will force Q12 Collector to be LOW which is the condition you want as indicated in post 1 (drops to 0v (this is when the display powers on as it should)).
You have control signal problem some where on the board.

Why is Q33 biasing Q12 anyway? I mean is Q33 needed if Q12 turns on when base is .6v which is could get without Q33?

[budm]

Re: Need help identifying smd

Q33 Collector may be used for driving other circuit beside Q12 otherwise Q33 is not needed since the two transistors are acting as double inverter circuit.

[caphair]

Re: Need help identifying smd

I shorted B to E of Q33 and now it powers on every time with the difference that the display turns white briefly before the logo and screen pop up. Is it safe to continue using this way?

Also didn't think to short B to E of Q33 I was thinking shorting B of Q33 to B of Q12. Would this yield same outcome as now?

[caphair]

Re: Need help identifying smd

Spoke too soon. Now I just get a white screen when turning on with B to E shorted on Q33

[budm]

Re: Need help identifying smd

I shorted B to E of Q33 and now it powers on every time with the difference that the display turns white briefly before the logo and screen pop up. Is it safe to continue using this way?

Also didn't think to short B to E of Q33 I was thinking shorting B of Q33 to B of Q12. Would this yield same outcome as now?

No, you do not want to do that.
The whole purpose of switching those transistors is to have proper sequencing on so you'll not see what you are seeing. Like I indicate, you have bad command signal going on with the processor.