Final update for the time being as I have to leave soon now:
That short turned out to be intermittent. I hope it was just due to something shorting out on the bench that won't happen when the casing is back on because you all know what a bitch it can be to trace intermittent faults. Anyway, that fault has now disappeared, so I took some voltage measurements before the 20W resistor got to hot (from 19'C to 60'C takes about 1.50s now) and I have:
61.7
12.7
5.8
0
-5.8
-12.7
-62.4
This is with the psu board plugged into the scope and all power connections made except for the VHT stuff.
The correct figures according to the manual should be:
60
12.7
6
0
-6
-12.7
-60
So very close! Looks like the main transformer may be ok after all.
Dimitrij, I will have to run these latest checks you suggest next week now as I have to leave on family matters and have no choice other than divorce. The PSU is now putting out near-enough the correct voltages in the 6-60VDC range as required when connected up to the scope and the transformer is virtually silent. It's just the power resistor heating that's causing concern. If you think of anything else, please leave your thoughts here. If not, I'll proceed with your checks on my return. many thanks again.
Missing voltages don't necessarily indicate shorts. They can also indicate open circuit to the source. Review solder joints and connections around the transformer pins. Possible damage in recent removal activity.
The main chopper is a TO-3 cased BJT with a closely-finned heatsink bolted to the top of it. By the time the resistor starts to emit a scorching smell, the chopper hasn't even had the chance to get barely warm.
Yes, it's exactly 20 ohms as specified. But please don't ask me to do any other checks for the next few days as I'm staying over 300 miles away at present.
OK, when you get back to it, put together a bulb tester. Take the 20 ohm straight out and put a 100 watt lightbulb (incandescent) there.
Then you start disconnecting things.
That is probably the only way to troubleshoot this. You ain't finding anything with the ohmmeter, but it isn't shutting down. Something is not showing up unless under voltage. Ohmmeters can't detect that.
Hhmmm. As I've said before, I'm reluctant to replace that power resistor with anything higher rated. At the moment it's acting as a robust detector that something isn't right. I don't want to replace it and then find the excess energy has burned out the transformer primary instead!
Dimitrij has already given me some steps to follow for resonance checks when I get back and I'm trying to keep the suggestions made here in an orderly queue, so thanks for your input which is appreciated, but no further test ideas from anyone for the time being, please!!!
Dimitrij, I think you may have missed this I posted elsewhere so I'm re- posting it here now for you personally:
"Final update for the time being as I have to leave soon now:
That short turned out to be intermittent. I hope it was just due to something shorting out on the bench that won't happen when the casing is back on because you all know what a bitch it can be to trace intermittent faults. Anyway, that fault has now disappeared, so I took some voltage measurements before the 20W resistor got to hot (from 19'C to 60'C takes about 1.50s now) and I have:
61.7 12.7 5.8 0
-5.8 -12.7 -62.4
This is with the psu board plugged into the scope and all power connections made except for the VHT stuff.
The correct figures according to the manual should be:
60 12.7 6
0
-6 -12.7 -60
So very close! Looks like the main transformer may be ok after all."
Since you were planning to be away for a while, I was in no hurry to reply right away. I've seen your other post too, and obviously the transformer must be ok.
I think that, from the major power-carrying components point of view, your power supply is now "almost ok". The "power train" clearly works, otherwise you couldn't get correct output voltages under load.
But the fact that the power resistor still overheats, hints to some timing being slightly wrong. It can no longer be "completely" wrong, as was the case with the slow diode, but it's not yet "right" either.
There is still the question with V1808. You said it looks ok, and it tested ok with a multimeter, but that's not really indicative of its true behavior under full load at high frequencies. If it has degraded for any reason ("lost its switching speed") then the resistor R1814 would be running at a higher load than normal. Not many times higher, but about double or triple. That would be somewhat consistent with your observation of it running too hot after a few minutes. You should now have (hopefully) a few spare UF4007s, so if in doubt, replace V1808.
If you find out that the replacement of V1808 makes a (little) change for the better (slightly lower load on R1814), then replace V1809 too. It would in this case be likely that those BY208-1000s have all degraded and became out-of-spec. They all have the same type and age.
Actually it's possible to test the condition of V1808 in circuit, without replacing it, but the test is tricky. You would need to see, on an oscilloscope, the voltage waveform across R1814. It should be basically a flat line, with short surge-like spikes at some 20 kHz intervals. All the pulses must be polarized in one direction only. The left-hand pin (on the schematic) of R1814 must be positive. There must be no spikes in the reverse direction. If there are any (the polarity would be alternating), then V1808 is degraded and no longer operable at full speed and needs replacement(, and so does V1809 likely as well).
