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

[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.

[tv john]

Hi guys,I had similar fault on a Bush LCD37TV016HD , could it be the same chassis ? U8 was hot and cut out to standby after time.It is a TIP31 shape (OLD REGULATOR FROM DONKEYS AGO ) and I bolted it to the chassis with extension leads and an old TIP31 isolating kit. This was ages ago,but I remember I only had to remove the centre section of the back cover.

John.

[Contrast John]

Hi John
Think these are two different chassis with different types of regulators.
The AP1501 regulator used in this model is a 5 pin switched mode type. Although in a similar shaped package to a TIP 31 only a short tab with no drill hole was available for mounting, the device being designed for surface mounting on the pcb with the copper print doing the job of a heatsink. I managed to find some spec on the AP1501 and found it rated at 3 amps which ruled out replacing it with a more standard 3pin 7805 type 5 Volt regulator rated around 1.5 amps. Like yourself I considered re-siting the AP1501, but was put off by the likelihood of extension wires introducing an inductance into the circuit and affecting the operating frequency of the IC. This left only the option of soldering a heatsink onto the tab which fortunately worked quite well.
Would be usefull to know if anyone has found or used a better rated device than a 7805 with perhaps a rating of 5 Amps or so, that would be handy to use in these circumstances and would allow re-siting with wires, it not being a switch mode type.
Regards
John.

[Turnip]

Hi John,
Considering efficiency of linear compared to buck regulators, a linear might need a considerable heat sink.

Tried it a while ago on some of those 10inch tubed portables when original buck regs became obsolete,  before the noisy replacements became available, needed nearly a square foot of blackened aluminium as the drop was about 5 volts at 3 amps. Never had one back though - Chris.

[Contrast John]

Hi Chris
Yeah, that was the basic idea. Attach it to a huge sheet of metal (space permitting) and test with moist finger whilst looking out for steam! It would be nice for once to be able to buy a standard off the shelf part to do the job, instead of spending hours researching and eventually modifying circuit designs.
Always up for the challenge though.
Regards
John.

[Turnip]

Well said John,

As both you and I are still in the biz, whilst some others, still awaiting approved mods have taken up more profitable occupations, doubtless requiring little original thought - well, and so on.

Penuary is us, think to invent a radio with rubber bands - again  - Chris.