Samsung LA32B350F no backlight

[beechman35]

Re: Samsung LA32B350F no backlight

Will test with variable power supply and limiting resistor out of cct to see what voltage the zener is at. Not sure if we have 15V SMD if not, we could temp install an axial to test the TV. Be in touch

Thanks
Rob

[budm]

Re: Samsung LA32B350F no backlight

Even 12V will do, we just want to see the Base Voltage will rise up.
If you analyze the circuit you will see that for the Voltage at the BASE of the transistor to be at 0.70V, there must be very low resistance.

[beechman35]

Re: Samsung LA32B350F no backlight

Hi Bud, OK, here is the latest with some crazy theories so bare with me. We pulled the 15 volt zener and tested it, found regulating properly so refitted it. Now to save putting the board in and out of the TV, I decided to rig up a variable DC supply to the collector of the opto coupler with a 1 amp diode bridging the opto to simulate it turned on and powered up the bench supply. low and behold, 15 volts at the emitter of QB801. Pulled Fuse FM802S and fed a second low voltage DC supply into the junction of resistor RB807 and RB816 to simulate the divided sensing voltage from the PFC. The figure I calculated if the PFC was running at 400VDC (approx. value) would be 2.78 volts. At this sensing voltage, the regulator kept running. I increased the sensing voltage until the op amp tripped and this happened at 3.6 V ( converted over the voltage divider network, this would be approx. 510V PFC). When the op amp tripped, the result at QB801 was exactly what we saw when installed in the TV, Collector volts at 18, 0.7 on the base and 0.5 on the emitter. All this at the moment makes me think that when the PSU is installed, the PFC is actually overvolting and tripping the voltage sensor (UB802) and shutting down the PFC controller. A quick check of RP809,810,811,812,813 and 814 the values appear good in circuit. I am looking at the PFC controller (UP801) as the possible culprit.
Have you seen a failed controller cause an overvolt ( all be it a very quick one) or should I look somewhere else.

Thanks again

Regards

Rob

[budm]

Re: Samsung LA32B350F no backlight

That does not make sense.
But if you look at the PDF in post 22, there is nothing that can pull down the Base Voltage to 0.7V even with the Latching circuit is activated (the two transistors are fully on) sine there is no need to to make the Emitter Voltage to be at 0V to stop the circuit from working, the out put Voltage at the Emitter just have to be below the Operating Voltage requirement.
Second with 0V at the Emitter, that will not make the Op Amp to run so the detection circuit is not active until you have 15V at the Emitter. Did you replace the Zener? Since it is SMD by just heating it and removing it may have cause it to function for now.
The PFC circuits do fail and produce higher Voltage than it should.
What you need to d then is to put everything back and install new Zener just incase, then monitor the PFC Voltage to see if it did went over the trip point or not.

[CapLeaker]

Re: Samsung LA32B350F no backlight

I'd say more measurements are needed as just about every idea got shot down mid air. Use budm's pdf in post #22 and fill in more voltage measurements at critical points he circled.

[beechman35]

Re: Samsung LA32B350F no backlight

Just refitted the power supply into TV and still no go. Looked at previous post and realised that the results of testing image did not post so will repost it here. Will do some more testing shortly with a storage CRO attached to see if the PFC is tripping overvolt. From what I can see from the two transistors (QB802 and 803), this looks like a crowbar circuit that will trip and latch the power supply off if it senses overvolt. CCT is reset with power cycling. (I think).

Will attach results again and do more soon, (have to do some real work now, Electronic engine control of a jet engine is playing up, this one pays the bills).

[budm]

Re: Samsung LA32B350F no backlight

"From what I can see from the two transistors (QB802 and 803), this looks like a crowbar circuit that will trip and latch the power supply off if it senses overvolt. " That is correct, it is latching circuit that will clamp down the Voltage at around 5V or so which will be too low for the PFC IC (FAN 7530) controller to work.
Vth (stop) Voltage is 8.5V typ. (page 5 of the spec sheet).

[beechman35]

Re: Samsung LA32B350F no backlight

Hi Bud,

Sorry for the delay in replying, but had to fix a problem with engine controller. OK, This is interesting after reading your last post. You say that in the overvolt condition, the regulator circuit will clamp the voltage to around 5 volt. This is not happening and the system is clamping to Zero volts (well 0.7 to be precise). Lots of trialling but no real success, except for something interesting. If I remove the PNP transistor (QB802) the regulator appears to work on by bench set up with the 2 variable power supplies. When the sense voltage is increased to the trip point ( comparing the Vref signal to the variable signal on pins 2 and 3 of UB802) the power supply folds back to 10volts output on the emitter of QB801. I thought,"Gold we have a win". replaced the PNP transistor and no difference, back to dropping the power supply off completely.

More testing to do......

