Transformer shot! (was scope SMPS/ capacitor venting)

OMFG

Yes, I read your posts on sed, you didn't tell them this was a Philips scope, and some replies implied Philips made televisions, jeez.

Philips/Fluke made excellent test equipment and many products beat Tektronix into the ground but being European never made it across the pond in large numbers. I've worked in both design and repair facilities and the folk who couldn't repair ANYTHING were the design guys because they always spent 3 days criticizing the original design, another week debating it with their colleagues and then gave it to a repair tech to fix (in about 30 minutes). You obviously fit into the design mentality, so good luck and give the scope to someone who will appreciate it.

Hi Dimitrij, Actually I did unsolder it in the the end as I wanted to inspect the underside for any clues as its specifications. Sadly there were none visible all around the device. Clearly all was not well with it and I'm inclined to believe that it's not an original part; just another poor replacement. When I removed it from circuit I was happy to give it 2W worth of power and see what happened. It just got hotter and hotter until I had to pull the plug at 120'C because my probe won't go any higher. In contrast, a known good 20ohm 2W resistor fed with the same 6.3V rose to 70'C and stopped at that. I have now replaced the old brown 'thing' with a 6W ceramic power resistor which after half an hour of running hooked up to the scope and under load only gets up to 40'C which I'm entirely happy with. The transformer is making a nice gentle, barely audible hiss; not that rasping sound it used to before your diode was replaced; all the DC Vouts are fine and there's no acrid scorching smell coming from the board any more; just a pleasant warm 'smell' of electronics behaving themselves. :) I now feel confident enough to re-insert the board and connect up the VHT and EHT and heater supplies - then we'll see if there's any life in the tube! But that will have to wait until tomorrow or I'm looking at divorce again, apparently. :( Later....

From previous traffic on this issue:

"If these are the maroon-colored parts, they are Philips flame-proof parts designed to run with body surface temperatures in excess of

175C.

"The long preformed leads are thin dia steel, with poor thermal conductivity, in order to reduce thermal conduction to the printed wiring.

"Your real concern should be the temperature of film caps and insulators in the immediate viscinity, which have a lower tolerance to overtemperatures. They should not touch.

"Book hot spot limits for Philips PR01, PR02 and PR03 is between 220 and 250C, depending on the series. This is typical for later metal glaze films. Book derating for normal use is linear, to zero watts at

150C ambient."

Larger bodied resistors with the same power ratings may run at a lower body surface temperature, but this will be of little value if their bulk results in reduced air flow or physical contact with nearby components.

This position should not be filled with a wire-wound component. Metal glaze, at certain values and body sizes, offers a reduced self inductance, allowing them to function in higher frequency snubber circuitry.

RL

So you're saying these ARE the original factory parts? Wow!

I think I can now see how that first diode met its end. Having its own radiant 1 bar fire juxtaposed to it would have lowered its junction barrier height - we all remember how temperature affects p-n junctions - enabling increased current flow and the eventual destruction of the diode through thermal runaway. It was asking for trouble having that resistor running at such a high temperature in this particular position.

BTW, there's an identically-sized power resistor at right angles to the one I've now binned and after a half hour's running, it tops out at only

33'C. I know it's got different working conditions, but in this case it was correctly rated for its job - unlike the other one. Looks like a design error after all, then.

Not a problem with the new component.

Yes, I'm aware of the perils of parasitic inductance in w/w resistors; the one I've selected doesn't suffer from it.

Be nice to see some traces tomorrow (one can but hope!)

Hi

Looks like you've got it going back to normal operation, finally!

The whole thing might just have been victim of a resistor with a "hot channel" (old carbon resistors will sometimes do this without reason).

Or an improperly replaced one - likely the same result. No matter if it broke down with a low resistance path or if it got replaced improperly.

You can check if that was the case (if you still kept the resistor) by hooking it up to a power supply with an ammeter. If you see it heat up and then suddenly the amperage rising and the resistor starting to heat even faster (as if that was an NTC in there) then it very likely has developed a hot channel (a small area not quite unlike an arc path that conducts lots of current as it overheats).

If that's the case, it may explain several things. For once, this resistor was intended to provide a field reset function for the inductor. The circuit would likely require the reset to be quite "complete" (the field decaying to zero) between switching cycles.

