Help with Tatung TLM-1905 LCD no power at all

[jbshocks]

Re: Help with Tatung TLM-1905 LCD no power at all

diode check 470 one direction and 143 the other ohms 89.8 one direction and .46 on 20k scale other direction. By the looks of it I did not change that little cap. I will see if I have one. It doesn't look bulged in person

[budm]

Re: Help with Tatung TLM-1905 LCD no power at all

In the last picture, i can see clamping Zener Diode ZD102 (on the bottom side of the board), can you take the resistance reading?
By the way, did you also replace this small cap, it has bulging top. I do not see any soldering works on its two solder pads.

[jbshocks]

Re: Help with Tatung TLM-1905 LCD no power at all

see if this picture is better

[jbshocks]

Re: Help with Tatung TLM-1905 LCD no power at all

removed fuse. no voltage on either pin or across them. I will try to get another picture later.

[budm]

Re: Help with Tatung TLM-1905 LCD no power at all

I just want to make sure that it is not in the safety shutdown mode due to shorted circuit in the inverter circuits, that white fuse feeds the B+ to the inverter circuits.

[jbshocks]

Re: Help with Tatung TLM-1905 LCD no power at all

OK, lift up one leg of the white fuse and then check the voltage on those two pins as shown in red circle. You can run the board by itself just have the power cord attach to it.
Can you also take the straight shot of the bottom side of the board, the pictures you posted are tilted and it is hard for me to trace out the connections.

So un solder the fuse and test for power? I ca do that after work but I don't think there will be anything. There is no sign of power on that side of the main yellow transformer.

[budm]

Re: Help with Tatung TLM-1905 LCD no power at all

OK, lift up one leg of the white fuse and then check the voltage on those two pins as shown in red circle. You can run the board by itself just have the power cord attach to it.
Can you also take the straight shot of the bottom side of the board, the pictures you posted are tilted and it is hard for me to trace out the connections.

[jbshocks]

Re: Help with Tatung TLM-1905 LCD no power at all

The 3.5 I get on the little 8 pin chip sg3842gs that I replaced. I also get 3.5 across the small 22uf cap. I get nothing on any pin of the white connector going to the other board.

[budm]

Re: Help with Tatung TLM-1905 LCD no power at all

That is an Optoisolator for the voltage regulation feedback circuits.
"I get 3.3v between pin 5 and 7 " Are these the pins of the long white connector that the wires go to the main board?

[jbshocks]

Re: Help with Tatung TLM-1905 LCD no power at all

what about this little chip? It is P721F, call a switching power supply. How would I test it. Could it be the cause? I have power on the two legs with in the zone that power works.

[jbshocks]

Re: Help with Tatung TLM-1905 LCD no power at all

I changed out the chip and the little 22uf cap. Symptoms the same. Only difference now is that I get 3.3v between pin 5 and 7. Still no voltage on board connector.

[jbshocks]

Re: Help with Tatung TLM-1905 LCD no power at all

Thanks Rtech, I will order one up from digikey and give it a shot.

[Rtech]

Re: Help with Tatung TLM-1905 LCD no power at all

The required start-up current is typically only 5uA. This
ultra-low start-up current allows designers to supply the
start-up power required by the SG384xG using a
high-resistance and a low-wattage start-up resistor.Thats from the datasheet,so on initial switch on some current is drawn from the Rectifier Bridge to the VCC Pin to start
off the sequence to get the chip running.If the chip is faulty then you could get exactly the symptoms you have.

[jbshocks]

Re: Help with Tatung TLM-1905 LCD no power at all

diode reads 500 one direction and nothing in the other in diode test mode. This is out of circuit. I could easily change out the chip. My concern is that I have no voltage on the chip vcc

[Rtech]

Re: Help with Tatung TLM-1905 LCD no power at all

At first look at the datasheet, then 15V DC needed between 5 and 7,which is the same as across the cap,and you have nothing.You need to check the diode connected to the +ve side of the cap, undo one end and check either with resistance or diode test on the meter...POWER OFF for this.You can replace the cap and try that as well, otherwise as these transformers do not usually fail...I have never seen one fail..you would need to replace the chip I suspect.

[jbshocks]

Re: Help with Tatung TLM-1905 LCD no power at all

FYI, found this thread. https://www.badcaps.net/forum/showthread.php?t=14042 Same power supply same symptoms. I tested pin 5 to 7 nothing. in fact tested pin 5 to all pins. Only pin 3 was non zero and fluctuated between .8 and .9v. Unfortunately no resolution was posted.

[jbshocks]

Re: Help with Tatung TLM-1905 LCD no power at all

chip says sg3842gs. I can see on the board it goes to pin 7, verified with an ohm meter.

[Rtech]

Re: Help with Tatung TLM-1905 LCD no power at all

Provided that is the Start up cap !! then you need to identify the part code written on U101,on the bottom of the board, as this looks to be the SMPS chip,you then need to find the datasheet for that,see what pin that cap goes to etc.Also there should be a diode connected to the positive side of that cap,the other side of the diode going to the transformer ??

[jbshocks]

Re: Help with Tatung TLM-1905 LCD no power at all

across the big cap is 169 v DC. Here is a little better picture. I don't have a very good camera.

[Rtech]

Re: Help with Tatung TLM-1905 LCD no power at all

Across the Big cap you should have approx 155v DC IF your mains voltage is 110V AC ????