Update on H7826 Power Supply Repair

It has been a while since I last posted anything about what I have been up to. However, I have been busy trying to find out what is wrong with the H7826 power supply out of my TURBOchannel Extender.

I received advice that there were errors in my method for checking activity on the primary side and that the fault could be on the primary side. I spent some time reverse engineering the schematics for the primary side. Not being an electronics engineer I may not have drawn this logically, and there will be tracing errors. I will update the images as time goes on, so by the time you read this it may have been improved, but don’t count on it. Unfortunately the part numbers are not labelled on the boards, so I have labelled photographs with part numbers that I have made up myself.

This is the schematic for the main input section.

This is the schematic for the 50-19530 daughter board, which I think is for Power Factor Correction.

This is the schematic for the 5019572 daughter board, which controls the switching transistors. This one was done by Matt Burke, I have added labels to a photo and filled in a couple of missing bits.

Further probing with an oscilloscope revealed that the 555 on the 5019572 daughter board was not oscillating, which means the switching transistors are not being switched. I then found that the Vcc input to the 555 was not rising above about 0.2V. So the next thing was to see why this might be.

Unfortunately, while doing the probing around the 555 I think I managed to discharge the large smoothing capacitors near the 555. Thankfully I had already turned the power off, but there was a sharp crack from the spark. I could not find any physical damage anywhere.

I took the 5019572 out to test it on the bench with a bench power supply. I found that it drew far too much current, the current limiter on the bench power supply happened to be set to 1.2A, and that is what it drew. Clearly there was a short. I suspected the 555, partly because of the spark problem. When I took it out I found that it was indeed shorted across Vcc and Ground.

I replaced the 555 and then I found that its output oscillated. However the current draw was still high and the other IC, the UC3842 got very hot. I checked all the resistors, diodes and capacitors for shorts and they all appeared to be fine, so I suspect the UC3842 is also bad.

I now have to order a replacement UC3842 and see if I can get the daughter board to work correctly.

I am not sure if the short (or shorts) have been caused by the spark I provoked. However, the fact that Vcc on the 555 wouldn’t go above 0.2V when the board was installed suggests it may have already been shorted, or the UC3842 was already bad, but of course I won’t know until I have the daughter board working again and installed back on the main board.

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