The BY134 is a lower frequency part with 2uS recovery time and is probably unsuited to replacement of BY208-1000 in any of the snubber or conversion positions indicated on the schematic primary. It should be soft recovery, medium speed (200-600nS) avalanche-rated part with a minimum 800Vprv.
I'd avoid the use of anything advertised as 'ultrafast' (ie UF4007), as this circuit may need a modest recovery time in order to reduce power loss and EMI, but they could be used temporarily in troubleshooting.
Well, if that is true then beware! V1808, V1809 and V1811 are supposed to be very fast. Any slow (more than a microsecond) diode in these positions will likely cause symptoms akin to a heavy overload.
Particularly V1811, if replaced with any 1N400x, is likely to render the energy recovery circuit around L1806 as good as inoperative, thereby dumping the entire energy from the switcher harmonics into R1814, which will cause it to overheat fast.
Please recheck L1806 (both windings) for turn-to-turn shorts (with a signal generator), and if any of the 3 diodes (V1808, V1809, V1811) looks like it had previously been replaced (possibly improperly replaced), consider replacing all 3 of them together, using the proper parts.
Use fast soft-recovery diodes rated for 1kV here. If you can't find any, use ultrafast ones. They're maybe not optimal from an EMI standpoint here, but at least they should work well enough for testing.
If you can't find a BY208-1000 replacement, a MUR4100E should work.
Check C1806 for dielectric breakdown. It should be able to withstand at least 500 V (or something in that ballpark). If it doesn't, replace.
Don't underestimate that L1806 energy recovery circuit. Although it doesn't by itself transfer any power to the load, this supply heavily relies on it for proper resonant operation of the main transformer. It must be working properly before you can test the main transformer waveform and have any chance of making correct measurements.
Besides, your description of heavy switching noise on V1806 (when you tried to measure the base drive waveform), up to the point of the waveform being unrecognizable in the noise, seems to indicate that the L1806 circuit is shorted at high frequencies. This can be a result of either a winding short in L1806 or a breakdown in one of its diodes or some of these diodes being replaced by a generic slow silicon diode.
As you say, this really doesn't tally up. The ratios look ok (see my other post), but the absolute values are obviously junk.
After all, Ohm's law still holds, even for reactive impedances, and 60 volts divided by 3.7 Ohms is 16 amps, which would be WAY too much for a small transformer winding's magnetization current. That would indicate very heavy overload, most probably due to a short circuit inside one of the windings.
But there's something that makes me distrust those impedance numbers - and that is your use of 100 kHz as the testing frequency.
First of all, did you really use 100 kHz as written? Most LCR meters have 100 Hz, 1 kHz, 10 kHz and 100 kHz signals. Did you perchance use the 100 Hz one instead? 100 Hz would be so low that the inductive part might not even register properly.
Second, 100 kHz is not a good choice. The reason for this is the self-resonance frequency (SRF) of your transformer. All coils and transformers in the real world are not just coils, but LC circuits. The L part is (obviously) provided by the winding itself and the C part is the stray capacitance between the wires in the winding. Being really an LC circuit, a winding has a resonance frequency, like any "true" LC circuit would have. This is called the SRF of the winding. To make matters worse, a transformer that has multiple windings wound with different geometries and wire diameters has multiple SRFs, one for each winding.
Windings with few turns of loosely packed thick wire have high SRF values, while windings with many turns of densely packed thin wire will have much lower SRFs.
Your particular transformer has 2 high-voltage windings for the kV outputs. They have lots of densely packed thin wire, so their SRFs will be very low. I don't know exactly how low, but I'm sure that they will be much lower than 100 kHz, and that's what makes 100 kHz an unsuitable choice for testing.
If fact, if you try to operate a winding above its SRF (let's say the winding has a 20 kHz SRF and you try to apply 100 kHz), then the winding will no longer behave like an inductor, but it will behave like a capacitor instead. I know, this seems crazy, but that's how a winding behaves above its SRF.