Unfortunately this test is difficult, because you can't connect a scope ground to R1814! This is a very fast switching signal that runs at high power and reaches voltages of some 800 V in normal operation! Even if you disconnect both mains grounds and "float" both the scope AND the power supply, and even if you power both the scope AND the power supply from two SEPARATE isolation transformers in order to increase isolation and minimize the stray capacitance via mains, this test would still be very dangerous and I would definitely not advise trying. Using two scope channels in "subtract" mode might work, but only if you have two high voltage probes rated for 1 kV, and only if both probes are exactly identical and the compensation of both channels is precisely matched to each other (a rather unlikely condition that requires some effort to achieve). To be honest, to do this test properly, you would need an isolated high-voltage differential probe. Unless you have one, don't even bother trying, to replace the diode is easier and much safer.
Ok, so much for the other BY208s in snubber circuits. Replace and see.
The other open question is that of the resonance capacitors (C1807 and C1808). As I noted in another post, they may be degraded and it may be difficult to test for this condition properly (LCR meter won't likely show the problem). Again, if you can get known good spares, they can easily be replaced, but the spares must be rated for resonant operation. "Typical" film capacitors are not designed for this use.
Foil capacitors with Polypropylene isolation rated for continuous resonant duty like the "FKP 1" type should work well here, and so may "MKP 4C" type too, to some extent, but only the 630 V DC rated ones, and only if two are used in series like in the original schematic ("MKP 4C" with lower ratings would hit its high frequency AC limits).
So, "FKP 1" rated at 400 V or 630 V DC (two 33 nF in series) or rated at
1000 V DC (one single 15 nF) or "MKP 4C" rated at 630 V DC (two 33 nF in series), would be feasible replacement candidates, but not many others due to the high loading requirement in resonant operation.
If yours turn out to be degraded, and you replace the 30 nF originals (now probably unobtainable) with 33 nF, you may need to re-adjust the resonance frequency somewhat.
Also, the frequency adjustment may be slightly out of resonance (maybe the previous repairer has misadjusted it and component parameters can also drift over the years). Again, a misadjusted frequency, especially if it has been set too high rather than too low (compared to the true resonance frequency of the LC circuit) can cause the dissipation resistor to overheat (so a little low is better than a little high).
A resonant circuit driven too slow (below resonance), will pull reacive power (will have a power factor below unity), but the direction of the phase shift will be inductive. If driven a too fast (above resonance), it will appear capacitive instead. Please note that the square-to-sine conversion circuitry, especially the snubbers, will have lower stress from peak currents when driving an inductive load than when driving a capacitive load, so an inductive load is "easier" on them.
Please read the instructions in the service manual (I've also copied the relevant part in my other post), and also note that the service manual clearly advises to always adjust the frequency "from below" and never "from above" ("use 170 V mains, then set the trimmer to lowest possible frequency, and slowly raise it until the output voltage regulation can just be obtained, but no more than this"). So the designers from Philips must have preferred this design to run rather slightly below resonance than slightly above it, and they must have had good reasons to write the adjustment instructions in such a way, as to prevent an accidental "too high" frequency setting.
Note that any frequency adjustment should be done with the correct dummy load connected in order to avoid entering a "light-load" mode.
But could you please make a complete list of found faults and your replacements, and post it here:
I mean, you posted at the very beginning (long before finding the slow diode) that you've found and replaced some obviously defective parts, but I can't remember if you ever posted, exactly which ones they were.
Also, you have indicated other things that may impair reliability (like capacitors with pieces of film isolation flaking off), and again, you didn't seem to indicate the exact schematic part numbers.
As you may well know, to troubleshoot anything properly and reliably, and to be able to assess the likely chains of cause and effect, one needs to know the history of the repairs, as completely as possible, and also anything obviously (visually or otherwise) suspicious too.
Therefore please make some lists, and take particular care to make them complete, to leave nothing out, and to indicate each and every listed part's schematic part number (important, since others can't see your board and need the exact numbers to identify the parts in question).
- one list with all previous repairs that you have found: which parts were replaced in the past, as visible from manual solder joints, and where the replacements were of different type from the original, clearly indicate the exact types of replacements.
- one list with all of your repairs: which parts you found defective and what exact parts (exact type and manufacturer) you have replaced them with.
