flatron L1915s

[sabre504]

Hi have this board from a 19" Flatron started out with no power .
I have changed all the caps for rubycon YXA YXG but am still getting no power output from the CW201 connection that goes to the switchs and logic board .
Power is going in as far as the main cap showing 339v (UK 240v AC)

I know the logic board and switchs work as i rigged up a temp power supply and had them working .
Any suggestions on what to check next as am coming up short or am missing something .

Thanks in advance for any input

[alexanna]

Re: flatron L1915s

Did you also change the small 50v start up caps around the SMPS coil?

[sabre504]

Re: flatron L1915s

Yes changed them all

[alexanna]

Re: flatron L1915s

Do any of the voltages come back with the logic board disconected?

[Rtech]

Re: flatron L1915s

PSU/Inverter ONLY,with BLs plugged in,power on,put 3volts Dc into the BL_ON and earth pins,and prove the Power supply is correct,then you can move on from there.

[jetadm123]

Re: flatron L1915s

Sounds like the power supply isn't starting up. Try measuring the VCC of the PWM chip, U101, pin 7. Ground is pin 5. Start voltage for the chip should be approx. 12V. If not, try checking the diode connected to VCC pin for shorts. We know that the startup cap is good, since you changed it. Also check resistor(s) on the VCC line for correct value and check the mosfet, Q101, for shorts.

[sabre504]

Re: flatron L1915s

NO power whether logic board connected or not ,
I will check the other suggestions and report back later .

Thanks

[sabre504]

Re: flatron L1915s

Hi all tested U101 no power going in or out,
Q101 tests ok showing 25 ohms on each leg ,
having fun trying to test resistors tho some reading open then reading resistance if i swap leads about ??
Am going to desolder them and test off the board .

[PlainBill]

Re: flatron L1915s

Hi all tested U101 no power going in or out,
Q101 tests ok showing 25 ohms on each leg ,
having fun trying to test resistors tho some reading open then reading resistance if i swap leads about ??
Am going to desolder them and test off the board .

Stop!!! What the dickens are you doing? IF Q101 is reading 25 ohms on each leg something is seriously wrong. If no power is going into U101, again, something is seriously wrong. There is no need to test resistors at this point.

1. With the board disconnected from the logic card, resting on an insulating surface, and AC power connected, CAREFULLY measure the voltage across C101. It should be 1.4 times line voltage (if you are in the USA or Canada 120 x 1.4 = 165).

2. If you are not getting any voltage across C101, check the fuse. If it is good, CAREFULLY check for AC across the center two leads of BD101. If that reads zero, report and wait for further directions.

3. If you ARE getting the correct voltage across C101, measure the voltage at U101 pin 7, using the negative lead of C101 as the ground point.

4. If U101 pin 7 still reads 0 volts measure the voltage at U101 pin 7. If that also reads 0 volts wait several minutes (or more) for C101 to discharge, then unsolder 1 end of R108 and measure it's resistance. If correct, report and wait for further directions.

By the way, the excellent pictures makes helping a LOT easier.

PlainBill

[sabre504]

Re: flatron L1915s

Thanks for the slap Plain bill
I must have not been totally with it when i was testing i will start from scratch .
But i am getting the correct voltage at the main cap so will go from there
Thanks again for the input

[PlainBill]

Re: flatron L1915s

Thanks for the slap Plain bill
I must have not been totally with it when i was testing i will start from scratch .
But i am getting the correct voltage at the main cap so will go from there
Thanks again for the input

There was an error in my earlier post. It should read "4. If U101 pin 7 still reads 0 volts measure the voltage at U101 pin 1. If that also reads 0 volts wait several minutes (or more) for C101 to discharge, then unsolder 1 end of R108 and measure it's resistance. If correct, report and wait for further directions."

Before the SMPS starts, U101 pin 7 (Vcc) gets it's power from U101, pin 1. If U101, pin 1 is low, either R108 is defective, or U101 is defective.

PlainBill

[sabre504]

Re: flatron L1915s

Hi got around to starting from scratch all test ok up to u101
Pin 7 showing no voltage but pin 1 has 283 volts so must assume u101 is
not working so will get another but am going to check my working LG board and see if it has one i can borrow to make sure.
Will report back when i have done this

Thanks all

[sabre504]

Re: flatron L1915s

Found another fan7601 taken off a working board
Pin 7 showing only 8.5 volts , checked R108 (47k) comes out just under 50k on my meter .

No power at the CW201 connector

[jetadm123]

Re: flatron L1915s

Hi got around to starting from scratch all test ok up to u101
Pin 7 showing no voltage but pin 1 has 283 volts so must assume u101 is
not working so will get another but am going to check my working LG board and see if it has one i can borrow to make sure.
Will report back when i have done this

Thanks all

From what I can see from your photos, it looks like pin 1 of U101 is connected to the main cap through resistor R108. 283V seems a bit low. If your mains voltage is 240VAC, then you should see approx. 240x1.4=336VDC across the main cap.

[PlainBill]

Re: flatron L1915s

From what I can see from your photos, it looks like pin 1 of U101 is connected to the main cap through resistor R108. 283V seems a bit low. If your mains voltage is 240VAC, then you should see approx. 240x1.4=336VDC across the main cap.

Note that he was measuring the voltage at pin 1 of the SMPS controller. The filtered mains voltage is dropped through a 47K resistor, so 283 is not unusual. A 50 volt drop across a 47K resistor means the smps controller is drawing a little over 1 ma.

PlainBill

[PlainBill]

Re: flatron L1915s

Found another fan7601 taken off a working board
Pin 7 showing only 8.5 volts , checked R108 (47k) comes out just under 50k on my meter .

No power at the CW201 connector

8.5 volts at pin 7 is within the acceptable operating range, but too low for startup to occur. At this point it's hard to tell if the smps controller is trying to start, or was damaged when you removed it from a working board.

Points to check - pin 6 of the SMPS controller. If you see intermittent activity, the SMPS is trying to start, but failing. Possibilities are an excessive load, or a shorted diode. There is also a possibility that D102 is bad.

Unsolder one end of J1. It provides power for the inverter. If the problem is the inverter, that will remove the load.

If that checks out, remove D102. If the voltage at pin 7 starts jumping up and down, D102 is probably leaky.

While the chances are extremely remote, check for any solder shorts from replacing the caps.

PlainBill

[sabre504]

Re: flatron L1915s

Ok thanks will check these out and post back .

Will recheck the solder on the caps and reflow to make sure .

[retiredcaps]

Re: flatron L1915s

While the chances are extremely remote, check for any solder shorts from replacing the caps.

I just did a Benq repair yesterday. After I replaced the new c5707 transistors I tested them with a multimeter and noticed a short. I rechecked my soldering and saw a solder bridge. Luckily I checked my work otherwise I would be redoing all the work again.

[sabre504]

Re: flatron L1915s

HI well rechecked everything ,
again the pin 7 on the ic is showing 0 volts so am going to put this to one side while i order some new ones.

Thanks for all the input so far will update when the new parts arrive

[bcstedt]

Re: flatron L1915s

I'm working on the same unit. The back-light will flash on about 5 seconds after power is applied, just long enough to see that the monitor is working.

A quick visual reveals one bulging cap (C203 Samxon 680u 25V 105C) and the board is baked dark brown between T301 and T302 (centered around HS301 HS302). I'll take some measurements and update as I go along. Thanks for all the info here.