Ok so I'm trying to repair my own tools. I'm learning as I go, by watching all the videos and reading all the beginner materials I can. I have a unit when functioning normally takes mains 110V and goes through an internal transformer to output 17V AC to the board in the picture. The unit has an over voltage light, and reset switch on the front panel. Normally when I set the speed control pot at minimum I get 1.8V DC, speed control at mid point 11.2V DC, and speed control at maximum produces 34.36V DC. Keep in mind also that as resistance is placed on the DC hand piece that the unit will compensate to maintain constant rpm up to and including 45,000 rpm. The unit when turned on was functioning fine, then all of a sudden the hand piece goes to 45,000 rpm and thats all it will do. The over voltage light came on also. The issue I'm having is that the company that repairs these units, says the board is too old, and they won't repair it.
I started by desoldering every component on the board, identifying the component and then researching how to test it after buying a new Fluke 87V. I replaced the BDW93C TO-220 transistor with a NTE54 because it looked overheated on the heat sink side (bluish color). I also replaced the D1047 transistor because I had one in the parts bin. I replaced the LM358N op amp. I checked every other component except the TL431 (because I could not find out how to test it). re soldered everything back in, and when I turned the unit on it outputted 0V DC at min, 10.9V DC at mid, and 12.90V DC at max. When I connected the hand piece, it ran, but when any speed change was introduced, it would cycle up and down. Then as I was testing, it went wide open to 50V DC and the over voltage light came on again.
I want to fix this and learn from it. What did I miss? Any ideas why it's doing this?
Thanks
taco
(Navy Veteran)