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Trouble shooting your power supply part 2
Now that you have finished testing with the directions from part 1, let us move on. If all the pass transistors and the drive transistor check out and you replaced the bad ones, then we need to do something with the regulator.
Most of this writing is pertaining to the 723 14 pin IC voltage regulator. The little chips are cheap to replace and I know of no way to test them. So I just pull them out and replace them. Always make sure that the filter capacitors have been drained of all measurable voltage. It seems to "kill" the chip when removing or replacing with voltage on the capacitors.
While you are in the unit, do a very thorough visual, everywhere. Take notice of any components that may have been hot or burned. Check solder connections and grounding screws.
Usually if a pass transistor shorts out, from the emitter to the collector, it takes out the 723 voltage regulator. Some times a bad switch or a dirty switch can cause such a spike as to kill the 723. And in turn, one of the pass transistors shorts.
There are times that the SCR in the over voltage protection or crow-bar circuit can die. There are also times that the zener diode can give up in a shorted configuration and cause the SCR to be in conduct mode all the time. When this happens, upon turning the unit on, the positive is held at ground. This will cause the input fuse to open providing the fuse is of the proper value. In this case, it is fairly simple to follow the wires that are connected to the SCR and you should be able to locate the zener diode on the circuit board. Lift one end of the diode and test it. Here is a link to finding out what an SCR is all about: SCR click here
If you have done all this, put the cover back on and secure it. Then plug the unit in and turn it on. Measure the voltage at the output and see if it is acceptable. If not, then we have to look at the unit in more detail.
The mods page goes into detail as to a problem that a certain brand has.
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