About Conceptronic 90W Power Supply (Part 2)
The first part can be accessed in the below link:
I replaced yesterday the defect components by all new parts from China after almost 4 weeks of waiting. And I noticed that to my shame the Conceptronic 90W Power Supply didn’t behave as I had hoped for. Although my article in waiting for the parts arriving proved to be a good way to analyse exploded controllers I now come to the conclusion that the transformer sadly must be partly defect. Somehow the fuse should have protected the transformer but didn’t. And this is how I came to that conclusion.
I found a loose primary wire that was hidden under the glued 400 Volt Buffer Elco, and after soldering it back to the pin on the board the Supply finally came to life.
From the left to right are the first 2 wires. The DC Run Vcc voltage for the LD7575 controller.
Above : Principle schematic of the Conceptronic 90 Watt universal PSU.
But on the controller OUT was a shorted diode (SD1 above schematic) without any marking besides its Cathode marking and so I removed it without replacing. (without any consequences so far. Possibly only for some kind of protection).
Also the low output impedance measured on pin OUT of the LD7575 is a direct cause of the internal pull-low resistor to prevent any floating conditions.
A 5Volt 0.11A bright Led light worked but it burned about one second, went Off for a moment and then gave light again for a second etc. But my 60 Watt light Bulb only briefly went on when the PSU was switched on. And the secondary 5 Volt voltage was with 5.4 Volt a bit too high. So first I checked the transformer again with my Blue Ring tester.
The transformer had one coil for DC run controller Vcc purposes. And it had only 1 or 2 flickering leds with the Blue Ring test, which is perfectly normal! And measured with my Digital meter on diode test it beeped for low resistance check. And the second and last coil wire (also connected to the gnd of the Buffer elco) of this first coil had one or two extra wire(s) that seemed to go into the transformer but I was unable to identify where to?
The other and second primary coil was not connected to the first coil and had one twisted wire pair coming out of the transformer top, and 3 bottom wires of which one only was connected to the transformer bottom itself. The Twisted wire pair of this second primary coil went to the Anode of special MURF840 diode. The most left bottom coil wire was plus via the diode coming from the + of the diode bridge, and the most right wire was connected to the Collector of IGBfet 20N60C3. And all Blue coil tests were okay except the most left coil bottom pin (+ in 300Volt) if checked against the Twisted wire pair.
But one can easily oversee this because the second coil has 4 connections of which one wire is only going to a bottom Transformer solder spot. And from the 5 possible Blue ring tests only the one from left pin against the twisted wire pair fails!
The next thing I assumed was that Vcc was oscillating. And measuring this with my digital meter proved that the voltage went fluctuating from 12 to about 22 Volt. So I checked the bottom components again and noticed that diode SR11 in serie with the Vcc pin of the LD7575 controller was somehow high impedance in 2 directions what shouldn’t be the case. No problem for the Vcc of the LD7575 controller because it handles up to max 30V DC.
After replacing it by a 1N4007 the power supply indeed worked uninterrupted. Without fluctuations continuously but with an secondary output USB voltage of about 5.4 Volt still a bit to high.But worse the 60 Watt light bulb was now burning about 30 Watt or so. But the secondary display and the configuration switch worked fine because I could set the output voltage to any voltage from 12 Volt to 24 Volt. So in conclusion I have to confess that the Conceptronic transformer is kaput beyond repair. Too bad that fuse (5A !) and other protective parts didn’t save the transformer what one should have expected.
Else it would have been a fantastic power supply repair. And sure it cost me some time and some new parts. But I now can use this partly functioning Power supply to test any LD7575 controller components and I am also new knowledge richer! And maybe I’ll find one of these days another Conceptronic 90 Watt PSU with a good functioning transformer for replacement. Because the other parts are functioning perfectly. But I hope this article explains how and what is, and what is not repairable. But one only can be 100% sure afterwards when every error is fixed and only if every bad component can be replaced. And old ways that prove to be wrong, lead ultimately to new and better ways of doing things.
Albert van Bemmelen, Weert, The Netherlands.
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