Regards
Rob

[budm]

Re: Samsung LA32B350F no backlight

Since it is latching circuit, if it is actually in latching mode then you should see around 0.6V at the Emitter and Base of those two transistors and it should be there to keep it in latching mode.
we need to find out the PFC Voltage is tripping the latch or not. You can also remove the resistor RM869 which feed the B+ from Emitter of the QB801 so the PFC IC will not be running which in turn will not produce the Over Voltage error signal to trip the Latch circuit then the Voltage should be present at the Emitter, you should put 1K Ohm load on the Emitter of the QB801.

[beechman35]

Re: Samsung LA32B350F no backlight

Hi Bud,
Confusion reigns supreme right now. I pulled RP823, the zero ohm link that feeds the PFC chip ( I saw you mentioned RM869 but that is the current sense resistor for the switch mode,) soldered a 1K resistor onto the output of the reg and powered up. 0.7 volt output from the reg. I then pulled one of the chain resistor group (RB804) so there was no voltage sense going to the regulator and now have 15 volts at the emitter of QB801. (obviously no lights due to link still removed). If I measure the voltage at the main cap, I still only have 330V and this should be below the over volt cutoff. I have checked in circuit RB807, 808,817,803,804,805 and 806 and the divider appears good. I am thinking of pulling all the divider network resistors and checking out of cct now as this is all I can think of.
Let me know if you see anything I have not.

Regards

Rob S

[budm]

Re: Samsung LA32B350F no backlight

"I pulled RP823, the zero ohm link that feeds the PFC chip ( I saw you mentioned RM869 but that is the current sense resistor for the switch mode,)" Sorry for the typo.
So some how it at 330V is still trip the protection circuit, so if up put RB804 back in place and check the Voltage across CN805 to see what you have.
I see that there are 3 resistors, the two of them (RB808, 817) are in parallel so it looks like they are do some Voltage trimming for the sensing latching circuit.
May be those resistors in the sensing circuits drifted in value.

[beechman35]

Re: Samsung LA32B350F no backlight

OK, RB804 refitted. RP823 still removed and wire soldered onto CB805 (again I assumed you meant this cap to measure divider volts)
TV in standby 1.02V, TV switched on, very quick pulse to over 1.5V then settles at 1.2 V. I havnt measured with DSO as yet, but I amd suspect of voltage overshoot somehow. To follow on from this, I measured the main cap voltage CP815 and what I saw was interesting. The voltage with AC supplied is 330V, when TV switched on, very quick kick up and then settles on 328V, but I left the meter connected to cap and removed the AC and the voltage rapidly dropped (like in 2 seconds) to 7V.
I would have thought the cap would hold for a while? Am I low in capacitance or is something else dragging it down? Or is the paranoia getting to me?

Thanks again

Rob

[budm]

Re: Samsung LA32B350F no backlight

The standby power supply is always on so it draws current from the filter cap all the time until the Voltage drops down low enough for the STBY SMPS IC to run.
I do not expect 1.5V is enough to trip the protection circuit.
The ref Voltage for the comparator circuit is set to 2.5V by the UB801 2.5V Vref IC (KA431S). So the trip Voltage has to be above 2.5V.

[budm]

Re: Samsung LA32B350F no backlight

BTW, I see now how the Voltage is pulled down to <1V at the BASE, when it latches you have Vce-sat of QB803 (it is turned on when output of UB802 (LM393 series dual comparator IC with open Collector outputs) pin 1 goes high) + Veb of VB802.

[beechman35]

Re: Samsung LA32B350F no backlight

I see all your saying and agree with all. I have checked the voltage ref chip and the voltage is 2.48V so reference is good. All I can think of is a spike somewhere is getting into the comparator CCT and tripping it off line before the regulator can start. I am going to pull all the voltage divider resistors and test out of circuit.

Will let you know

Regards

Rob

[budm]

Re: Samsung LA32B350F no backlight

This is a good head scratch-er.

[beechman35]

Re: Samsung LA32B350F no backlight

Hi Guys, an update on the TV. Found a power supply on Ebay for 35 dollars from a cracked screen unit. Installed and away it goes. Great picture no dead pixels so pretty happy about that. Shame I couldn't get the original supply working but I learnt a heap about switch modes and PFC's which is sure to help me on other repairs. Thanks to all that helped, especially Bud.

[budm]

Re: Samsung LA32B350F no backlight

Thanks for the repair follow-up.

[mrobo]

Re: Samsung LA32B350F no backlight

I got probably same problem as you I appreciate any help, by me are the voltages so
1. Power On/Off 0,76V <--> 0V
2. 5,15V
3. GND
4. 13.03V
5. GND
6. GND
7. 5,15V
8. 5,15V
9. GND
10. 13.03V
11. 13.03V
12. InverterOn/Off 5,11V <--> 0V
13. E-PWM 2,03V <--> 0V
14. DET_5V 0,12V <--> 0V
15. GND
16. GND

on big filterring capacitor i measure 309-310V I have repaced it for another one but get same voltage results

[Agent24]

Re: Samsung LA32B350F no backlight

If voltage is too low the PFC circuit is probably not working or not getting enable signal