- one list with all parts that currently look suspicious or for whatever reason seem to be of questionable integrity.
It would be nice if you could make a printout of the schematic, and mark all those items in color (like for example yellow for previous repairs, circled twice if the repair was inexact, red for those you replaced, and blue for the suspicious ones), and then scan and post the color- annotated schematic somewhere for us to see.
To avoid "... and what else was there?" or "... and what about part XYZ?", please make sure that this annotation is really complete. Trying to get such information one question at a time can be frustrating.
Yes, I bought 20 of those faster diodes to be on the safe side. :)
[live power resistor procedure testing snipped]
Actually I did do this a while back without knowing the risks! As you can see, I survived to tell the tale. All I was seeing was about 30V of noise across that resistor but that was before I was informed of the importance of hooking the supply up to a load, so the test was probably invalid.
Certainly can do that, yes.
Is there any way of *definitively* testing such a capacitor against all its possible failure modes? And I'd be interested to know where you get this figure of 800V you mention from?
Fortunately this is one aspect I pretty much totally understand. As an old-style radio ham of more decades than I care to recall, the concepts of resonance, reactance, impedance, power factor and phase shift are like second nature so please don't go to any trouble explaining the finer points in extreme detail; there's absolutely no need. BTW, your explanations are unusually clear and thorough, I've noticed. If you don't already, you really should edit or author technical manuals. It's an all- too rare talent nowadays.
I think you may possibly be getting mixed up with a different repair here, Dimitrij. I do have some flaky capacitors to replace when I return and I'll note which ones I change for your information. As for what previous technicians may have done, I have no idea what if anything has been replaced - apart from that one obvious diode. I got absolutely no background information on this scope, it was given to me for nothing by some guy who was emigrating so its past will now always remain a mystery. It's a pity, because this obviously adds another set of unknowns into troubleshooting the thing, but it's just something I'll have to live with I guess. In all honesty, this repair is proving to be a 'baptism of fire' for me in the world of SMPSs of which I admit I know very little (yet a lot more than I did 3 months ago!) :)
Ok. Usually most people who ask here understand DC parameters well enough, but rarely get to consider impedance, phase angles and such.
As for the 800 V, that was mostly a guess. Basically I've taken 320 V of the storage capacitor, added to that another 300 V of the resonant circuit (when the power transistor is off and it's being swung in the other direction) plus the voltage rise from the winding reset from the primary of L1806 (which is actually unknown since I don't know the ratio between primary and secondary, the secondary being at 320 V), which I guessed to be somewhere in the 200 V ballpark.
That's 320 V + 300 V + 200 V = 820 V, likely even to be more because the
300 V may reach up to 320 and the 200 is only a guess and may likely end up higher than that, plus there may be some 50 V from L1804 adding up in the same polarity, so even a 900 V total won't be out of the question.
That would be consistent with the rating of the BU208 power transistor, which has a 1500 V absolute maximum collector rating when driven from a low-impedance base drive signal.
As for definitely testing the resonance caps: I'm somewhat at a loss.
First thing, you can measure the capacitance, that an obvious test. If the capacitance is wrong, they're can't be working properly.
But reduced current handling ability comes from an increase in ESR and in the dissipation factor. To measure them, you would need to run the cap at the intended target frequency (and preferably at a realistic voltage too).
LCR+ESR meters can measure the dissipation factor and ESR, but those intended for electrolytics will often measure only ESR and also may have trouble testing such small foil capacitors like 33 or 15 nF.
Also, I don't know the target numbers for ESR and dissipation here, so one would need to compare them against a known good pair somehow.
An other way I can think of, would be to run them at resonance with the transformer, and measure both frequency and "Q". But that's also not meaningful unless one has a known good reference value for Q.
I think that the most realistic test would be to sweep the resonant circuit with a signal generator and watch the waveform. If the resonance frequency looks right (in the 20 kHz ballpark) and a signal generator is able to drive it from a high 600 Ohm source impedance to a significant amplitude without much "sagging" (that is, the resonant circuit presents little load to the generator), it's probably OK.
Also, even with a dummy load connected, the stray capacitance of an oscilloscope, when hanging off the loose end of a power circuit with some 800 to 900 V worth of HF on it, would probably cause so much undue capacitive loading that the power supply circuitry would hardly handle it. That may have been the reason why you just got noise (the overload from the hanging scope may have affected the over-current shutdown of the power supply controller). As I said, the proper way would be with an isolated high voltage differential probe (such a probe would present very little stray parasitics) or maybe with a well matched pair of (identically compensated) HV probes in subtract mode.
