I know I said I was going to mothball this scope for the time being, but I keep thinking of new things to try and just can't leave the damn thing alone. It struck me I ought to next check out the main transformer because if that's toast, the whole psu might as well be binned. I think I
*may* be on to something.
Here's the circuit again:
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The transformer in question is T1801. I've pumped in a test signal into its primary winding (2V p-p sine wave @20kHz Zs=600 ohms) and scoped all
15 remaining pins. Every single one of them is producing clean sine waves at various amplitudes just as you would expect, except one, which is giving a really thick, noisy trace. On the diagram, this is the 5th one down on the right hand side (immediately below the 1kV out pin.) It's not marked on this diagram, but is supposed to be 0V from another schematic I have. Someone has replaced C1826 & C1827 at some time and the replacements don't look good for more than 50V max to me, and that's not enough since the pd across them will be much more (the parts list specifies a 500V rating!) They're not showing short-circuit, so I'm wondering if they've failed open maybe that could account for the symptoms I've been getting: terrible noise in the primary circuit and the rapid heating of R1814 (the 20 ohm power resistor intended to dissipate the switching energy). Any thoughts?
As this terminal has no ground reference, with the loads removed, you can only measure its winding voltage by measuring between other pins that are connected to the winding, or by introducing the ground connection on one of them.
The amplitudes measured will not be various - they will be scaled as you expect them to be, as reflected by schematic output voltages or labels. Measuring with a high impedance scope, it is possible to see a waveform that is simply capacitive transfer, if you're not making rational measurement observations.
C1827 is 'Select On Test' (SOT) to trim some operating parameter in the functioning unit - possibly the HT tolerance under load or even just display noise. You'd need a manual to figure this out. The more often you mention the actual model number (PM3264 ?), the more likely it is that you'll get someone to cough one up. It's $20 from jetecnet.
Ceramic disc caps of this size were typically 500VDC rated at that time. Only smaller square plate types were less (and some offshore stuff). Lower voltages will generally be marked as such, or colour coded for identification in the mfrs spec sheet.
You still haven't identified the diode substitutions made in the primary, or reported on secondary winding amplitudes loaded/unloaded.
You still need to report IC pin3 voltage when loaded. (it is reported as normal, when unloaded, indicating a regulated condition exists.)
Thank you for your observations. I can only reiterate that testing under mains power is not practically possible, loaded or unloaded, given that I only have about 15s before that power resistor starts to burn up in either case. I really have to try to crack this by other means. There is no way on earth the subbed caps are anywhere remotely close to
500V rating; they'd be lucky to withstand a tenth of that. I'm going to pull them out later today and replace them with a properly rated part then I'll scope for noise again. I suspect it's this noise, fed back, that's causing the primary circuit to fire falsely regardless of the signal from the pwm chip; the excess switching energy therefrom showing up as excessive heat in the power resistor which is only rated to dissipate the *expected* switching energy from a properly functioning primary circuit. Anyway, we'll see....
Your concern for this resistor's health is touching, but it really is misplaced, as the part is easily replaced, relocated, temporarilly beefed up or whatever.
If you know what you want to do, in 15seconds, you can do a lot, particularly if you're prepared to repeat the process at intervals, until all the necessary info is accurately recorded.
By the way......I very strongly doubt that this assembly was issued from the factory with transistor sockets. I would be suspicious of any substitutions made here, as they are ripe for error, even if only for part lead identification.
The low voltage square-plate ceramics (63V to 150V) referred to elsewhere are visibly in use on this unit, in the photos supplied. The location of the two suspect parts is not visible/identifiable.
If you have a bill of materials, listing a 500VDC requirement, perhaps you might distribute this as well? Wondering where it came from, if not from a service manual? It's possible that tempcos may be specified, if only indirectly, in the part numbers listed.
Philips didn't pick part numbers out of a hat, or junk box.
Nice idea, but it's in series with the main transformer primary winding, and I'd rather that resistor go up in smoke than the transformer. If I replace it with a higher power one of the same value, it puts the primary winding and other components at risk of fatal damage.
It takes about 5 minutes to fully cool down and then give me another 15s of probing. I could of course let it cool for just 90s, but then I'd hardly have time to get a probe in place before it got too hot again. I simply haven't the patience to work that way - and the fumes off that resistor are not pleasant.
Maybe. But the main signal boards are all socketed, so the fact that the psu board is too is not suspicious.
Yes, they're tucked away tightly next to the side of the transformer with the top prongs sticking out of it. You can only see one, but someone has used two in parallel to make up the required 560pF I would guess, but I can't see without pulling them - which I will of course. BTW, there's another variant of this schematic which uses just one 560pF 500V ceramic instead of the fixed and variable combo shown on the schematic I've previously posted, so perhaps Philips themselves had problems with this part of the design before!
