It would probably be worth doing some further checks on those caps to make sure that they are not any part of the problem - if indeed there actually is one. I'm not so sure now that I haven't somehow led myself up the garden path, as I said elsewhere in the thread.
The actual type of cap is a high voltage pulse tolerant disc ceramic. I think that these are single layer, aren't they ? Usually, if they do anything, they get an arc track across them, and ultimately the plastic coating splits. These are showing no signs of distress in any way. As to changing them for a different type, I would consider that to be a big no. I would never advocate using substitute parts anywhere in the primary side of a switcher, particularly where high voltages and the magnetics are involved. These things work by the skin of their teeth in the first place, and any parts that I replace, are always sourced to be either dead ringers for the originals wherever possible, or at worst, something of identical construction and spec, from a different manufacturer.
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Didn't you walk right into the problem? It seems that by a yet unknown cause the circuit sometimes runs into some state that makes it oscillate or in some other way fries the resistors. This kind of intermitted faults are the most difficult to solve as most of the times there seems to be no problem. Even your observations with the scope may mislead you as connecting the probe may change the circuit enough to change its behavior.
The low capacitance, high voltage disc-ceramics sometimes used for flyback tuning in TV LOPT stage have been known to fail, ut its usually visible.
Sometimes the ceramic disc cracks and is usually visible because the resin dip also cracks with it, sometimes they track around the edge of the ceramic disc and burn a little "volcano" through the coating.
The company have now come back to me, and accepted the quotes that I have given them for quantity repair on this supply, so it will now be worth spending the time to trace it out.
Yes, that's the type of cap that it is, and those modes of failure are my experience of them also. These show no signs of anything like that, and check ok for leakage - although not actually tested for this at high voltage - and capacitance.
Yes, there may indeed be some kind of intermittent problem. I will put them all back up again on Monday, and see how they perform this time. As to my 'scope muddying the waters, it's generally pretty well behaved in that respect. It's a high quality 100 MHz job, which is always used with a x 10 low capacitance probe. If that is having much of an effect on the circuit, then it must for sure be a pretty poor design. To be honest, I still think that the problem lies with me somehow. Something that I did differently between the first and second times that I tried them, but I'm not sure what ...
Every example that I've seen so far, has the resistors badly discoloured and the print and substrate scorched. On some, one of the resistors has been open, so the network has not been doing the job it was put there for, at all. They are 3 watt resistors, and even when the supply *appears* to be running correctly, they get hot enough for you to say that they are probably dissipating a good 3 watts, and maybe a bit more, so I would guess that you would have to say that from that angle, it's a badly designed bit of circuitry. I am fairly convinced that the purpose of the network is to attenuate the big spike that occurs on the leading edge of the switching waveform. This would tie in with why they have used about the biggest film resistors they could get, rather than using a higher power wirewound type, which would have a fair bit of inductance. I would also surmise that they have used two x 150 ohm resistors rather than a single 330 ohm, to try to spread the dissipation a bit.
When you replace the resistors, they still run hot, with no visible signs on the 'scope of any 'RF' on the waveform, so you'd have to say that it *is* working correctly. What led to this thread in the first place was that when I was initially evaluating these supplies for the company that wants them repairing, after I replaced the resistors, they ran very hot when the supply was loaded, but seemed to just run 'acceptably' hot when it was idling. Likewise, when loaded, the switching FET got very hot, but was almost cold at idle.
However, when I next tried them - same conditions for i/p voltage and load, as far as I was aware - they now seemed to be working much better in that the resistors were just acceptably hot for all conditions, loaded or not, and the FET remained cool also. So this has now led me to believe that it must have been something I was doing differently - and wrongly - when it was running very hot. So, a mistake ? Yes, probably. As you say, we all make them, and this has got to be one of the easiest trades for doing it in.
I've just heard from the company that they are sending a bunch more up this week, so if there's some more of this type amongst them, I'll have some more 'untouched' ones to see what happens this time.
There's allways low inductance thick film resistors, but they have to be generously rated to minimise heating - they were a constant hassle in the video O/P stage in one of the TCE CTVs, running too hot and repeated thermal cycling took its toll on the pins attachment pads.
Carbon film is less inductive. Carbon comp would be ideal, but most EEs these days don't know they exist. Low power SMD metal film resistors do a lot better at UHF and Microwave frequencies. We used them at 10 GHz, with no problems. Larger, high power metal film on a round core are a spiral of metal.
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