Switching Power Supply repair PSS-28G from Power Switch Corp. 25.4V @ 6A

[PCS Electronics]

Switching power supply failed. It is 25.4V 6A unit out of some telecom equipment. Two primary drive transistors to the HF transformer are shorted. They are 2SC2625. Everything else seems fine. I repair a lot of power equipment with FETs. I will often power up the units without the transistors installed to verify that the gate drives are good before replacing the transistors. But these are bipolar NPNs so I know I am dealing with a current not a voltage. When I put my isolated scope probes on the base to emitter I am getting alternating push-pull spikes from 0 to 6 volts at 24 khz. Voltage might be ok (transistors are not installed) but the frequency seems low. Buss voltage is 360 volts so I think it should run a lot faster. Is there a better way to check this? Should I just throw some transistors in and try it?

[CJay]

What's the controller chip, does 24KHz make sense for it?

[TheMG]

It could very well be the frequency the power supply is designed to operate at. Many older SMPS designs often ran in the range of 20-50kHz. It also may not operate at a fixed frequency depending on design topology.

[PCS Electronics]

The PWM chip is a Fujitsu MB3759. Data sheet says 40khz. Is it because there are two outputs which alternate which would be double the frequency?

[CJay]

Pretty sure that's functionally identical with TL494, 24KHz isn't unreasonable for one of those

[PCS Electronics]

Ok I have ordered the transistors. When they come in I will put them in and stand back. Thank you for the help.

[George Edmonds]

Hi

Replace all the diodes around the switching devices at the same time

Initially test it with a light load and a 100W incandescent light bulb in series with the supply hot/live.

This will current limit the line current supply and prevent it immediately self destroying again

George