In a transformer, where there are multiple windings, there are also multiple SRFs, so at some test frequency, some windings may happen to be below their respective SRFs, while some other windings may be above their respective SRFs, depending on how you choose the test frequency.
If any winding happens to be above its SRF, then it will behave like a capacitor. As you know, capacitors behave more or less like a short circuit at high frequencies, and an above-SRF winding will behave like that too. That is, it will look (from an impedance measurement) like if it was heavily overloaded or even shorted out altogether.
So your 100 kHz measurements indicated very low impedances, like some winding was shorted out. But then you also have 2 high voltage windings in there, which would have been way above SRF at 100 kHz frequency, so they will effectively behave like shorted even if they were perfectly fine otherwise. At 100 kHz they're no longer inductors, they're likely just capacitors instead.
Now, to test transformer winding impedances, you need to select a reasonable test frequency. It must obviously be lower than any SRF of any winding - otherwise the transformer will appear overloaded. If you don't know the SRFs' values, you can measure them out with a signal generator and an oscilloscope. But you don't need to. Normally no transformer is operated above its SRF (it would not work very well if one tried), so you can assume the normally intended frequency of operation to be a "safe" choice that is unlikely to hit SRF limits.
Your transformer is probably supposed to run at something like 20 kHz in normal resonant operation, so 20 kHz should be ok. But because it has high voltage windings, it may be very close to the HV winding's SRF. Indeed Philips engineers may even have chosen to run the transformer not below, but essentially right at SRF. They may have selected the resonance capacitors for the primary in such a way that the primary (together with the resonance capacitors) would have a resonant frequency which closely matches the self-resonance of one of the high voltage windings, being just a little bit below to account for tolerances.
If that's the case, you should use a lower frequency for testing impedances. Most LCR meters don't offer 20 kHz anyway, just 10 kHz and
100 kHz as "nearest neighbors". 100 kHz won't do, so use 10 kHz. That should give you realistic impedances (which you can manually multiply by
2 to get to the in-circuit conditions).
Yes, definitely 100kHz. Not my preferred choice, but the only option given the meter I used which was actually a capacitor ESR meter.
[SRF remarks noted]
Very good point. I admit I never considered that possibility.
Yes, it might be illuminating to sweep a range of frequencies and note any resonances, I can see the value of that. Unfortunately, an LCR meter is one item of test equipment I don't have, so it would have to be sig gen and scope in combination. Anyway, it's do-able. Many thanks for your observations as always.
Thank you. If you've had the chance to read my follow up to JC (I think it was) then you'll be aware that one of those BY208 diodes was replaced by a BY134. If the design is that critical of the speed of the diodes it uses then maybe it won't function properly as you suggest. I can only imagine the technician who replaced it was unaware of the critical nature of the part concerned. I'm kind of unhappy with this design overall, to be honest. It was critically appraised on s.e.d and found generally unsatisfactory. I'm strongly tempted to just save the transformers, junk everything else and start afresh with a modern design. The rest of the scope is mint and untouched, it's only the psu section that's been butchered around and shows signs of burning and scorching in places. Maybe the best thing to do would be to bin it? :-/
Switching supplies are all designed with the diode (and other component) parameters in mind, its how they function efficiently. Bunging any old diode in is asking for trouble. Same as the ESR and temp rating of the caps used. Seriously you need to sit back and chill. These scopes were very well designed and I would say exceptionally reliable. I'd like to see you build a replacement. I've designed switching supplies, everything needs to be right or it goes wrong fast.
FR107G seems to be the current equivalent for the BY208-1000
Well, the modern ones may well be super reliable, but this old thing is very dated and shows many signs of its age and the scars of previous faults and questionable repairs. I wouldn't attempt another resonant converter; there must be something simpler with fewer critical parameters, surely.
Thank you, JC. You and Dimitrij have both given me some subs for suitable replacements which I'm quite happy to go along with. But I'm not prepared to spend much more time on this repair, to be honest. I'd relish the prospect of a comprehensive re-design. Even if it's beyond me at this stage I'd learn a lot from it.
