EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: gmc on November 17, 2022, 10:32:28 pm
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I'm busy trying to repair a SMPS from a IBM XT Computer from the 80's and unfortunately there is no schematic so my first order of the day is to reverse engineer the PCB.
I've been looking at the high voltage transformer which I removed and was beeping out and thought it was faulty as 2 of the wires are not connected together. I thought they were another winding.
I thought this a sure sign of a failure. Was just about to rip it apart (This looks like a point of no return as there is no way to reassemble) to confirm my suspisions when I traced the 2 suspect wires.
One goes to earth and the other to 380V DC in - so they can't possible be joined! |O
What kind of sorcery is this if they are not connected to any of the other coils. Is there some magic about transformers I'm missing?
I've attached some pics of the transformer - I know it's going to be impossible to probably tell from the pics what's going on but maybe someone has seen something like this and can offer a reason why this is done? Only possible thing I can think of is it's for noise reduction?
I've drawn a symbol of how the coils are connected (not 100% sure about the pri/sec sides) but the interesting ones are bottom left.
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Must go somewhere, otherwise you might have found the problem.
Was any of the primary transistors shorted/blown?
A huge current spike like that can damage the coil if the wire is thin enough.
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They could well be electrostatic screens for EMC reduction.
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I've rechecked and they are definitely not connected to any other pins on the transformer. I
I've checked all the transistors, diodes etc and can't find anything that is shortted.
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They could well be electrostatic screens for EMC reduction.
I think you might be right, this is the only possible explanation.
I think it's time to get a differential probe and start checking through the circuit.