I could find no further info on the actual model I have, but I discovered the PM3262 is identical except it has only 2 channels, so I downloaded the plans for that instead. It contains a rough description of how the switcher section works. Here's the link (ignore the 4112 bit, it's an error, you get the 3262 trust me!)
One thought has come to mind. Common advice on powering up a scope after long terms out of service involves the use of a variac to 'reform electrolytics'. This advice may be counterproductive with some switch-mode power supplies, which may never operate normally (for more than ~20mS) at their low-line dropout voltage. It's just not something that they are required to do, nor do they see this condition in normal service. It's usually avoided by introducing considerable hysterisis in the UVLO and start-up thresholds.
If you're confident that the high voltage bulk electrolytics are functional, you should probably just apply normal mains input to the fully loaded psu. This may allow it to function normally, if no stupid substitutions or errors have been created by earlier meddlers.
Based on your response (or lack there of) to earlier suggestions, I can only suggest now that you review previous posts on this subject. I personally couldn't go farther in troubleshooting this unit without running through those steps or gathering that information.
Already double-checked that yesterday, just as you suggested above. Exactly the same thing happens as when the smps board is out and on the bench and tested separately.
The original thread was packed with suggestions I'd already tried from people who clearly hadn't read all the posts. I don't blame them for that, of course, it became rather long and convoluted and I had trouble keeping track of it myself. I shall continue to work on it as time permits, but it's a challenge for me having zero experience with switchers before and not knowing what's been done to it by previous technicians. The fact that it's been 'got at' by others adds an extra layer of unknowns to an already tricky set of problems.
did you disconnect everything frmo the secondary side and fire up the prima ry side and see if the resistor still get hot.
if not, you have a fault on one of the secondaries, reconnect them one by o ne.
if it still gets hot, you have a fault on the pri side or a bad xformer per haps in the form of a shorted winding. Use yopur 2v drive on the pri to tr y to isolate a shorted winding,,,how??? I'm not sure. See if there is any winding that when you intentionsally short it, nothing much happens.
Yes, I'm pretty much on the same page as you with those suggestions. I've sorted through about 500 caps today and not one of them was a 560pF!! Could not believe it but still have a couple of thousand or so left so maybe there's still hope. I'm using an old valve HP sig gen for this purpose as it puts out a healthy 75V p-p maximum thereby getting closer to the operational voltage than a modern solid state one would be capable of. Anyway, I'll report back in a day or so....
It looks like the circuit is tricky and you aren't getting anywhere yet. The caps you mentioned won't do anything with the board outside the scope. Even if they were completely shorted, as long as the board is on your table and with no loads connected, they won't be noticeable. So you can skip them for now.
I think that you should first try to pinpoint, in which general direction to look for the fault, that is:
- in the power circuitry
- in the feedback, control and regulation circuitry
- in the output circuitry (rectifiers and capacitors)
- in the auxiliary supply (diodes V1816 to V1819 and caps)
If the fault appears in the power circuitry, it is likely
- due to dielectric breakdown (capacitors and transformer)
- due to diode breakdown
- due to operation outside of resonance (also control related)
To this end, you'll need a way to slowly power up this thing at controlled, limited power conditions and try to see what works (to a limited extent, given the conditions) and what doesn't.
I'll try to include some step by step instructions, but please be aware that this is mostly instructions for the desperate, so please take care to exercise more caution than you might otherwise.
You'll need a 12V load, an isolated variac, and a signal generator capable of outputting a square wave with variable duty cycle. The 12V load should be easy to observe visually, therefore preferably a lamp. It should be substantial (not a christmas tree light), but still sort of within the capabilities of this supply under heavily reduced voltage conditions (so not a car headlight either). A car taillight lamp is, I think, Okay, but a brake light lamp is maybe already too much.
You will also (actually first and foremost) need to pay attention to good safety practices, and also don't increase any voltage too fast and don't "take shortcuts". Patience is a virtue and "I'll just quickly do ..." has no place when working with a defective and live power supply!
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First, disconnect L1803 and R1807 and R1804. This will make the controller inoperative and allow you to drive V1806 externally.
Connect the signal generator to V1806 B-E (in parallel with R1812) A signal generator with 50 Ohm output, set to +/- 5 V peak square wave should easily drive the transistor into full turn-off and into full saturation.
Connect the board (mains input) to the isolated variac (or to an isolation transformer that is itself being fed from a variac), but keep the voltage at zero first.