As for the BY134, sorry, I must have overlooked that somehow, or maybe it did not register in my memory right away. Anyway, it's just as bad a choice as a 1N4007 and its ilk. It's designed for mains rectification and doesn't even make an attempt at being fast.
No use in an active snubber or energy recovery circuit whose task it is to "strip out" the high frequency components from a square wave.
Get rid of it, and while you're at it, consider the condition of the other two (V1808+9) identical ones. Sometimes a person who does an improper repair will try swapping nearby components hoping that another one might be "less critical". So if you see signs of unprofessional manual soldering on them, take that whole trinity and replace them.
Same with C1806. If it looks suspicious, does not pass a withstand test at some 105% of its rated voltage or shows high ESR, change it too.
BY208-1000s are hard to come by nowadays, so here is a list of some more modern candidates: MUR1100E, BYV26E, UF4007. They should fit, and even though they are faster than the original BY208-1000, they should work.
There are also: RGP30M (slightly large, modest speed), UF5408 (slightly large), MUR4100E (slightly large), STTH112U (smd), US1M (smd) BYG23M (smd). They may or may not fit due to size and space constraints, and the SMD ones would likely need some wire leads soldered on (won't look professional, but hey, if others are too hard to come by, that's ok).
Once you've fixed that botched repair on the energy recovery circuit, connect a taillight lamp to the 12.7 V output, test it again and tell your results here (make sure you put all the proper parts back in, before you switch it on, this supply may be unforgiving if any parts are missing and it's powered on).
As for the design being "generally unsatisfactory", let me disagree. Resonant converters do have a well earned place in the world of power electronics, but the design of them is, in a way, a black art. They have lots of pitfalls for the unwary and not so many engineers can actually design them properly and they tend to use special components (inductive ones in particular) that would be rather unsuitable for other topologies too. Yet they do have certain benefits, low noise operation that is suitable for sensitive measurement instruments, being one of them. They are not so easy to understand, compared to "simple" flyback topology supplies - so people go screaming "this is too complex" or "this uses too many parts". In fact your supply's energy recovery circuit is actually a little unregulated flyback converter of its own! But so far (and considering the design's day and age), all the parts that I've seen in that schematic seem to me to have a good reason for their existence.
Will do. I'm guessing the tech who replaced that diode was solely concerned with its voltage rating. In all honesty, I'd have been the same before this speed importance was drawn to my attention in this thread.
That one actually looks fine appearance-wise, but I'll test it electrically of course.
Will do. I'll order the parts tomorrow if I can't find any in my spares bin.
I read somewhere that resonant converters are poorly understood by engineers who don't specialise in them and that accurate, detailed literature on them is not easy to find. So it's very valuable to have knowledgeable people like yourself and others here who do understand how they work; otherwise I'd have nowhere to turn for advice on how to proceed with this! I'm going to work through the steps you've outlined here and elsewhere and hope they work. But if the problem remains, I shall definitely be mothballing it for the foreseeable future. My patience isn't infinite! :)
I should perhaps have been more specific and stated that V1811 on the schematic is the diode that was incorrectly replaced by that lower grade part. Anyway, replacements now on order; will report back in a few days.
A gross failure in this part would blow a fuse, hence it is the least suspect in that regard only - the fuse doesn't open.
It is actually the only one that is involved in power transfer - seeing double input voltage stress and peak/average conversion currents; the other two are snubbers/clamps.
If it's slow, it looks like a short when the power transistor is trying to turn on, stressing the current snubber around L1804.
Well it seems it *is* far too slow if I understand Dimitrij correctly. Anyway, I've managed to source one of his suggested substitutes, the UF4007 type quite cheaply online so we'll find out before the end of this week if the wrong replacement part has been responsible for the problems I've experienced.
