Transformer shot! (was scope SMPS/ capacitor venting)

If you've got UF4007s, then what are you waiting for? Polyester film caps in low voltage circuitry are the last things to suspect. Their perfomance is most easily assessed in the working unit.

After making whatever node tests are made convenient by the transformer's absence, stop screwing around and reassemble the unit.

No benefit is obtained by running the unit unloaded unless the loaded outputs produce non-typical loading effects, as measured on the transformer output windings and rectified outputs.

Unstable waveforms will produce the same voltage ratios as a steady signal. The present switching circuitry is an excellent signal generator for the application, having survived all insults so far.

RL

Even if there are bits flaking off them?? That's the case here!

Obviously a professional technician just wants to get each unit fixed as soon as possible so as to get on to the next one and maximise his income. But I'm just a hobbyist and my motivations are not at all the same. Of course I'd like to get this up and running, but if I don't *learn* something from the experience, then it'll be next to worthless AFAIC. So you might see it as screwing around to run these side-by-side diode tests from your perspective, but I really don't. This unit is beyond economic repair, but I'm still working on it - for a little while longer anyway - whereas a professional service person could not afford the time on what he would see as a basket-case.

Well, that's how I look at it. But I do remember back when I was a teenage r working on what was then new technology (transistorized TVs) and the boss trying to get me to check the "Goldenrods" (RCAs service bulletins back th en). I didn't want to because I wanted to track the problem down myself.

Even today on a slow day, I'll spend a lot more time on something that isn' t economically worth the effort just to solve the puzzle. Even us old griz zled veterans aren't immune to such things.

Well I'm pretty old and grizzled myself! Just getting stuck back into troubleshooting again after a 30yr. lay-off. So much has changed! Anyway, the plan was to replace one suspect part after another one at a time and test in between each replacement so as to identify the specific part which is at fault (the new caps arrived today, btw). However, I only got as far as replacing that diode (the by134) with Dimitrij's suggested

4007 and *something* has *definitely* changed. The 20ohm power resistor is warming up *much* more slowly and the hissing noise has gone. The limiting factor now is not the 20ohm resistor, but the improvised dummy load (a 30ohm 40W w/w resistor) which gets too hot long before the 20ohm circuit-board part. In fact even with the dummy load disconnected, the 20ohm resistor doesn't get hot in a hurry like it did before. So like I say, *something* has changed and that something can only be the diode swap. Can't believe it would make that much difference, surely? More testing as soon as I find a better DL...

Decided to use the scope itself as the 'dummy load' by plugging the psu back into it. It now takes 1m 25s for the power resistor to reach 50'C whereas previously it was just under 15s., so an unmistakable improvement. Does anyone know what temp I should expect this resistor to run at, BTW? I mean if they're good for 70'C I could leave it powered up longer and see if it tops out before reaching that.

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.

RL

It's been many years since I worked on power supplies that had large wattag e resistors in it, but I do remember some 10 watters running hot enough to sizzle water or spit off them, and that's when they were running normally. That would put it over 100C I guess.

It's important to remember what hat you're wearing, when you're performing specific tasks.

If this guy had originally stated that he had a Philips scope and that he just wanted to sniff it's perfume, I would never have bothered responding.

RL

As for the transformer test: This is actually quite a generic test that is often used to test inter-turn winding isolation under high voltage conditions by the manufacturers of motors, inductors and transformers. Normally they use a specialized piece of equipment called an IWT, and the test is routinely performed in production. Unfortunately an IWT is expensive (like $2500 and up), and rather specialized, so the typical repairman won't have access to one unless he works at a place where they are commonly used.

The test frequency actually doesn't matter, and most common IWTs won't go all that high. In the past, when IWTs still had a tube screen, they needed a steady repetition rate in order to display the trace. So 50 Hz (or whatever your country's line frequency is) was not unusual. Nowadays they all have flat screens and lots of sample memory, so the repeat rates are usually from "single shot" to maybe a dozen a second.

