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

[pjs2015]

I've been having a strange issue that I've haven't seen in the searches I've done. I have a Samsung LN40B640R3F which has a BN44-00264A power supply/inverter board which I think is the problem. If I turn the TV on, it works, but then eventually goes off after a few minutes. Then it won't come on again unless I unplug it for a while.

I checked the standby voltage after I've left it unplugged for a while and then turned it on, and the voltage starts normal and then slowly drops to zero.

Could this be a simple capacitor problem? I visually checked them and they all look good. All the components look good. No burn marks or damage anywhere, as far as I can tell.

[ReeceyBurger123]

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

I had the same issue on the 37" version of this Psu intermittent 5vstby. Could never solve it ended up replacing the whole unit its not the lytic caps btw.

[pjs2015]

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

I had the same issue on the 37" version of this Psu intermittent 5vstby. Could never solve it ended up replacing the whole unit its not the lytic caps btw.

Hey, thanks for the reply! Was yours intermittent in that sometimes it would work and other times not, or like mine? In my case, it's actually reliable in that if I leave it unplugged for a while, then plug it in, the TV will turn on, but then the voltage drops and then the TV turns off. So, not sure if that would be intermittent or not.

[Moreno83]

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

Is it always after a few minutes? If so , turn it on and use a hairdryer on the caps from like 20 cm's away to keep the caps warm and see if it stays on for a longer period.

If not do the same on the main board. Is this also the model that the PS is connected to the main board and from the main board to the T-con? ( just 2 cables inside the tv ).

[pjs2015]

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

Is it always after a few minutes? If so , turn it on and use a hairdryer on the caps from like 20 cm's away to keep the caps warm and see if it stays on for a longer period.

If not do the same on the main board. Is this also the model that the PS is connected to the main board and from the main board to the T-con? ( just 2 cables inside the tv ).

Moreno, yep that's how it is connected. The time on depends on the time unplugged, strangely enough. For example, if I have it unplugged for a week it, then plug it in and turn on, will stay on for hours, but after it goes off, it won't come back on (and no standby light) unless I unplug it for another period of time. I will try the hair dryer. Thanks for the suggestion.

EDIT: Actually I have to leave it unplugged for a while before trying the hair dryer because I had it plugged in recently and watched the standby voltage drop to zero, and now the standby pin won't get any voltage unless it's left unplugged for a while. As another example, yesterday, I watched the voltage drop to zero, I unplugged for about 3 hrs, then plugged it back in. The standby voltage was 1.2V and dropped to zero over about a minute or two. If I leave unplugged long enough, it will have enough voltage to turn back on though like I said.

[tom66]

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

There is a common fault on this model caused by leaky zener diodes that causes issues with the standby/run supply. The zener diodes are in series and on the primary side and are used to limit the peak voltage of the controller Vcc.

[ReeceyBurger123]

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

Hey, thanks for the reply! Was yours intermittent in that sometimes it would work and other times not, or like mine? In my case, it's actually reliable in that if I leave it unplugged for a while, then plug it in, the TV will turn on, but then the voltage drops and then the TV turns off. So, not sure if that would be intermittent or not.

Yep mine was intermitent and no affected by heat.

[budm]

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

See the diagram, the two Zener Diodes that Tom mentions are marked in the diagram, you should check the Voltage output at the Emitter of QB801 when it is working and when it is not.

[pjs2015]

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

There is a common fault on this model caused by leaky zener diodes that causes issues with the standby/run supply. The zener diodes are in series and on the primary side and are used to limit the peak voltage of the controller Vcc.

Wow, I took out one of what I think are the zeners you are talking about and it does appear to be leaky. The zener is at ZM803 on the board in series with ZM805. They are both 12V 1N4742A zeners. When I hooked it up to a test circuit, it was at 12.13V. Couple hours later, it's now at 12.22V and still rising. Seems consistant with my issue. Thanks tom66!

[pjs2015]

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

See the diagram, the two Zener Diodes that Tom mentions are marked in the diagram, you should check the Voltage output at the Emitter of QB801 when it is working and when it is not.

Aw crap. Ignore my previous post then. The one I was testing isn't one of the ones that are circled. I guess what I was seeing was normal then. Thought it was supposed to hold the voltage. I see those circled ones on my board. Thanks for the diagram! Huge help! I will check what you said bud. Appreciate the help!

[gnuorder]

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

I don't see the two zeners in series. I just see zb801. Do you mean the switching diode db802 as well? I've got the same problem.

[pjs2015]

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

I don't see the two zeners in series. I just see zb801. Do you mean the switching diode db802 as well? I've got the same problem.

Here's a picture of them that I made, corresponding to the circles from the schematic as well as an arrow showing which pin is the emitter on QB801. I haven't gone any further with it. I was going to try to replace those components but haven't gotten around to it.

[gnuorder]

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

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?

[pjs2015]

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

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?

Yeah I see what you are saying. Not sure what was meant then. I was thinking about replacing the diodes, but I wasn't able to verify if they were defective. There's a TV repairman that I'm going to be taking it to, who will look at it for a low price. I'll post back if he finds anything.

[budm]

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

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?

The Emitter of QB801 is the output side of that linear regulator circuit, the Zener diode/or Zener diode + diode at the BASE of QB801 sets the output Voltage of the circuit, so if the Emitter output drops to <1V then you need to verify of the input Voltage at the Collector is still present or not, if it is then check the DCV at the Base of QB801 to see if it is still around 15VDC or not.

[pjs2015]

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

Update: Took my TV to a repairman and he had a good power/inverter board that solved the problem. Simple board replacement -- 40 bucks. Still curious as to what specific components were faulty.

[Andrew F. Ali]

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

If you still have your non-working board and really curious as to what the specific components faulty, and you have a meter, take a look again at the previous posts and make resistance tests and compare the boards.

[budm]

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

Update: Took my TV to a repairman and he had a good power/inverter board that solved the problem. Simple board replacement -- 40 bucks. Still curious as to what specific components were faulty.

$40 for repair shop, that is very low price, I hope he can stay in business.

[gnuorder]

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

The Emitter of QB801 is the output side of that linear regulator circuit, the Zener diode/or Zener diode + diode at the BASE of QB801 sets the output Voltage of the circuit, so if the Emitter output drops to <1V then you need to verify of the input Voltage at the Collector is still present or not, if it is then check the DCV at the Base of QB801 to see if it is still around 15VDC or not.

I get 0v at the collector and 8.9v at the base. Before it cuts out I get 15.26 at the collector and 17.35 at the base.

[budm]

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

I get 0v at the collector and 8.9v at the base. Before it cuts out I get 15.26 at the collector and 17.35 at the base.

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