I don't think this diode is the culprit, TBH. Just out of curiosity I hooked it up and tested it this afternoon. The faster diodes turned up so I thought it might be instructive to compare them. The main flaw in my test is that I'm unable to replicate actual working conditions. I just hooked up each diode in series with a 1k resistor and fed the arrangement from my 600ohm sig gen using 10VAC p-p. Slow recovery was certainly visible on the scope with the BY134, but it wasn't *that* bad. In fact it was still able to function as a viable rectifier right up to nearly
600kHz. There were no signs of slow recovery with the UF4007 of course, but the difference at 20kHz, whilst still noticeable, is unlikely to be causing the issues I've experienced. But as I say, it was in no way a scientific test and only when the new diode is in circuit will we know for sure. I won't be holding my breath!
Ok, there are other simpler ways to test windings under high voltage :)
See below for a simple test circuit that would be easily doable with a few common parts and works like an IWT (impulse winding tester):
It needs a power supply (can be just a mains isolation transformer with rectifier and capacitor) and it's intended to show the resonance waveform on an oscilloscope at realistic rated voltage conditions.
The MOSFET (any 400 or 500 V type with less than 1 Ohm Rdson) is driven with a square wave from a signal generator (frequency should be slow enough to allow the cap to recharge, some 50 to 100 Hz) and discharges a capacitor from 320 V (rectified isolated mains) into the inductor under test. Under discharge conditions the capacitor and the inductor form a resonant circuit and slowly "ring down".
The resistor heats up with prolonged operation, obviously, since it has full supply voltage across it, so that's why it's rated 10W.
The waveform is measured (due to the high voltages involved) with a 400 V rated 10:1 oscilloscope probe. It should give a reasonably reliable indication whether an inductor (or a transformer) is good for use at full mains voltage or not.
The circuit works similarly to a commercially available IWT and it's intended to be connected to the primary of a transformer. The waveform should look like a typical IWT waveform (search for "impulse winding tester" in Google Images to see what it looks like).
Here's a good looking waveform example:
A shorted (or otherwise overloaded) coil will decay very fast or even hardly resonate at all. A good one will resonate for many cycles.
A failing one with significant corona discharge may look like this:
This test should be easy to do, and should be able to settle the question if the transformer is "shot" with reasonable confidence.
As always, when working with high voltages, pay attention to safety!
Well, I don't think that it's the main culprit either. But it may impair the working of the energy recovery circuit far enough to make it inefficient, forcing it to dump too much power into the resistor. If everything else was well, that might still have worked to some extent.
But you're trying to troubleshoot it, and something is obviously wrong that causes the resonant circuit to appear as too low impedance. Either the transformer is broken or the output circuits (rectifiers) or the whole thing is operated on wrong frequency too far out of resonance.
If the energy recovery circuit was working well, it should be able to protect the resonant circuit, even at some overload, by diverting the energy back into the main capacitor. That would allow you more time to "probe around", checking what is the cause of the overload.
Slow diodes usually become worse with rising currents, so one that is able to drive an 1k resistor from a signal generator may just as well behave like an RF short circuit if one tries to push significant amps through it. So it's really difficult to compare.
Anyway, while I don't thing that it's enough, I was hoping that making that part work efficiently again would at least lower the load on the resistor to some extent, and give you more time for such more complex things like resonance frequency measurements or even adjustments.
Also, as for testing the transformer (out-of-circuit, with a poor man's IWT equivalent), see my other post.
Many thanks as ever for your thoughts, Dimitrij. One question on your other post before I forget: your schematic shows pulsing the transformer input at 100Hz, so we're just testing the primary winding in this instance, right? We're not concerned in this test about what's coming out of the secondaries? I assume so because 100Hz is so far off its intended frequency range but would be grateful if you'd confirm if I have this right.
I fully agree with your observations on my diode test's shortcomings.
The only other thing I'm waiting for is some replacement caps for the original tropical fish types that don't look very healthy. They test okay at low voltage but may be misbehaving badly at closer to their working conditions. They're in really poor shape visually and I could certainly believe THEY might be responsible for the issues I've had. They should be here tomorrow or Thursday so by the end of this week, I should have some firm results one way or the other.
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