Since you've told us in the past that you have a CRT oscilloscope (you posted a picture of a noisy signal on a switching transistor that was shown on such an instrument), a test circuit should have some impulse rate that is reasonably fast that you'll be able to see a steady trace on the screen. That's where my 100 Hz came from. You can obviously go lower, the circuit has no lowest limit, but because of the resistor in the charging circuit it won't likely be able to go much faster. 10k is already a low resistance for 300-something volts and 10W is also quite considerable, so making it faster would mean making it beefy and power hungry too, and these side effects would outweigh the benefits.

For the actual test, one single impulse would theoretically be enough.

In practice however you'll want a repeating pulse train for 2 reasons. First, in order to see it (unless you also have a digital scope to capture a single pulse), and second, in order to see the state of the isolation properly (usually broken isolation will arc in some sort of semi-irregular fashion and that may not be visible with only one try).

The test is done in such a way that the coil (under test) and the resonance capacitor (inside the IWT) are connected together while at the same time a very fast charging circuit "charges" this LC resonant circuit to a preset voltage and then immediately disconnects itself. The LC tank is then allowed to "ring down" naturally without outside interference and the ringing waveform is observed.

The frequency of the ring wave is determined by L and C, the duration by the coil's resistive losses. A defective coil (shorted or with arcing isolation) will have a very low "Q", so there will be basically no ring wave, just a fast decay from the charging peak down to zero.

In your case, you can use 15 nF for the cap, so that it will match the same conditions like in the actual power supply. This means that the ring wave will have not only the full starting voltage but also the actual "correct" resonance frequency. With these conditions you can of course take wave shape measurements with an oscilloscope on any output of the transformer, not just on the primary. They all should show the same shape and the voltages should all be realistic (but of course brief, since each pulse doesn't last very long). The cap and charging circuit should of course be connected only to the primary, to make for realistic test conditions.

With enough pulses per second you should get a bright steady trace.

The 10 W resistor should allow for continuous duty operation, so you can take your time looking at the scope. Lower wattages will also do, but will need limited test duration with cool-down periods for the resistor.

Regards Dimitrij

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.

RL

[...]

Ah! I kind of suspected it would be, hence my query...

OK, I follow that...

[...]

Thanks for the details, Dimitrij. I think I can cover for all of that without any trouble. You do explain things with remarkable clarity I must say. It may yet not be necessary if my component replacements succeed, but it's good to already have the steps to follow should they fail.

[...]

Oh, Dimitrij, I meant to say your diode replacement has made a bigger improvement to the psu than we expected. The hissing noise has almost gone and the power resistor now takes almost a minute and a half to reach

50'C instead of less than 15 seconds using the old incorrect BY134 diode. I now have sufficient time to do some probing around under mains power! First up I plan to test the rectified outputs from the long secondary winding to see if they are anywhere near the 6V-60V range they should be. I'll report back with the results tomorrow. Many thanks for that!

Wow, that's quite something! I wonder how this thing ever worked in its previous state. Given the change you describe, that square-to-sine wave circuit must have been as "good" as completely non-operational.

Now that it looks like it's "almost working", the supply might even run again in the scope (to some extent at least), but the fact that the resistor still slowly heats up a little, may indicate that it's slightly out of resonance now. I mean, the frequency is not completely wrong, but it might be just somewhat off-center.

No exact idea yet, but I think I'm beginning to see a pattern:

Here's my guess, not sure wild or not, so take it with a grain of salt and the usual precautions of a power supply repairman :)

It looks like the thing may have drifted a little bit out of resonance over the years. That can happen, electronic parts age and tolerances slowly increase. Running out-of-resonance, the power factor of the resonant circuit was probably no longer close to one, but instead the resonant circuit began to pull reactive power. If you try to drive an LC circuit with a frequency that is slightly wrong, the driving source will still force the LC into its frequency, but there will be a phase shift between voltage and current. The further off the frequency, the larger the phase shift will become. A phase shift means that a load is no longer purely resistive, but also reactive (either capacitive or inductive depending on direction) and so the power factor gets lower. As the power factor gets lower, the total current draw increases (imagine a constant current due to resistive load plus an additional current due to the reactive part of the load, which increases).

