Help required for an old Alba 19" combi TV

[Max_UK]

TV is an Alba ATVD91186P (Unable to locate circuit schematic)

A friend found that their kitchen TV was no longer working.
A check of the power board showed that 1 of the 2 secondary paralleled schottky diodes (SB5100 - 50p each) was dead short.
Both diodes changed & set tested via variac - isolating transformer & 70W load limiting lamp.
Standby led now lit, & set turned on via remote.
Brief screen flash & then set shut down with load limiting lamp flashing very briefly. (Sec voltage steady at 12.66V until shut down)
This prompted me to suspect one of the four 470uF 25V capacitors on the secondary side - all changed but no improvement.

Then decided to try my lv power supply directly on the 12V secondary side of the psu.
Everything worked normally at 12 V 1.80 A. (Set is nominally rated at 45W on mains side)
Screen illuminated, picture & sound normal, remote working correctly. Ran for 2 hours without any problems or excessive heat.
So it seems that the ccfl strips & drivers are ok. As is the main board.

Options are:
Throw the whole thing in the bin & admit defeat
Adapt the set to run on a 12V input direct from a power brick (approx £7), but it would be more satisfying to repair the fault.
Fit a used replacement psu board, available for £20-30 but not economically viable.(TV was only a cheapy originally & now virtually worthless)
Hopefully, one of the great brains on this forum will say "Simples, the fault is?"

Have watched a lot of smps videos, but none with this fault.
My thinking is that as the original fault was a shorted diode, the fault must be on the secondary side?
(But that works fine with direct 12v power to it).
If a primary fault, would the module not be permanently damaged? & not re-settable.
If a protection shut down, why when the load seems normal?

Sorry to be a little long winded, but grateful for any feedback.
Max.
 
 

[SilverSolder]

How long between set turns on and stops working - does it work for a few minutes, a few seconds, or does it overload and die within milliseconds?

[keymaster]

remove the lamp limiting and the variac .
the variac is not something to use with an smps power supply and the lamp limiting trick is only until you bring the psu back to live, after that ,it will only cause problems when you put a load to the psu.

[shakalnokturn]

From photos seen on the www, I'd check the small C3(?) capacitor, looks like it would be PWM supply decoupling and is in a hot spot on PCB.

[Max_UK]

Thanks for taking the trouble to reply guys.
The psu would shut down as soon as the ccfl drivers powered up, approx 5 secs.
Special thanks to keymaster, you were correct.. I was being over cautious.
Following your advice, I upped the lamps to 110 watts (still too chicken to run without them) & the set powered up normally.
I then had the confidence to flick the lamp overide switch (which also dropped the power drawn from approx 30W to 27W)
(Also tested a couple of other smps with various lamp combinations & they both shut down on protection when the lamp wattage was too low)
The only reason I was using the variac was to have a "normal" mains voltage of about 230 at the set.
As can be seen from the picture, without the variac, my isolation transformer is outputting 250 volts.

So for future reference, I now know to use at least 2.5 x smps wattage for the current protection lamps & not be such a wimp.
It's been a useful learning curve in the repair & operation of smps & as always, eevblog has come to the rescue.
Thanks again, Max.

[SilverSolder]

Nice work! 

Another gremlin bites the dust...