Help with Tatung TLM-1905 LCD no power at all

[jbshocks]

I have a Tatung LCD monitor that does nothing at all. No LED on power button nothing at all. It had some bad caps which I replaced and that did nothing. I hooked the back lights to a different brand power supply and the display itself works fine. With power plugged in I have voltage on the large yellow transformer on the plug side nearest the big filter cap. Around 60v. I would think that mean the bridge rectifier is fine as well as filter on the output side of the transformer I have nothing. No voltage at all. I checked the resistance on the output and it is roughly 1 ohm. Would that indicate the transformer is dead? It does not seem to have been hot. There is no writing on the transformer to determine a proper replacement. Am I on the right track? Are these transformers some what universal? Any idea what I could replace it with? I would be tempted to buy a board but I can't seem to locate one. It is a LM004 Rev 3.

[selldoor]

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

Have you checked the fuse at F1,?

Check for continuity - post result in ohms
Check voltage on each side Black probe on ground screw probe each side with red
Meter on 200vDC

[jbshocks]

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

F1 checks fine

[jbshocks]

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

Where circled in black I got voltage. Red circles, nothing.

[selldoor]

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

Have you checked the fuse at F1,?

Check for continuity - post result in ohms
Check voltage on each side Black probe on ground screw probe each side with red
Meter on 200vDC

I didnt say but you need to post result of voltage test.

[retiredcaps]

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

When troubleshooting consumer electronics, one of the most important voltages to look for is standby DC voltage. This is typcially 5V DC.

In consumer electronics, we rarely need to measure AC.

List pin number and DC voltage for the connector like so (black probe goes on a ground screw on chassis).

pin 1 = 0.0
pin 2 = 5.0

[retiredcaps]

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

See budm's lcd troubleshooting guide in my sig file link in the above post for the basics.

[jbshocks]

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

I didnt say but you need to post result of voltage test.

on 200v DC tested ground to each side of F1. Nothing. 0. Also tested ground to each pin connector. All 0. Ground screw to input side of transformer 63 v. I will try to get better pictures and read the guide.

[selldoor]

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

Between the BIG cap and the transformer there is a small cap - what voltage is on that meter on vdc200 probe on each leg joint

[jbshocks]

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

Between the BIG cap and the transformer there is a small cap - what voltage is on that meter on vdc200 probe on each leg joint

it is c109 and it has 64.5 on each leg to ground.

Also testing with meter on 200 vdc the area in orange is the only area of the board with any power at all.

[Rtech]

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

But this ground is NOT the same as the other one you used.You must measure across the small cap itself,Black to the side where the stripe, is red to the other side.

[jbshocks]

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

But this ground is NOT the same as the other one you used.You must measure across the small cap itself,Black to the side where the stripe, is red to the other side.

Measuring across c109 (small cap) 0 v dc

[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 ????

[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

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

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

[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.

[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

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

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.