Now my guess, what may have happened:

The thing drifted over the years, and the total RMS current was slowly increasing because the frequency wandered away from resonance and the power factor of the resonant circuit was going down.

With the RMS current becoming larger, the load on the diode also became larger (it depends on the total RMS current of the LC circuit, no matter whether that's resistive or reactive), and the diode heated up more.

Sooner or later, after many years, the diode finally overheated and shorted out, immediately blowing the fuse. Somebody saw this and replaced it. But he did not know, with what speed grade to replace it properly, so he put in a particularly slow one without thinking.

Now with the slow diode in place, it no longer blew the fuse, but instead the energy recovery circuit became barely operational.

It still ran for a while, but without the square-to-sine conversion, it was driving the (now no longer really resonant) main transformer with a rather square-ish looking waveform.

That waveform caused large current spikes in the resonant capacitors (you know, capacitors don't like being driven with a square wave, charge current peaks go through the roof if one tries to do that).

These peaks, plus possibly the low power factor and reactive current (the frequency may or may not have been re-adjusted after the diode repair) were now stressing the "new" diode (that was wrong anyway) and also the resonance capacitors. The capacitors did not like this additional stress (and they may have drifted over the years already).

When you stress film capacitors with large repetitive current spikes, it slowly erodes and embrittles the foil electrodes inside. The capacitor still tests OK on an LCR meter and even on an ESR meter, it may still look like working, but under full load conditions it can no longer sustain large currents. It becomes like as if someone has put an inrush current limiting device on it, it can no longer supply peak loads (people who repair photoflash units often find this fault in the HV trigger capacitor, it tests with a correct capacitance, but can no longer supply a strong current pulse for triggering).

Now there were probably two "processes" going on, accelerating each other. The resonant capacitors (C1807, C1808) were degrading and letting the frequency drift ever more out of resonance. The wrong diode degraded too. When you try to feed a slow diode with large high frequency current peaks, it can also degrade even more and become even slower and more like a high-frequency short circuit. Both things probably started slowly, but were accelerating each other until something really broke.

Now you've replaced the diode, so it should be OK again from the diode point of view. But the resonant capacitors may have degraded and may now be in a pitiable state. They are difficult to test because the problem usually means good LCR meter readings, but much reduced power handling capability only when running at full power.

My advice would be to replace them anyway. This sort of degradation is difficult to test, so better safe now than sorry later.

With new capacitors, the resonance frequency will somewhat change (you know, component tolerances, degradation of old ones, slightly different values of new ones...). The change won't be drastic, but it may be significant. Therefore I would advise you to measure the new resonant frequency and then readjust the power supply's working frequency if it happens to be different (R1827 is the FREQ trimmer).

To measure, you sweep the primary winding with a signal generator (with the transformer in circuit and connected, but the transistor V1806 disconnected and no loads attached to the outputs). Look for maximum amplitude with a scope and measure the frequency with a counter. Compare the measured value with the one that the circuit runs at (fully reassembled, with dummy load attached to avoid "light-load" mode).

If they deviate, VERY SLOWLY and VERY CAREFULLY readjust R1827*. The service manual says how to do it. See chapter 3.4.4.2.1

"This potentiometer is a factory adjustment control. THE SETTING OF THIS POTENTIOMETER MUST NOT BE DISTURBED UNLESS IT IS ABSOLUTELY IMPOSSIBLE TO SET THE 12.7 V WITH THE AID OF POTENTIOMETER R1826* (FEEDBACK). Adjusting procedure:

- Set the main input voltage to 220 V.

