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Dimitrij Klingbeil <nospam@no-address.com>: Feb 28 03:01AM +0100 On 25.02.2016 16:51, Cursitor Doom wrote: > 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.   Hi   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.     1.   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.     2.   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.     3.   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.   Regards Dimitrij

Dimitrij Klingbeil <nospam@no-address.com>: Feb 28 03:33PM +0100 On 27.02.2016 01:42, Cursitor Doom wrote:   > 60 12.7 6 0 -6 -12.7 -60   > So very close! Looks like the main transformer may be ok after all."   > Making progress! :)   Hi   Noted your progress :)   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.   Regards Dimitrij

Cursitor Doom <curd@notformail.com>: Feb 28 03:18PM On Sun, 28 Feb 2016 03:01:02 +0100, Dimitrij Klingbeil wrote:   > 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.   Yes, I bought 20 of those faster diodes to be on the safe side. :)     > 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.   [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.   > Ok, so much for the other BY208s in snubber circuits. Replace and see.   Certainly can do that, yes.   > 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).   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?   > 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.   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.

Cursitor Doom <curd@notformail.com>: Feb 28 03:28PM On Sun, 28 Feb 2016 15:33:02 +0100, Dimitrij Klingbeil wrote:   > 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.   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!) :)

N_Cook <diverse@tcp.co.uk>: Feb 28 11:26AM In hindsight it should have been obvious why no lead mode. There is an error on the Mesa schematic of Mesa Boogie 3 on Elektrotanya. The lead-switch should take the common LDR1 - 4 line, to ground and not directly shorting those 4 leds as in the diagram, doh!

jurb6006@gmail.com: Feb 27 02:19PM -0800 Just to let you know I just found this 3212 to have a bad transformer. It is a little different as it runs on 24 volts, but it is bad.   I finally hooked it up to all its loads and it let out a little smoke when excited by a power amp at 10 KHz. Couldn't see where the smoke came from so I had a rally good look and looked at the transformer. The outside insulation appears burnt, not too bad but bad enough I guess.   Thing is, this thing has no shorts in it. Being a self oscillator of course the bad transformer stops it, not enough feedback.   I know it is a different unit, but this proves the transformers can go bad. I'd've never thunkit. Like, flybacks in TVs went bad but they had 30 KV floating around in there. The highest supply on this one is 1,500 volts.   Anyway, just lettin' y'know that it is probably more of a possibility than I thought before. If you find out unequivocably that your trasnsformer is bad let us know and I will take that into consideration if buying another one that does not have basic operation.   I don't like dead scopes anyway, 'I like to see a trace before putting the money down, but this one got thrown in on a package deal type thing so it won't break the bank.

mroberds@att.net: Feb 27 05:56PM > En el artículo <naqubs$uaf$1@dont-email.me>, mroberds@att.net escribió:   >> Taking mine apart,   > You didn't have to go to that much trouble, but thank you.   I use it a lot in fan-only mode, so it was about ready for its annual dusting anyway. :)   > There's 4 ceramic cores with two heat levels selected by a switch > (heat level 1 = 2 cores operating, heat level 2 = 4 cores operating)   Same here.   >> The part I have looks like their "AUT-P" series (with the white >> plastic tab)   > That sounds like a resettable one.   It is. There is no user-resettable button; you just have to unplug the heater and let it cool off.   > The ceramic element is held in place in a hard plastic frame, so isn't > going to be allowed to get very hot.   Same here. On mine, the frame also extends back towards the intake to hold the fan motor and make a venturi/shroud for the fan blade.   Matt Roberds

Mike Tomlinson <mike@jasper.org.uk>: Feb 27 06:56PM >On mine, the frame also extends back towards the intake to >hold the fan motor and make a venturi/shroud for the fan blade.   Yes, same here. It's all one piece. The section holding the ceramic element is square, then there's a short cylinder to the fan housing which is square. The lot fixes to the back half of the enclosure with 4 screws.   I suspect this is pretty much a commodity part made in China and badged with OEM logos as required.   I've ordered the 95C switch, pretty sure that'll do the trick. Thanks again for your help, have a virtual pint on me :)   -- (\_/) (='.'=) Bunny says: Windows 10? Nein danke! (")_(")

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