About switching power supplies

[badbaud]

Bally black power supplies, Tatung switching power supplies, Games of Nevada switching power supplies, and many more have one common feature that may be missed when repairing them.

Power up the defective supply then turn it off, measure the voltage on the two (or single) main capacitor. If you are reading around 160 VDC but the supply puts out no voltage then the rest of the electronics on the board is not getting voltage. It just sits on the capacitor and takes a long time to discharge (I use a 2.4k 3W resistor to discharge the capacitor(s)).

The common factor in all of these supplies is the use of a high Ohm resistor the trickles some voltage to the rest of the electronics to tell the main MOSFET to fire up and provide the output voltages.

These resistors can change value, or in some cases open up, and cause the supply not to work.

For example, on the black power supply there are two 150k 1/2w resistors that will change value and cause the MOSFET not to fire up. On another brand of power supply it was a 560k resistor. The Tatung supply I just fixed had two 100k resistors and two 270k resistors (one not really used, follow the traces on the board) that had changed value, one of the 100k's had opened up.

On a Power One supply it was a 390k resistor.

These resistors are located near the input circuitry of the supply.

So a clue is the main caps charge up and the DC voltage is still on them long after the supply is turned off.

Remove the high Ohm resistors in the area and measure them with your Ohm meter to verify this is the problem.

A open resistor may indicate one of the smaller transistors that fire up the main MOSFET may be shorted so check that also.