Set the generator to the nominal resonance frequency that this board is expected to run at (value was mentioned somewhere in the previous thread), square wave, duty cycle somewhere around 30 - 35 %, 5V peak.
Connect the lamp to the 12.7 V output and some voltmeters to other outputs for reference.
Turn on the signal generator, make sure the waveform is right.
VERY SLOWLY start raising the input voltage with the variac. Don't go higher than about 70 V at any time.
Watch out for unexpected problems (heat, significant increases in primary current consumption, other signs of overload!
Try to power it up so far that you can see the lamp faintly beginning to light, but not much further than that. Watch for overload conditions. If you notice shorted rectifier diodes in the output (they would heat up) or capacitors (also), replace them and try again. Don't let the dissipation resistor (R1814) overheat, reduce voltage if needed. If you can't get the voltage up enough for the lamp to glow (because something else overheats first), try a small 3 volt lamp from a pocket light instead. It should allow the test to continue at much lower voltages.
Now, try to adjust it into exact resonance (it won't automatically be there with your "first try" coarse signal generator setting). Slowly and carefully adjust the signal generator frequency (still with 35 % duty cycle constant) a little up and a little down, try to get a feeling for the direction and sensitivity, and then set it for maximum lamp brightness (maximum brightness = resonance frequency peak). Keep the frequency there. If you (at any point) notice the lamp becoming too bright, wind the variac down. Don't ever let it burn out!
If you cannot get good resonance, even at low voltage, check C1806 and C1807. They may be open-circuit. Capacitors do not always fail shorted, sometimes they fail open, and sometimes they even fail with a weak (too high resistance) connection inside (this won't be noticed on a LCR meter). Especially foil capacitors at high impulse loads sometimes will do that. If in doubt, replace always both together.
Make sure that you really have resonance - check the waveform at the transformer. It should be a sinewave. Possibly with some distortion, that's okay, but still mostly a recognizable sinewave. This test is better performed with the small 3 volt lamp at a low variac voltage. If you cannot get a reasonable waveform, it means that some component in the power circuit is most likely shorted out completely. It may (or may not) be a diode. Also don't forget V1816 and its mates, C1819 and C1821. If you found a problem in this area, just disconnect all those 4 diodes for now, you can replace them later.
Always remember to switch it off (wind the variac full down) when changing lamps, in this mode the board is not supposed to run with no load at all, at any time and for any time duration. Not even for a fraction of a second.
Now put the 12 V lamp back in and dial the duty cycle on the signal generator very low (preferably around 3 %, at least smaller than 5 %). Make sure that you have the correct polarity (so that the switching transistor duty cycle is really 3 % and not 97 %)!
Now slowly raise the variac again. Keep an eye on the lamp, another on heat dissipation. Make sure the variac is isolated (or connected through an isolation transformer) because you will want to try reaching the nominal mains voltage now.
Keep an ammeter in the power input. Watch out for sudden increases in current draw. Keep an eye on the lamp and on the voltmeter(s) on the output(s).
Watch out for power "inversion" behavior. This is important! Watch out for behavior, where the lamp suddenly becomes darker instead of brighter as you increase the input voltage, although the current draw has increased. If this happens, then it's likely that you have a problem in the power circuit due to dielectric breakdown (or due to a diode breaking down). Set the voltage where this "just happens" and identify the part that is in process of breaking down (it may start hissing, smoking, getting hot and showing similar signs of distress).
If you found one, replace and repeat, looking for others. Whenever you have replaced something, go back to low voltage and 33% duty cycle and readjust for best resonance (because it may have changed when you replaced a part).
If you cannot get the input voltage back to full mains, even with minimal duty cycle (3 % or slightly less) on the signal generator, because something overheats too fast, and you cannot get the lamp to light (even a little), then something is wrong in the power section. It is most likely some isolation in some part breaking down or a part in the power section being shorted. Disconnect all rectifier diodes from all transformer outputs except the ones where the lamp is (V1831 + V1834) double-check or replace those and see if the problem goes away.
If you can now get it back to full mains without anything smoking, count yourself lucky. This would mean that the power circuitry is basically okay, and that therefore the problem must be elsewhere (in the control circuitry most likely).
At full mains, but still at 3 % duty cycle, the lamp will probably light very dimly (maybe hardly visible). That's ok.
Now try very slowly and carefully (and considering isolation and good safety practices of course, after all there are high voltages now) to increase the duty cycle in order to get the lamp to light up with "normal" brightness. Remember that there is no regulation and no feedback, so don't switch anything in "hard" steps and don't burn the lamp out. With the load suddenly disappearing (burned out lamp) the board would fail catastrophically and immediately, faster than you can turn the power off!
