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| Cursitor Doom <curd@notformail.com>: Jan 23 04:53PM On Fri, 22 Jan 2016 12:11:31 +0000, Chris wrote: > It can be so hard to tell sometimes. Try scoping the output from the > transformer (taking suitable precautions of course!) and look for any > irregularities that might indicate internal arcing. Well I've just scoped the B/E junction of the chopper transistor and it's just ALL noise; no discernable control pulses at all, so clearly this thing has issues beyond what this medium can assist with, I'm sorry to say. :( Anyway, must do some shopping right now or there'll be hell to pay. |
| "Ralph Mowery" <rmowery28146@earthlink.net>: Jan 23 12:03PM -0500 "Cursitor Doom" <curd@notformail.com> wrote in message news:n808ru$u87$1@dont-email.me... > resistor, makes no sense at all to me. It shouldn't even be getting warm. > The resistor in question is R1814 just to the right of the chopper > transistor on the schematic: As the calculated power is less than 3/4 of a watt it should not be getting hot. It may be that as this looks to be in a high frequency part of the circuit the meter may not be giving the correct voltage. I would think it would be some AC value, or maybe puslating DC as there is a diode there. May show up as a better value if you measure across the coil that the diode and resistor is across. Might even need a scope to show the true value. |
| Dimitrij Klingbeil <nospam@no-address.com>: Jan 23 09:03PM +0100 On 23.01.2016 17:13, Cursitor Doom wrote: > this heating effect is 3.6V and given that this looks like about a 4W > resistor, makes no sense at all to me. It shouldn't even be getting > warm. Don't get deceived by a DC voltage measurement on that resistor. It's very unlikely to be realistic unless you have a true RMS multimeter with a very wide bandwidth (like hundreds of kHz to lower MHz). The resistor is in the reset circuit of the inductor L1804. It't there to "reset" (dissipate the energy of) the field, so it gets to dissipate whatever this inductor has stored during the switching cycle as soon as V1806 turns off. In this circuit, the resistor has a highly uneven load that basically comes in the form of very narrow and energetic pulses. A DC average voltage may indeed be low, but the true RMS voltage (and therefore the equivalent heating value) will not be. It's likely that the resistor may be getting pulsed with a couple of amps worth of current (and corresponding voltages according to Ohm's law), but the pulses will be narrow. Simple voltmeters won't register that and even some true RMS voltmeters ("typical cheap" ones) may have insufficient bandwidth to register the pulses accurately. Dimitrij |
| Cursitor Doom <curd@notformail.com>: Jan 23 08:48PM On Sat, 23 Jan 2016 21:03:32 +0100, Dimitrij Klingbeil wrote: Thanks for that, Dimitrij. Well, I can scope it for a better view of what's happening, because certainly the voltage readings don't make sense so your explanation rings true with what I'm seeing here. I have to say I've never probed one of these before. I'm familiar with how they work at a block diagram level, but some of the actual circuit topology is completely alien to me. It's a big obstacle. So that's another thing to try tomorrow. I'll also recheck the B/E junction of the chopper with the scope's filtering tweaked; see if I can get rid of that noise.... |
| Cursitor Doom <curd@notformail.com>: Jan 24 03:13PM Yes, the voltage across that power resistor looks very different when scoped. No clean pulses at all; just constant NOISE and lots of it up to about 20 or more volts so no wonder it was getting hot. I also scoped the pwm output from the controller chip (TP.2 on the diagram) and it looks like it's outputting a pulse train but is totally overwhelmed by noise which I would imagine must be virtually saturating V1812 resulting in a close to 100% duty cycle at no load. Clearly major stability issues here. :( |
| John-Del <ohger1s@aol.com>: Jan 24 07:45AM -0800 On Sunday, January 24, 2016 at 10:16:22 AM UTC-5, Cursitor Doom wrote: > overwhelmed by noise which I would imagine must be virtually saturating > V1812 resulting in a close to 100% duty cycle at no load. > Clearly major stability issues here. :( At the risk of raising the ire of some contributors, I've found over my 45 plus years in the trenches that electrolytic capacitors can pass an in circuit ESR test and still be bad. It is an extraordinarily low percentage to be sure (although in Mitsu DM boards it's typical), but it's still a probability. If I had that supply on the bench, I'd pull every cap and test for ESR, value, leakage, and dielectric absorption. |
| avagadro7@gmail.com: Jan 23 03:15PM -0800 http://goo.gl/vWyckv |
| davidrsmith.uk@googlemail.com: Jan 23 02:11PM -0800 Yes, I appreciate that this thread is almost ... 20 ... years old. But just for giggles, I can confirm that we had the same model VCR from 1986 to 1992, and whenever sunlight would fall on the remote sensor, it would startup and record .... On Friday, 27 November 1998 08:00:00 UTC, Franc Zabkar wrote: |
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