P.S. That voltage estimate has probably surprised you. Unless one looks at the circuit schematic and adds all the voltages from all the storage elements (inductors / capacitors), considering timing and phase, it may not be obvious that the thing was intended to run at such high voltage levels. But there's a reason why they used a 1500 V transistor in it.
Thanks again, Dimitrij. You're obviously an expert on the little understood world of resonant converters so when you say try this or that, I make a point of paying extra attention. I liked your theory on the resistor heating due to this supply running out of resonance as a result of component values changing over time; in fact I'm currently pinning my hopes on it. It's a pity I'm stuck here for a few more days with my revolting in-laws but it'll be the first thing I do on my return!
Somewhere I have a big old valve/tube capacitor tester capable of simulating realistic high voltage working conditions. It'd be interesting to know what kind of checks it's capable of performing if it's still in working order and if I can find it among the towering piles of obsolete test equipment I have here (a couple of million pounds worth of gear at new prices adjusted for inflation) I may possibly hook it up and give it a shot.
How about those 'Octopus' component testers? They subject the part under examination to sweeping test voltages over the expected working range and you look for any signs of breakdown on an oscilloscope in X=Y mode. I guess this method is about as good as it gets?
Isn't this just another example of the unsatisfactory nature of this resonant converter design? If the thing is *that* fussy that a little bit of stray capacitance can catastrophically destabilise it, then AFAICS it's a fundamentally unreliable topology and it would be better to have used one of the non-resonant forms of converter. Unless there's some compelling reason I may be unaware of not to for oscilloscope power supplies, of course.
Please don't rely in my advice too much. While I do design electronics, I'm very far from being an expert in this particular field. I've never actually designed a resonant power supply, unless you count one little
3W prototype based on a modified Royer / Baxandall structure.
It may be relatively easy to look at a ready-made schematic and try to guess various upper and lower limits based on parts and topology (like "signal X cannot be higher than Y volts, otherwise part Z breaks down" or "ratio of transformer X cannot be above or below A:B, otherwise the ratings of part Y would be exceeded"), but that's not expertise by any stretch of the definition. A lot may be intuition, but that's no expertise either.
I've had to look up, what an "Octopus component tester" is. Apparently a transformer with some provisions for routing the voltage and current signals of the load to an oscilloscope, making a simple AC curve tracer.
I don't think that you'll need one here. It can test for breakdown, but in your case that's unlikely (the capacitor would be buzzing and arcing and the supply sure wouldn't work "almost normally"). It won't see the problems that are likely to be important in an LC circuit.
The cap must have the correct capacitance. Any LCR meter or any common pocket multimeter with a capacitance function can measure this. This is a basic prerequisite that should always be tested first and if the capacitance is wrong, no further tests will be necessary anyway.
The foils inside the cap must have a reliable connection (deviation manifests itself as ESR, ESL, and the general inability to supply high impulse currents). This particular curse will sometimes plague the trigger capacitors from photoflash units (the flash won't trigger or will only trigger erratically while the capacitance value is still ok).
This is difficult to measure directly, but can be checked with another capacitor as a reference. You'll need a known good capacitor with the same value (in your case: 15 nF), but not necessarily with the same voltage (you can use a known good, but lower voltage one for testing). The test is only with a signal generator, so the cap won't be subject to a lot of stress.
Connect the known good capacitor to the original inductor (transformer primary) with no other loads attached. Sweep with a signal generator (use as much voltage as the signal generator can provide without much distortion, that usually won't be a very high voltage anyway) and look for resonance on a scope. Note the resonance frequency. Disconnect the known good cap and connect the original one instead. Check where the resonance is. If it's in the same place and the amplitude has not become lower, the cap is very likely good. If it disappears and you can only measure the inductor's SRF instead, (if the inductor has more or less the same resonance with or without a capacitor connected), then the capacitor is basically open-circuit or very high ESR. If the resonance has wandered away somewhere, especially upwards in frequency, then the cap is most likely degraded and not a good candidate for full power resonant use either. Same thing if the amplitude has dropped much.
If your resonant caps turn out to be good, that most likely leaves only the snubber diodes and a possible frequency misadjustment as the likely causes.
If you check the resonance with a signal generator and scope against a known good 15 nF, and it suddenly wanders way, or the amplitude drops, then you'll need to find replacement capacitors. Fortunately, if you put "WIMA FKP1 33nF" into ebay search, there seem to be many available.
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