Carefully set the duty cycle to normal lamp brightness and let it run at nominal input voltage (variac at 100 %) and normal lamp voltage (around
12 V) for a while and watch out for signs of electrical and thermal overload. Don't touch parts now, they are at 310 V!
If you can let it to run in this way for a while, then the power circuitry is definitely good and the original problem is not in the power part. You will have to troubleshoot the control and regulation circuits later.
Drop the variac voltage somewhere low, measure the resonance frequency in this (semi-working) state and write the value down. You may need it later.
Carefully wind everything down, first variac to zero, then signal generator off, and reassemble the board back.
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Post here, how far you came and if you could really find a problem in the power circuitry or if it was good and the control part is the next in line.
@other regulars here: If you notice something amiss with the steps above, or something that is likely to be wrong, please indicate that.
LOL! I like your thinking. Actually when I returned to the task today and fished out another tub of caps, the very first one came out at 535pF - close enough to 560pF IMV - and didn't go up in smoke when I put 270VAC across it. Neither did its capacitance change afterwards, so I've got myself a replacement part at last. Funny how these things eventually work themselves out. :)
Dimitrij, you've gone to an awful lot of trouble here for which I'm extremely grateful. The main impediment at this time is that my fancy, high-end Marconi sig gen is temporarily out of order. Perhaps I could rig up something to do the same job from a 555 timer, but that's another story. I followed everything you said - except for one important thing: you say this board has to be under load at *all* times when mains powered or it will be catastrophically destroyed (or words to that effect). I'm a complete switcher novice, so there have been *many* times when I've probed this board under power with no load connected; I didn't know any better. Before I go any further I need to know if I've therefore toasted the board. Which components exactly catastrophically fail under no-load conditions? Thanks again for all your help.
Well this afternoon I removed the dodgy looking caps someone had soldered in. They crumbled into dust in the process; certainly not 500V rating no way. Soldered in a replacement. Anyway, as predicted by two of our gurus, no improvement. So I've just pumped in 10VAC p-p sinewave @ 20kHz to the primary winding and scoped all the secondaries. From these recorded voltages I shall now work out the implied winding ratios at the various tapping points and see if anything shows up as requiring further investigation (shorted turns and whatnot). Never give up, eh?
In that rather long description I posted a method for slowly bringing up the board "from zero up to where a lamp lights up". Please note that the first step was to make the board's internal switching controller inoperative (by disconnecting its start-up power supply and the power transistor driver transformer) and using a signal generator for timing. In this particular state of affairs the board (obviously) has no automatic control, feedback or regulation whatsoever. The only feedback "control" is through your eyesight (lamp brightness) and the reaction of your hands ("Turn the duty cycle and the voltage down quick NOW!"). This is not a particularly fast nor a particularly reliable method of SMPS control, :) so I put in some extra words about exercising caution. When running the SMPS in this "mode", you need to understand that the power flow in it is governed by two aspects.
The switcher at a fixed duty cycle (that was set on a signal generator manually) and at a fixed voltage (also manual from a variac) will basically be pumping out a fixed amount of power (a fixed amount of energy per unit time). Normally the power would be controlled (and automatically reduced very fast if needed) by the SMPS controller chip, but in full manual mode that's obviously not possible.
Now this "flow" of power is balanced by the dissipation in the lamp - and only there, because there is nothing else where to put the power into and no immediate way to reduce it either. As long as the lamp is still OK, whenever the SMPS power "flow" gets higher, the output capacitors will charge up, the output voltages will therefore rise, the lamp will light brighter at higher voltage, pulling more power. By its increased dissipation, the lamp power will balance the SMPS power, and restore a net zero balance, preventing the voltage from rising further.
But if the lamp breaks, the SMPS will suddenly find itself without a load, and with the power having "nowhere to go". Without automatic control the power won't get reduced automatically, so it will continue flowing into the output capacitors and charging them up. But they don't have a very big capacitance and in a fraction of a second, their voltages would rise way outside their ratings, causing some spectacular bang somewhere. This happens so fast that you won't have a realistic chance to turn down the operating parameters (voltage and duty cycle) manually in time before something breaks. Power supply controller ICs can handle power control and shutdown on a sub-millisecond basis, but human reaction times and hand movements - unlikely.
As you see, the "keep it loaded with no interruption" is a particular precaution that applied only to a particular mode of operation - full manual power setting with no automatic feedback control. Yes, it's tricky, and when you have an "uncontrolled" (unstable) SMPS waiting to do something stupid as soon as the power balance gets upset, that's where caution is definitely needed.
In "normal" circumstances, with the controller IC working and performing its natural protective functions a power supply is way more robust and can work with no load just fine.
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