Samsung LA40A650A1F no 5.3v standby

[maxvalutech]

Power board BN44-00197a has 126v on hot section nothing on cold side. Have checked transformer and coils for continuity and OK . did replace 6 caps on top rhs of the low voltag e area although originals all look OK . the set apparently working normally was turned off but wouldn't start next day no light no clicking nothing. Board has no sign of any problem. Located in country Australia and components not readily available and prices for another board not justified these days. Have schematic and understand basics. Any experience of this problem appreciate your comments.

[budm]

Re: Samsung LA40A650A1F no 5.3v standby

So what DCV do you have between the two legs of the main filter cap?
What GND did you use to measure that 126V? Which test point is that that gives you the 126V: AC? DC?
It is impossible to have 126V reading on the power supply board, you must be using the wrong GND ref.

[maxvalutech]

Re: Samsung LA40A650A1F no 5.3v standby

Thank you for reply. CP802 and 803 side by side read 343v DC. I only have phone to put up any photos and not the best camera!!

[maxvalutech]

Re: Samsung LA40A650A1F no 5.3v standby

Was using board as earth sorry. Now 126v DC on DM801 as measured between grey side of it and neg leg CP803.

[budm]

Re: Samsung LA40A650A1F no 5.3v standby

So you do have Voltage to run the stby power supply, the fuse F801 T3.15A is good.

[maxvalutech]

Re: Samsung LA40A650A1F no 5.3v standby

Hi yes FI801 good. The voltage certainly on hotside
Caps on 5.3v line seem likely but replaced and as well this wasn't the usual thing with warnings etc just went from good to bad overnight.

[budm]

Re: Samsung LA40A650A1F no 5.3v standby

Do you get stby Voltage if the main board is not connected to the power supply board?

[maxvalutech]

Re: Samsung LA40A650A1F no 5.3v standby

No joy with that. I also jumpered pins 1 and 3 just in case but no luck. How do I test DM805 on the heatsink? In circuit? My experience was really with valve TVs when the screen virtually told us what was wrong. Did transistors as well but only recent mucking about with success was with self evident faults such as bad caps burnouts etc. I do this now rather than scrap an otherwise good unit and if they go pass them onto other old fogies who appreciate the bigger screens.

[budm]

Re: Samsung LA40A650A1F no 5.3v standby

You check the Diodes DM801, DM803 whci are for the switched 13V, DM805 for the STB5.3V use Diode mode on the meter and also test with Ohms mode, no need to remove them unless the readings look bad.
You also need to check ZDM802, ZDM803 Diodes, the caps CM802, 814, 815 capacitance and ESR.

[maxvalutech]

Re: Samsung LA40A650A1F no 5.3v standby

Thanks again. I have concentrated on output side because of 126v DC to earth reading at jumper RM801 and now note that is a minus voltage and i really have no idea what the voltage should be at that jumper. Caps you mention are rated 50v. Will try and get diodes checked difficult to get at some.

[Alastair E]

Re: Samsung LA40A650A1F no 5.3v standby

The hot side is floating at 150 odd volts with ref to chassis. For hot side checks use filter-cap neg term as neg for meter.

Cold side will give usual readings ref to chassis....

[maxvalutech]

Re: Samsung LA40A650A1F no 5.3v standby

Been busy watching Aussie Open tennis etc!! Do not have any vcc at all by looks of it as no voltage of any type on ICM802 or following components inc DM806. The 126v is on ICI806 and CI822 ref to filter cond leg. Half components are embedded in board and these include overload circuit. Will keep working back.

[budm]

Re: Samsung LA40A650A1F no 5.3v standby

you cannot use the chassis GND as the GND ref when making the DCV reding in the HOT side of the circuit, you need to use the Negative leg of the main filter cap as the GND ref for your meter.
So what DCV do you have between the two legs of the main filter cap in the HOT side of the circuit? around 325VDC? We need this Voltage first.
You need to concentrate in the STBY power supply section first.

[maxvalutech]

Re: Samsung LA40A650A1F no 5.3v standby

Measurement on main filter caps is 343v but can't get any readings on input side of standby circuit as stated above. No vcc from ICM nor any reading on any of the 8 legs. Now trying to trace back to other components earlier in circuit to see where voltage stops - should there be about 15v feed to the standby circuit and could brown out circuitry affect? Just trying to get a handle on basics but lack of test points and voltage info not shown except for 5.3 and 12 audio.

[budm]

Re: Samsung LA40A650A1F no 5.3v standby

You said fuse FI801 is good, then you should have 343VDC with ref to the Negative legs of the main filter caps of either CP802 or 803, that fuse (page 3) feeds the Voltage to STBY transformer TM801 pin 6, so you should verify if you have the DCV on pin 6.
ICM802 STBY SMPS IC ICE2QS01 HV pin 4 and 5 are the startup Voltage receiving pins, what do you have on that PIN?
What do you have on VCC pin 7 which the cap CM814 is charged by by the HV pin until it reaches the VCC ON THRESOLD of 22VDC for the circuit t start up, onec that happen the AUX winding (PIN 1 ,2) of the TM801 will provide the running Voltage.
See diagram page 2 and 3 to see how it works.

[maxvalutech]

Re: Samsung LA40A650A1F no 5.3v standby

There is 343v DC on DM804 so it seems that section to transformer is OK. But nothing to any pin of ICM802 which picks up its voltage from PFC via 3 in line resistors. Has the ICM gone bad as I would think input voltage should be readable at least if some other component after it was bad.

[budm]

Re: Samsung LA40A650A1F no 5.3v standby

So the resistance of those three 3.9K resistors connected in series to provide the 22VDC start up Voltage to pin 4/5 are OK? Check the resistance between HV pin 4 and the GND pin 8 to see what resistance you are getting, it just a simple Voltage divider to drop the 325VDC (230vac x 1.414 = 325VDC on the main filter cap), you AC line Voltage must be higher than 230VDC for you to have 343VDC on the main filter cap, your meter is OK and in calibration?

[maxvalutech]

Re: Samsung LA40A650A1F no 5.3v standby

Thanks to you getting somewhere now. First two resistors read spot on the third reads open circuit! Not used to these embedded components the good ones have a very small black block in between ends third one nothing. No reading between pins 1 and 8 on ICM. Over here we are nom 240v ac .

[budm]

Re: Samsung LA40A650A1F no 5.3v standby

Great, we just need to get that startup Voltage to that IC running.

[maxvalutech]

Re: Samsung LA40A650A1F no 5.3v standby

I will check a couple of boards I have to see if there is a close match I can solder onto the PCB and then advise the result - here's hoping!!