- Turn R1827* (FREQ) fully anti-clockwise.

- Check that the voltage on the positive pole of C1831* is 12.7 V +/-

100 mV; if necessary; readjust potentiometer R1826* (FEEDBACK).

- Set the main input voltage to 170 V.

- Check that the voltage on the positive pole of C1831* is 12.7 V +/-

100 mV; if necessary; readjust potentiometer R1827* (FREQ)."

If you've had to readjust "FREQ", better re-test with 220 V afterwards, when you are finished, and double-check the "FEEDBACK" setting again. Readjust "FEEDBACK" again if there is a voltage mismatch on 12.7 V.

Note that in the service manual that you linked to, the part names are different, like C1843 instead of C1831 on the schematic. But the descriptions are reasonable and the meaning seems to be the same. I've changed the text a little and written the schematic numbers instead. I've marked the changed item numbers with an "*" and written the descriptive names (FREQ and FEEDBACK) next to them.

Regards Dimitrij

Lots of stuff in commercial gear runs at 70 C or even hotter. I don't like to see parts running that hot. Especially in something that might get buried in a shield housing deep in the bowels of some piece of gear like a scope. But, 70C is not insanely hot for a power resistor. Of course, I have NO IDEA how hot it is actually supposed to get.

So, does the scope actually run correctly? That would probably indicate the transformer is fine, and maybe there is some load somewhere in the scope that is excessive, maybe a bad electrolytic? I've got a B&K scope here that blows a $13 power module after 15 minutes or so, and I've gotten tired of fixing it. Since every time the module popped, the interval got shorter, I'm strongly suspecting a bad electrolytic, but a quick visual inspection does not show anything obvious. I've long since replaced it with a Tek scope, so I'm just going to take it to the surplus shop.

Jon

I couldn't agree more. Coming from the germanium semiconductor generation where even slightly too much heat was terminal, I still like to go by the rule of burnt thumb: if if it burns your thumb it's too hot. In which case derate, derate, derate.

I didn't get the chance to find out! Began this morning trying to get some voltage readings off the psu outputs and there was nothing there to read. To cut a long story short, further investigation reveals something has gone short-circuit on one of the signal boards. When the psu is removed and run from my make-shift dummy load, it's still 'fine' with its new diode (not quite right, but functioning to high degree). So clearly I jumped the gun slotting it back in the scope when it still wasn't 100% and now it's damaged something - typical! I'm running out of time now as we have to leave later to spend a few days with 'er mother 300 miles away and whilst I shall still have internet access there, I'm not allowed to take any test gear with me. Ain't life great?

Sounds more like you're getting closer to root cause.

Troubleshoot the (unidentified?) signal board.

RL

Your wonderful description of resonant circuits reminds me of an experience I had as an apprentice. I was given the job of trying to lay down a silicone insulating film generated by polymerising a silicone vapour in a high voltage AC plasma. (I don't remember why it had to be AC.)

We didn't have any special HV AC supplies but stores did have a signal generator and powerful audio amplifier. I managed to scrounge a large open-centred coil (meant to generate a magnetic field around a bell-jar) and a collection of high voltage capacitors, waxed paper in steel cans with ceramic terminals on top.

With these I built a series LC circuit which generated a satisfactory plasma. I'm afraid I don't recollect any measurements. Health-and-safety consisted of large hand-written warning notices!

However despite being distracted by my plasma I also noticed that the poor capacitors, excellent with DC no doubt, didn't like the AC current they were subjected to, and bulged, leaked and fizzed. The experiment was terminated abruptly!

Mike.

I'm guessing you mean like this:

I still have a couple of dozen of this type here.

I'm afraid not. It didn't happen yesterday when I first hooked the psu back into the scope so this is a fresh fault - and probably my fault for not testing the psu's output voltages properly before plugging it back in. :(

May have to wait til the latter part of next week; I have to leave later today for a 5-6 days due to family-in-law commitments.