View Full Version : Long Shot

6th July 2006, 11:27 PM
hey there

i think i have narrowed down my PSU problem

on the secondary transformer - the big one with two inputs and five outputs (including the large ground) i am getting about 113V AC on the inputs and 0V - 0.5v (DC or AC) on all outputs.

I have a feeling this is why i get no output on my PSU

Can anyone confirm my hunch?

on a longer shot, does anyone know what these transformers are or do i need a parts supply and rip one from that (are they interchnagable?)



7th July 2006, 04:25 PM
Not sure what machine we're talking about here but:

1. Check for an open-circuit primary on the transformer with an ohm-meter. It should be very close to a short circuit.
2. If the primary is OK, disconnect the secondary and measure the AC output. If you get nothing, disconnect the primary (so you don't toast yourself) and measure the secondaries with an ohm-meter. You'll need to know which winding is which of course.

Make sure you disconnect everything first, take care with high voltages, I'm not responsible etc etc.

7th July 2006, 05:29 PM
this is a normal switching power supply which was initally blown via a short through either the 5v or 12 v lines

the fried transistor was replace with no joy.

now testing the transformer - with AC voltage only connected (no load on outputs)) i am getting the outputs i have described when probing the transformer in circuit.



7th July 2006, 08:55 PM
OK I'm confused. Every switchmode power supply I've ever worked on rectifies the 240v ac in and then uses a transistor to rapidly switch the DC to the main transformer. They're not like your conventional transformer you get from Jaycar.

There's a whole heap of things that could be going wrong. Some switchmodes need some load before they'll start up. If the feedback circuitry from the secondary to the switching circuitry is bad, you might get a little "kick" from the supply when you initially apply the AC, but thats it.