Electronics > Repair

Failed 24V Switchmode PSU [Repair Complete]

(1/3) > >>

jaxbird:
It's served me well for a few years, so I thought worth having a go at getting it up and running again, but I have little experience with common faults, so all suggestions are very welcome :)

It failed totally in a non spectacular way, just stopped working.

Fuse is fine and only about 0.5V on the output without load.

I found a very bad capacitor (pretty much failed open)



Didn't have a size that would fit, so I bodged this one in for now:



But still very dead. I measured about 145V across the new capacitor.



The output capacitors also look a bit dodgy, but still measure ok, so I'll leave them until I know if I can get it running again.

More detail pictures coming...

jaxbird:
More pictures:













Much appreciate any suggestions on what to measure.

SeanB:
Look at high value resistors on the primary side ( anything over 47k and over 0.5W) and look around the controller for small low voltage caps that are either shorted or open. Your secondary side likely has bad caps as well.

Look up the TL494 datasheet and apply power to it and see if it is oscillating and driving the transistors with the board not connected to the mains. That way you see if the chip is faulty and if the transistors are working. properly. Most likely it is a faulty resistor that is not allowing it to start oscillating on the primary side, or a shorted capacitor. If no oscillation and all the resistors and low value emitter and base resistors along with the diodes you have low gain output transistors, or a shorted driver transistor on the small transformer.

tautech:
I fixed a similar PSU that the chip V+ supply 22uF E-cap had drifted to just a few uF and high ESR.
In short the chip did not have a to spec power supply and that was all!

jaxbird:
Thanks for the suggestions  :-+

I've been poking around a bit more and one of the two switching transistors appear to be faulty. I assume they are transistors as the pins are marked B - C - E.

So I desoldered them and using DMM diode measurement, it appears C and E are shorted on one of them (0.08V in both directions). The other one measures as I would expect from an NPN.

I believe they are NPN transistors, but I really have no idea what the specs are, no luck on Google searching for TK100A relating to a transistor and no brand markings I'm familiar with.

Navigation

[0] Message Index

[#] Next page