If the resistance is lower than normal, the field will decay slower, and a resistor with a hot channel would likely slow the winding reset long enough to be "incomplete" - that is, by the time that the next switching cycle arrives, there would be still significant remaining current flowing through the inductor and the diode.

This can be a condition that the power circuit might not like. There is only a moderately fast diode in there - the original BY208 is not really fast. As long as the winding reset is complete, that might not matter because the diode would already be "out of charge carriers" when the next pulse arrives, so there would not be much reverse recovery.

If the reset happens to be incomplete because of the resistor breaking down, then the diode would still be conducting when the pulse comes and that would bring its (quite long) reverse recovery time into equation. During that reverse recovery time the diode would pass current and the resistor (and subsequent circuitry too, up to the resonance LC) would be getting "hammered" with large 800 V voltage spikes from the switcher, thus it would be subjected to high peak currents because of the spikes.

Of course, hammering the resistor with enormous voltage spikes won't be very good for its reliability, and since it was already breaking down, would also accelerate the process a lot (and stress other parts too).

Dimitrij

Unless you've actually cracked one open, you will not be able to tell what the element construction is.

RL

No need. I have a VNA. Better than x-ray vision.

I know! It just seems so weird having the thing running like a top for the first time, not having to worry about overheating, not having that horrible burning smell and the rasping noise - plus all the voltages where they should be. :) But I couldn't have done it without you, Dimitrij. I'm enormously grateful for your assistance and your lengthy explanations have been a huge help to me in getting to grips with this 'alien technology' LOL.

Anyway, it's late here now so I'll report back tomorrow for what will hopefully be the last time.

Amen to that. Been quietly following this from the start and have learned a lot. Dimitri, are you a professional SMPS designer by any chance? I admire your carefully-considered 'measure twice, cut once' approach.

One thing I've learned the hard way. NEVER throw out any replaced components until the repair is done!

Yes! I can confirm it is now back up and running and early checks don't seem to show any other faults. I've got four nice bright sharp traces and all the buttons and knobs seem to do what they should (although I haven't exhaustively tested *every* single function at this time, of course). The only error I made was switching it back on from 'cold' - it clearly hasn't been on for a long time and I really should have brought it back to life on a variac but I forgot. There were some initial crackling noises and whatnot but they gradually subsided; some noisy knobs obviously as only to be expected, requiring lubing up but nothing else noteworthy.

Many thanks again to all who assisted. The kids at the local school electronics club will soon have a new toy to add to their test gear. :)

Good advice, but it's all fixed now (see my other post of a moment ago).

and everyone has a pair of pliers, or a hammer.

What did the network analyzer say about this particular resistor? What's the impedance and phase angle between 10MHz and 100MHz?

You didn't use it previously on the caps and transformers.....?

RL

Yes, but in the process of investigating the part, you destroy it.

Why on earth would I need to know that?? The frequency of operation here as you well know is only 20kHz.

Sorry, was there a cap I needed to know the SRF of? As for the transformer, what would the VNA have told me that the other metrology didn't? It was testing under the high working voltage levels I'd have liked at the time to check for breakdown; no VNA I know of will do that.

I should quit while you're still only slightly behind because you are beginning to sound like a troll. You know what they say: "when you're in a hole, stop digging."

Seeing as how everything's perfect now, I can only agree with you.

I just hope the little tykes at the orphings home enjoy their new toy and appreciate all the hard work and brain power went into its restoration.

Halleluyah!

RL

Are you sure that you have not mixed up the windings? Maybe the two

1.8V

windings are actually the 2 symmetrical "innermost" ones, the 3V ones are the "medium" ones and the 15V are the "outermost" windings? Your measured winding voltage ratios "1.8:3.0:13.0 volts" and the schematic output voltage ratios "6.7:13.4:60.7 volts" (I've added a little compensation for 0.7V silicon diodes) are (from a purely ratiometric point of view) not very far away from each other. In fact they are so close that the differences between the smaller ones can be easily explained by your measurement errors (how accurate was that 0.8V measurement anyway?) and the possibly intended uneven loading of the power rails in the scope.

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EdithLeman