Philips 42PFL8654 - no standby voltage - two shorted SB260 diodes - Power supply DPS-298 ticking noise

This Philips 8000 series of the generation 4 came in dead. It wasn't starting, no sign of life. This pointed to a stand-by power supply problem.

With the multi-meter in continuity mode I quickly located two shorted diodes on the secondary side of the stand-by supply. The diodes were Vishay SB260. I probed around the supply lines for shorts and found nothing. This is strange. Why did both of the diodes die? Maybe one blew first and the other would not hold on for much longer due to overload. I have never seen two diodes in parallel in any power supply so far.

I didn't have those diodes or similar on stock, but some BYV29 ultra fast diodes were available. One of these can cope with more current than two of the little ones, so I gave it a shot.

Plugged the TV in and it started just fine. The image came up all right. That was too easy, I thought. I left the TV running for about two minutes and suddenly, an intermittent ticking noise like from a Geiger counter came from the high voltage transformer, which feeds the backlight. A cold solder joint? I reflowed all the critical joints, even though none of them looked suspicious. As expected that didn't fix it.

So, those were the possibilities:

- The transformer had an intermittent short. I would expect it to go up in flames if that was the case.

- Some other weird problem with the transformer.

- At least one those blue ceramic 6kV capacitors was about to die:

I unsoldered all four caps and measured them with the capacitance meter. All had their specified values. Then I pulled out my funky Chinese DUOYI DY294 transistor and cap tester to check for break through voltage. This little device can produce high voltage and test whether a part withstands it or not, using 1mA leakage current.

All caps measured similar with about 1450V. I didn't know what to think about that. They are rated 6000V. It was outside the spec of the tester (max 1000V). 

As all caps were identical in that respect, I concluded that a fault was not measurable with my equipment. The output of the transformer produces much higher voltages, so it was still possible that at least one cap was faulty. Or I was on the wrong path altogether and the transformer was bad.

I checked eBay with the power supply board id DPS-298 and a number of repair kits came up! One contained the two diodes and the four caps. I ordered one, even though I was not convinced that the caps were the culprits. For 5€ there is not much harm done.

Compared to various images on the net and on eBay

like here:

or here:

my caps looked like new. I wonder why they burn up like this. I better not measured their temperature, for it is high voltage which either hurts me or destroys my meter.

This thread discusses the same problem my TV had:

http://www.badcaps.net/forum/showthread.php?t=35814

After a night's sleep I doubted that the four caps were responsible for the noise. I started measuring a little more. The inverter drives two windings in parallel. The primary side looked like it had a center tap, but the two coils are arranged symmetrically. The circuit is a mystery to me. The outer transformer pins are driven against ground and V+ by the FETs. The inner pins are coupled via 0.47µF to V+ and 0.47µF against ground. It must be some kind of resonance circuit.

I first located the noise in the transformer (because my mind said that it was logical) until I switched the light off to see if there were visible sparks. Nope. Nothing to see. Being blind, my ears overruled my mind and found the real noise source: The 0.47µF cap, which goes to ground from the test point.

The cap could not be the source of the problem, because it goes to ground and the measured pulse increased the voltage. However, the sudden voltage spike caused mechanical noise in the cap.
The two FETs are in a half-bridge configuration and they were my new suspects, because they control the voltage to the primary winding. Maybe one had intermittent failures?

After some searching, I found the circuit for another Delta power supply DPS-283 at Elektrotanya, which was close enough to the DPS-298. This design is missing the four caps on the secondary side, but the primary side seems identical.

For some reason, this resonance game had glitches. And this is what the clicking noise looks like on the scope. From flat 210V a spike of 230V.

This website offers a specific repair for the power supply and a number of spare parts for it:

http://www.electronic-doc.de/28-reparatur

The price for the FETs was too high for my taste and I found a cheaper source:

http://www.tv-ersatzteile.de/fqpf10n60ct.html

and ordered two pieces.

Screw that. It must be the transformer. Swapped all four caps and the high-side FET, which controls the positive voltage to the inverter transformer. Nope. Still noise after a some warm-up and bending the board changed the pattern. I shot freezer spray all over the inverter circuit and nothing reacted to it.

This power supply is an atrocious design. I know at least of four different regular failures by now (caps, diodes, clicking noise transformer, inverter control chip).

I gave up on this one.