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Wharfedale L32TA6A lcd tv - trips to standby after a few minutes.

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Contrast John:
Hi All
Guess this could be a stock fault on this model. A customer arrived with two identical sets both with the same problem. They`d run fine for the first half hour or so then trip to standby. Both sets would start again ok but would now run only 10-15 minutes before tripping back to standby. The next start would only give a 5 minute run before reverting to standby.
The problem was traced to thermal runaway in U30 (AP1501-50) a 12Volt in / 5Volt out Buck Regulator on the signals board. Its operating frequency appeared to fall as the temperature rose. Presumably this caused the temperature to rise further, creating a runaway effect until a thermal trip inside the IC was fired. Nearby capacitor CA100 470mF 25v looked stressed but was not in fact the cause of the problem as replacing it first didn`t provide a cure.
A permanent solution was provided by soldering a strip of copper heatsink (taken from an old Salora tv chassis) to the mounting tab of the regulator and bending it into a safe position. A similar strip of copper was soldered to the reverse side of the board underneath the body of the regulator. This made a dramatic difference to the operating temperature of the IC. Where it had been 'burn your finger' hot, it was now quite bearable to touch. Both sets were ran on test for several days with no problems before being returned to their owner.

John.

daz:
hi john
yes this is a very common problem on this set, i have seen quite a few over the past year or so,i too have resorted to attaching a heat sink to the reg which works great, it would be nice to find out what causes the reg to overheat, i must have spent days trying to find the fault but to no avail, perhaps someone knows the answer and will let us know?
daz

Contrast John:
Hi daz
Thanks for the reply. Spent a fair while wondering what I`d missed or why the fault had taken so long to appear from new - about 18 months in this case. Could only perhaps conclude that the thermal conductivity of the bond between pcb and IC had been lost with time. Would have liked to try another regulator in case it was internally faulty causing it to run hotter than normal, but couldn`t find a supplier. Think it was just a case of inadequate heatsinking (none!) in the first place, but guess like a lot of things these days I`ll never know.
Regards
John.

daz:
hi john
i have tried swopping the reg(as well as other things) from a working set, it wasnt causing the fault, and ive had the fault on very new sets as well as old, think ill put this fault down witchcraft and continue to fit the heatsinks.
daz
 

Contrast John:
Hi daz
Thanks for the info on the regulator swop. Like your theory on the cause of the problem -it would explain everything!
Regards
John.

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