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Interesting problem. Many great suggestions have been put on the table from different members so there is not much to add. The resister that is runnin g too hot is telling us something. Why is that resister not a happy camper? What is it connected to. There is a hiss or possibly plasma arcing and a r esister that is not happy. looking at a diagram to see what the unhappy res istor does in the power supply could provide a clue.
Yes, a schematic would be extremely useful but I don't have the precise one; just a similar model of scope with a similar ps section. Some parts are correct and in the right place, others aren't. You have to work with what you've got. Hopefully tomorrow I'll discover a few more clues...
For goodness sake, this is out of the PM3264 right? So what happens when you put it in the scope? I can't find your original posts on this but these are not very complicated supplies and pretty reliable. Main fail is the electrolytic caps, all of them will be useless, Philips blue caps suck, they were never low ESR in the first place and from your photo it looks like you haven't replaced any of them. The hissing is probably the
1.5Kv from the transformer. Its a good scope if you can get it working. Buy the manual.
I'm reluctant to do this until I'm sure the psu won't toast it.
No, because I've tested all of them with one of these:
and they all check out totally fine.
Thankfully I've now managed to get a circuit for the psu section off the net. That will be a *major* help. I'll be happy to buy the complete manual if I can get the thing at least partly working.
Do look for a short to (cold) ground on the secondary, and if you can isolate and open up that short (like by removing the shorted zener, if any) then bring it up briefly on a variac and 'scope the dc line(s) for noise. Remember to use an isolation transformer when servicing the primary circuit.
Mark Z.
Thanks, Mark. I'll try that in the morning along with a few other things as it's getting very late here (1.20am) so I'm going to call it a day for now.
Sorry for the delay. I'm having to do odd bits towards this project in between other chores.
OK, it IS a 20 ohm resistor and the voltage across it giving rise to 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.
The resistor in question is R1814 just to the right of the chopper transistor on the schematic:
That's all I know right now and I have to dash off again for a couple of hours....
cheers.
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.
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.
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
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....
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. :(
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 circ uit 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 probab ility.
If I had that supply on the bench, I'd pull every cap and test for ESR, val ue, leakage, and dielectric absorption.
o
5 plus years in the trenches that electrolytic capacitors can pass an in ci rcuit ESR test and still be bad. It is an extraordinarily low percentage t o be sure (although in Mitsu DM boards it's typical), but it's still a prob ability.
alue, leakage, and dielectric absorption.
Good thing this is a hobby project and not a repair with the customer breat hing down your neck. Your hot 20 ohms resister goes through a diode then th e collector of the main switching transistor. That transistor has a emitter resister that is of interest. Can not make out its part number or value fr om the schematic. I believe it is responsible for over current feed back to the controller IC for auto shot down protection. I will venture a guess it is in the 1 to 5 ohms range. Knowing it is okay would be a plus. It is tri cky to find a ground with the whole controller IC and main switch transisto r connected to the bridge rectifiers directly from the AC plug. One false m ove and there will be more than a hot 20 ohms resister to worry about. Howe ver the secondary side of the switching power supply is nice and isolated f or easy measurements. I see at least 6 or 7 DC outputs there ranging from 5 to 60 volts. None of these voltages will be correct however by comparing t hen a general idea of what percentage of their target. If +60 volts is meas uring +20 volts then all the rest of the DC outputs should be 33 percent of their target output voltages. By going through the other 6 or so DC output s you can see if they are reasonable or possible have a problem. I do not t hink this will pan out but at least you can say you made the measurements. Another last minute desperate grabbing at a straw , try unplugging the 14 K Volt cup off the CRT just in case it is gassy loading down the power suppl y. Unlikely as you would have seem in glowing but hey you never know.
It will run hot whenever operation is abnormal. Just doing its job.
RL
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