Samsung 740B plus won´t start

[selldoor]

Re: Samsung 740B plus won´t start

I think its still most likely that c107 - when you said you changed it was that for a previously used one. It needs to be a good cap with low esr.

[rogfanther]

Re: Samsung 740B plus won´t start

Hmm, well, to heat the opto with the soldering iron, I was near the C107 also ...

The original wasn´t that low ESR ( 0.45 ohms as per the datasheet ) so, as I didn´t find a low one around here, just used a general purpose one.

I´m waiting for the board to cool and giving time for all the caps to discharge, just to repeat the experiment. Will try to put some isolation between the heat and C107, otherwise will try transplanting the C107 from the good psu to the bad one.

[selldoor]

Re: Samsung 740B plus won´t start

OK - you could also check its not a bad joint on that D103 looks a bit odd in the pic.

[rogfanther]

Re: Samsung 740B plus won´t start

Ok, redid joint. I think it is a little odd because I had already replaced that diode.
Well : used the C107 from the "good" psu in the "bad" one, and it didn´t work . Tried also with another cap, from an ATX psu, but same thing.

Also, replaced the optocoupler with one from the parts box, still no change. If I heat the diode side of the opto ( near the CN2 connector ) , voltage climbs up and the controller start working.

Tomorrow I will see if the resistors around the TL431 measure ok, and maybe replace it.

[selldoor]

Re: Samsung 740B plus won´t start

Not much left to go at in that area - a bunch of caps that might work better when heated? How about heating further out and work in towards the opto perhaps start at the diodes above the caps.

[Rtech]

Re: Samsung 740B plus won´t start

Disconnect D104,105.106 and 107 from the transformer side,and see what happens to the Vcc then.If the Vcc then works every time, you have isolated the problem.The next thing, without a diagram I am assuming that C107 is fed from the transformer via D103,you have proved the cap ok, but the diode could be 'leaky', easy to test by swapping it with the one from your good board.

[rogfanther]

Re: Samsung 740B plus won´t start

@selldoor :
Will try heating the diodes. As of now, for it to work, I´ve been heating with the solder iron directly over the opto.

@rtech
I believe that disconnecting the diodes would be the same as not connecting the logic board flat cable ? Without the logic board, the supply seems to come up ok, with Vcc a little low ( around 10.7V ) that I suppose is due to no load. But will try to disconnect the diodes tomorrow.
I have already replaced the diode by a new one.( 1N4007, as it was ) .

[budm]

Re: Samsung 740B plus won´t start

Did you swap the ref diode IC102 (TL431)? It is used to regulate the output voltage by turning on the LED inside the OPTO which causes the opto-transistor on the receiving side to conduct more which will clamp down the output voltage if the output voltage rise above the ref point, or conduct less if the output voltage drops down.

[rogfanther]

Re: Samsung 740B plus won´t start

Good Morning,

Not yet. That will be done today.

[rogfanther]

Re: Samsung 740B plus won´t start

Hi,
Aaaand, ..... it works.

The captain´s log :

- Revised all solder joints. Defect remains.
- substitute the zeners and pwm - defect remains
- substitute the (already replaced ) 1000uF x 10V caps, just because they were a different brand from those that worked in the other psu. Still nothing.
- before replacing the TL 431, observed that the psu would start after just touching the meter probe to the 431 ref output.
- tried reflowing the 431´s solder joints again. Same thing.
- Then, replaced the TL431. Finally, the thing works. Left it rest for some hours to coold and discharge the caps. Tested ok. Repeated some times. Still ok.

So, I consider it is fixed. Will order good new caps for them today, so I don´t need to open this thing again in the next months. Then test it correctly.


Thanks you all for the help.

[selldoor]

Re: Samsung 740B plus won´t start

Great!! not sure how/why heating helped but at least it meant you were able to find the fault.
Well done.

[rogfanther]

Re: Samsung 740B plus won´t start

Thanks. Not let´s hope with the new, good caps it behaves itself.