Late 1970s SMPS Design Query

On a rainy day (Sun, 24 Jan 2016 12:34:24 -0000 (UTC)) it happened Cursitor Doom wrote in :

C18907 C1808 are in series with voltage divider resistors to tune the output transformer. V1808 and V1809 and associated RC is probably to limit peak voltages. V1811 feeds peaks back into the supply. C1804 47nF is in parallel with the supply filter caps C1802 C1803, probably for RF decoupling.

Thoughts not to be published... V1811 'boost diode'. Looks like a copy of an old TV output stage, xept those were better. misty too. temperature now way above zero next weather report

Yikes, who designed that?

Throw us a bone here, John. What's up with it?

Looks a little bit like something Tektronix would come up with but it is not drawn like a Tek schematic. Maybe HP.

Back then most of them were still using big iron power supplies.

I wish this one was, I'd have it fixed by now! It's actually from a old Philips analog scope I'm trying to get working. The SMPS board has obviously had some past issues, given there is clear evidence that it's been previously repaired at least once. I was thinking about maybe salvaging the re-usable parts like the transformer, inductors etc. and rebuilding them into a new board to a more modern design with new caps and semiconductors; a MOSFET for the chopper instead of the BJT and whatnot. This old design seems overly complicated and I'm sure something simpler could be made to work equally well?

Analyzing that is too much like work. But it does have an astonishing number of parts in bizarre arrangements.

The 7000 series Tek scopes had conceptually similar power supplies, but simpler circuits. Those supplies weren't especially reliable, either.

I never much liked the Philips scopes. Why not trash it and get a nice Rigol?

I recall you're a big Rigol fan, but I'm really struggling to accept that the Chinese can manufacture their own high quality gear. I associate high quality gear with W. Europe and N. America. Plus I'm into vintage electronics as a hobby and tend to collect old boat anchors. I can hardly move here for them, actually. Seriously.

Okaaayyy... Thank you, Jan. By all means feel free to expand and clarify if you happen to feel so inclined. ;-)

The Rigols seem to be at least as good as Tek, sometimes better. Easier to drive, fewer bugs, 1/3 the price. We have a lot of low-end Rigols, but we also have the 4-channel 1GHz scope, $9K, which is superb.

I can't use analog scopes any more. All the traces are the same color, and when the trigger stops everything disappears. That is so weird.

I have some old tube scopes. My favorite was the Tek 547, which was a piece of art; I have 5 or 6 of them, and a zillion plugins. I also have an HP 185, the first really good sampling scope. But I don't use them any more.

One of my bench scopes in my office is a Tek 11802, a 50 HGz sampling scope. It's all silicon except for the raster-scan CRT display. I love that scope. One day it will die, and I'll miss it.

On a sunny day (Sun, 24 Jan 2016 18:11:24 -0000 (UTC)) it happened Cursitor Doom wrote in :

Well, I dunno, you mention Philips scope. That confirms my idea that a TV designer was at work, like H output TV stage.

In some old Philips TVs the resonant caps went open, causing higher voltage than normal (K12 chassis I think it was) and then tripler (that is the cascade) breakdown. You had to exchange both the cascade AND the caps (C1807 C1808). If it are those long green or pink caps with purple or gray color ends do not trust them. Those are HV caps OK? That is why 2 in series and the voltage divider to keep the voltage equal. But that is a wild guess, there was once a service note from Philips for that (for TVs).

Anyways, plz tell us first what is wrong before we can make any suggestions.

I disagree that 'designing just a replacement' is simple. There are too many voltages and unknown currents and not many people have extensive experience with this sort of output stages. But it can be done, I did it, but alas, was that not 40 years ago...

Basically, there are huge amounts of NOISE present in places where there shouldn't be any and the thing appears to be running flat-out even when completely unloaded.

I've seen some of the stuff you've posted here over the years; very impressive for a hobbyist, so when you say that, I don't dismiss it! BTW, here's some photos of the board itself. You can see it's been seriously hot in places at some time in the past. Maybe it's just a rotten design as JL says.

You could sink a mass of time trying to upgrade it. You seem to really like this scope. Why not get one on eBay, with a working CTR, hence a working supply, and replace your supply with that one?

Indeed Win, it makes no economic sense to fix this thing up. I'm not especially attached to it, it's just one of 13 old analog scopes I have here in need of attention. My primary motivation is to *learn* something in the repair process about how smps work. Come to think of it I recall there's really very clear explanation at component level in your most excellent AoE book (the first edition) I really must take a closer look at it....

It's just a parallel resonant main transformer with a series resonant switch, to allow for pwm. This gives a comparatively well-regulated set of output voltages from one transformer, for postregulation, where required. With low dV/dT rectification and quasi-sinusoudal rectified current, this produces reduced EMI and diode stress. The HT supply for the CRT is generated by a multiplier.

The extra bits around the transistor collector serve as clamps and limiters under transient, overload and start-up conditions.

RL

On a sunny day (Sun, 24 Jan 2016 18:52:20 -0000 (UTC)) it happened Cursitor Doom wrote in :

But I am no hobbyist :-) Maybe now I am :-)

Strange Philips with Spraque capacitors (electrolytics). Philips made their own.

Not sure how it is today, but in Eindhoven from design concept to working board was 2 weeks IIRC. Try it some time.

I dunno - I found it highly offensive that when I scaled two channels on someone's fancy digital tek so I could subtract the one from the other (or invert and add) and look for discrepancies in the amplified signal the f-ing thing ignored my scaling and subtracted the absolute voltage of one from the absolute voltage of the other, making it f-near useless for that basic function that pretty much any old CRT scope with two or more channels can do.

