Samsung LN40B640R3F -- standby voltage slowly drops/drains to zero

[gnuorder]

Re: Samsung LN40B640R3F -- standby voltage slowly drops/drains to zero

It must be measurement error, Collector of QB801 is the input, the Base is connected to the Zener diode/diode and biased through the RB814 and OPTO PC802S, the Base Voltage cannot be higher than Collector Voltage, if the Collector is 0V then there will be no output at the Emitter of the QB801.
The setup is just a simple linear regulator with OPTO used as a switch.
You should check the solder joint of the transistor, OPTO, and the connection from the Collector all the way back to diode DM802.

http://www.radio-electronics.com/inf...ry-circuit.php

I did assume the middle connector of the transistor was the base so those may be reversed. Looking at the photo pjs2015, they are reversed and I found the opto. Solder connections look good for all but the opto which I haven't checked yet.

[budm]

Re: Samsung LN40B640R3F -- standby voltage slowly drops/drains to zero

I did assume the middle connector of the transistor was the base so those may be reversed. Looking at the photo pjs2015, they are reversed and I found the opto. Solder connections look good for all but the opto which I haven't checked yet.

Middle pin is Collector.
KTD1347.

[gnuorder]

Re: Samsung LN40B640R3F -- standby voltage slowly drops/drains to zero

So if the collector is going to 0v, the opto is switching off. I should measure the voltage between rb814 and rb815 to verify that it's shutting off.

[budm]

Re: Samsung LN40B640R3F -- standby voltage slowly drops/drains to zero

The Voltage should always be present at the Collector as long as there is AC feeding the TV, the Voltage is generated by the standby power supply power transformer, see circuit for details. I would check the DCV between the two legs of the filter cap CM804, check diode DM802, RM806, solder joints.

[gnuorder]

Re: Samsung LN40B640R3F -- standby voltage slowly drops/drains to zero

The Voltage should always be present at the Collector as long as there is AC feeding the TV, the Voltage is generated by the standby power supply power transformer, see circuit for details. I would check the DCV between the two legs of the filter cap CM804, check diode DM802, RM806, solder joints.

The voltage across CM804 was 8.89 but was slowly dropping, 8.50 last check. The voltage across RM806 was 0v.

[budm]

Re: Samsung LN40B640R3F -- standby voltage slowly drops/drains to zero

The voltage across CM804 was 8.89 but was slowly dropping, 8.50 last check. The voltage across RM806 was 0v.

That cannot be right, if it that low, the standby power supply will not be running since the shutdown Voltage for the stby IC UM801S (STRW6252) is around 8~9.8V. I expect the DCV on CM804 to be around 18V or so.
Must be error reading.
You do have stndby Voltage, right?

[gnuorder]

Re: Samsung LN40B640R3F -- standby voltage slowly drops/drains to zero

That cannot be right, if it that low, the standby power supply will not be running since the shutdown Voltage for the stby IC UM801S (STRW6252) is around 8~9.8V. I expect the DCV on CM804 to be around 18V or so.
Must be error reading.
You do have stndby Voltage, right?

Apparently not. I measure 0v at the connector between pins 3 and 4.

[budm]

Re: Samsung LN40B640R3F -- standby voltage slowly drops/drains to zero

Wait a minute!
So this is no longer true? The standby must have been working before.

I'm getting back to this TV. I'm getting 14.5v at the QB801 emitter when it's working and 0.22v when it's not.

I'm still confused about the two zeners in series. I only see one zener ZB802 and one diode array DB802. I ordered replacements for both. We'll see how it goes. Did you have any luck?

[gnuorder]

Re: Samsung LN40B640R3F -- standby voltage slowly drops/drains to zero

Wait a minute!
So this is no longer true?

Yes, that is true. When I started working on it today I was getting close to 15v off the emitter, now I'm getting 0.23V. I can let it sit for a while and start again. It will have to be tomorrow though.

[gnuorder]

Re: Samsung LN40B640R3F -- standby voltage slowly drops/drains to zero

Sorry it's been a while before I could get back to this. I measured today to remind myself. At first I have stb but it goes to 0 after I turn on the TV. It use to be slow but now much quicker. When I first plug the TV in, I get 5.15 at stb and it seems stable. Once it shuts off, I can't get stb to come back unless I unplug it a while. First plugged in today, I got 5.15v at stb, .22v at the emitter of QB801, 17.2v at the collector and 0 at the base. When I power on I got 15.35v at the base, 9.1v and dropping at the collector and .37v at the emitter. By the time I got to stb, it was at .15v so it's dropping much faster now. I measured CM804 between ground and DM803. Wait, I take that back. I have been measuring at DM802 thinking it was DM803. I just found DM803 so I'll have to retest the collector voltages after I let it sit for a while. I'm getting a stereo scope soon to make this easier. My eyesight is getting bad.

[gnuorder]

Re: Samsung LN40B640R3F -- standby voltage slowly drops/drains to zero

Wait, I take that back. I have been measuring at DM802 thinking it was DM803. .

Never mind, they are common.

[dick_barton]

Re: Samsung LN40B640R3F -- standby voltage slowly drops/drains to zero

What voltage do you have across CM808 (22uF 50V cap) when you lose 5VSTB?

[gnuorder]

Re: Samsung LN40B640R3F -- standby voltage slowly drops/drains to zero

What voltage do you have across CM808 (22uF 50V cap) when you lose 5VSTB?

It fluctuates between roughly 13v and 10v. Also, I don't get 5vstb at all any more.

[Davi.p]

Re: Samsung LN40B640R3F -- standby voltage slowly drops/drains to zero

Hi, i don't have seen circuits, nor followed your measurements, but i suggest you to replace el.caps on the secondary side, first to the standby circuit then to rest if it doesn't fix, el.caps can go short under load sometimes, psu goes in protection and you see voltages go down slowly on caps..

[gnuorder]

Re: Samsung LN40B640R3F -- standby voltage slowly drops/drains to zero

I get a brief voltage between DM802 and RM806 but that drops to zero. I still get around 8v on the other side of DM802. I can no longer get stb to come on even after letting the TV sit unplugged. I replaced CM804 and CM808 even though they didn't seem bad.

[Davi.p]

Re: Samsung LN40B640R3F -- standby voltage slowly drops/drains to zero

i told about changing el.caps on the secondary side...