24th February
written by Todd Harrison

This is Part 3.  You may want to read (part1) & (part2) first.

In this part I was going to probe the board to find where the signal was being lost for the controls. After mapping out all the connections and looking up the functionality of the ICs in the earlier postings I was confident I was going to track down the problem. I had the motor secured in my bench vise, the power supply and speed control board connected to the speed sensor feedback and mains 120v to a switch. When I turned on the 120v mains power I could smell something getting hot real fast. I had to work 30 second at a time and let the control board cool between tests. Something was not happy in the power supply! The heat was coming from a large 4watt 1.8K ohm resistor. I didn’t dare probe for more than 30 seconds because it was clearly getting too hot. But I pushed my luck too far (KA-BAM!).

READ —>:

Yes a trace turned into a make shift fuse and the bridge rectifier was also destroyed.

I cleaned up the PCB to get a good look at the carnage.

My guess is the bridge rectifier shorted across the mains and vaporized the traces to the 120v mains connection. Strange being the bridge rectifier wasn’t getting hot at all. The below is the old 10A bridge rectifier.  It tested as shorted between the middle pins which is where the 120v mains connect. You can’t have a dead short between these pins in a bridge rectifier without getting some magic smoke!

I had a salvaged bridge rectifier from an older teardown project so I checked it’s specs online. It was good enough for testing at 8A so I dropped it in.

Plus a big fat jumper wire to replace the vaporized trace.

Time for more testing. (NO I’m not going to test it using 120v mains when its in the vise! Don’t even ask, I’m just taking close-up photos.)

But now I’m in no mood to continue probing this “widow-maker” hot with live 120v mains. I had stopped by Circuit Specialists in Mesa, AZ earlier and picked up two of each IC on the board for just a few cents. I’m done probing around hot so I’m just going to replace each IC and stand back a few feet when I switch it on this time.

Darn, it still didn’t work even with all new ICs :( . It seems like all my repair projects lately have all been failures. This one really has me stumped. I know I could probe it more and find the problem being this board is so darn simple but with it overheating so fast I just don’t want to be near it  with probes. I know it’s not the bridge rectifier or any of the ICs. I made a jumpier across the fuse to make sure that wasn’t holding back on the current somehow. I have testing all the transistors in circuit and they seem fine.

So what next? I will find a known good 200V 3300uF capacitor and make sure that isn’t somehow the problem. I have found capacitors to be the problem with power supplies many times but normally they show damage and/or don’t test. If that doesn’t get it I will just start test components cold by removing the ones that are best tested out of circuit and hope I find a bad one.

I just don’t want to test this thing hot anymore. There is plenty of magic smoke left in this power supply and I’m not looking to let anymore of it out. Working with 120v mains right in front of your face is NO FUN! I don’t recommend it to anybody.

Just posted part 4.

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