As it happens my scope of the moment is a Philips 25 MHz that works pretty well (some signs that the input switches could stand a cleaning) which is not bad IMO for $100 (used, of course) 8-10 years ago, or something like that. The timebase does have a bunch of obsolete CRT-centric functions. The Heathkit 1010 is out of whack and I don't think it will really be worth bringing back, though I have not throughly investigated. But as a 10 MHz room-heater (tooobe scope, not just CRT toooobe, and the CRT is 2.5 or 3") it seems unlikely to be economical to resurrect. The Naval Surplus job that was lurking in the school's physics closet is probably worth more to a collector, if there are crazy collectors out there - I forget if I've actually checked it recently. Some of the accessory cables are still in the paper-foil wrap from Uncle Sugar IIRC - it obviously hasn't had a lot of use here, nor in whatever war it was supposed to be for.

On a sunny day (Sun, 24 Jan 2016 19:37:46 GMT) it happened Jan Panteltje wrote in :

OK, if you have an other scope, check the filter capacitors for any fast rising edges, if present replace those caps.

?? All the blue electrolytics are Philips's own! I think you can just about make it out from the photos.

On a sunny day (Sun, 24 Jan 2016 20:40:35 -0000 (UTC)) it happened Cursitor Doom wrote in :

The big vertical ones in the metal bracket holes are marked spraque.

Hi

Here's my basic understanding of it, however limited it may be (I'm also seeing this thing for the first time).

C1807 and C1808 form a series capacitor, with the 10M resistors for balancing (to prevent one of them from being overcharging by random leakage currents), so you can treat the whole C1807-C1808-R1817-R1818 thing as just being one 15 nF capacitor with a double voltage rating.

This compound capacitor, together with the T1801 transformer's primary forms a resonant LC circuit. Being resonant, this LC must be working with a waveform that closely approaches a sine wave (in contrast with typical switcher waveforms that are more square-like).

Now the whole thing in line between V1806 and T1801 is basically a power square to sine wave filter. V1806 is driving a square wave (being a switching transistor it can't do otherwise), so it takes some filtering to make a sine from that.

The main part of the square to sine conversion is probably done by L1806 (the primary winding of it, the one on the lower side in the schematic). Together with L1804, L1806 serves the same purpose as the series inductor in the classic Baxandall converter that is used in resonant CCFL drivers.

Like in the Baxandall, the inductor sees a square wave on one side, a sine wave on the other side, and eats the difference. The waveform on this inductor therefore looks like some sort of arithmetical difference between a square and a sine.

Only in contrast to the Baxandall, which uses a push-pull topology where one of the two switching transistors is always on, in your power supply, the V1806 transistor only provides conduction during one half cycle. This leads to the necessity to discharge the energy stored in L1806 during the other half cycle, during which the transistor does not conduct (in the Baxandall this part is handled implicitly by the other symmetrically arranged transistor coming on as soon as the first one turns off, but here, there is no second transistor). Therefore a secondary winding is provided in L1806 (the upper one on the schematic). This is a reset winding, and its function is to reset the field of L1806 during the off-cycle time. In the course of the reset, the stored energy is circulated back into the primary capacitors C1802+C1803 through the steering diode V1811, which thereby forms an energy recovery circuit.

Now because there is still some leakage inductance between the windings of L1806, there remains some little energy stored in the leakage inductance that the energy recovery circuit cannot recover and that would otherwise end up creating huge voltage spikes at turn-off. This energy from the leakage inductance is dissipated in the RCD snubber circuit formed by R1816, C1806 and V1809. The snubber eats the spikes in the capacitor C1806 and slowly dissipates their energy in the resistor R1816, thereby keeping the inductive transients of L1806's primary at bay.

Note also that L1806, together with its upper reset winding serves an additional duty as the power supply's overload protection. Should the main transformer (secondary or otherwise) for whatsoever reason become overloaded or shorted out, then L1806 becomes a current limiting element (like a fluorescent light ballast) that circulates energy back and forth from the primary capacitors and back to them. This runs the switcher at maximum rated current, but safely sends the energy back into the supply, preventing the switching transistor from overloading.

Now, for whatever reason, the power supply's designer must have figured out that L1806's inductance alone was insufficient to cleanly convert a square into a sine wave. Maybe L1806 has a little too much interwinding capacitance, and therefore it has ended up less efficient than desired when it comes to filtering the high frequency components (the steep rise times) of the switching square wave. So the designer has added L1804 in addition (likely making it smaller and of a construction with higher self-resonant frequency, at least that's my guess). Now this winding, too, needed a reset, but either the energies involved would have been much lower and/or possibly the reset did not need to be complete and significant DC field is allowed to remain in L1804 on average, so the designer chose to provide no energy recovery but a dissipative snubber instead, and made that snubber from V1808 and R1814.

L1804, having no way of energy recovery, is probably not directly involved with overload protection, but still, in the event of an overload, when the energy transferred over L1806 reaches its maximum, L1804 would also see both maximum average currents and maximum peaks.

Therefore, since the peaks are being dumped into R1814 for resistive dissipation, the fact that R1814 heats up too much, especially if there is no load on the outputs, could be an indication that something is overloaded down the line (in the main transformer or later) and the whole thing is operating in permanent overload protection mode. In this mode the main transformer would be out of resonance and L1806 would be circulating a lot of energy back into the input caps. Now what is really responsible for the overload is another matter, but it could be anything either in the main transformer itself (winding is suffering a dielectric breakdown, shorted or arcing) or later on (output rectifiers or output filters). Can you check the main transformer's waveforms for evidence of dielectric breakdown? (Careful with the high voltage!) If some isolation breaks down, the waveform would be unlike a sine wave, and would likely show places where the voltage "just drops for no reason".

Note that there are many guesses above, wild or otherwise, so please take my description with a grain of salt and, anyone here who happens to know more details, please correct what needs correction.

Dimitrij