Standby Transformer Voltage

[argonrepublic]

I am getting O Volts out of this. By the looks of the schematic, my voltages are not what they should be. I will try to upload schematic.

[ReeceyBurger123]

Re: Standby Transformer Voltage

Checked mains fuse any voltage on primary caps ?

[budm]

Re: Standby Transformer Voltage

When you have dead switching power supply you will need to check:
1) Main AC feeding the board.
2) Main AC fuse.
3) Ac getting to the rectifiers to convert AC to feed main filter cap.
4) DCV between the two legs of the main filter cap, for 120VAC Line you should have 165~170VDC, for 230V AC line,you should have about 325~330VDC.

If all of the above are OK, then you need to trace the B+ connection of the main filter cap to see if it goes to any fuse or fusible resistor to feed the STBY SMPS section.
You need to have at least the MAIN DC first. If all are present then you dig deeper in the STBY SMPS section.

[bucephalus]

Re: Standby Transformer Voltage

Hi

I have a desktop power supply unit (SMPS) which is not working and completely dead. I have tested this power supply by connecting ground to green wire but this power supply fails to turn on. There is no +5v standby voltage. On visual inspection there is no sign of any burnt component, blown fuse, short circuit or bad capacitor (like a swollen top or a leak in capacitor).

I have examined this power supply using a multimeter. There is a 230V AC on PCB track pad where AC input wires are connected. The fuse is working and intact. when I connect this unit to AC the first transformer, thyristor, bridge rectifier, and two big capacitors of Filter circuit all are working properly and there is approx. 170v DC on each of the capacitor.

I have also tested three NPN power transistors from the switching section (by removing them from PCB and testing them separately) which are working properly. I have no idea about the PWM switching IC SDC7500 and I have not tested it yet.

There is a key problem that i have diagnosed is that there is no voltage on the two legs of SM transformer Primary side and the Secondary side have seven legs and all of them shows full continuity, which I doubt is wrong but even after removing the transformer from PCB there is 0v on the primary side of the PCB tracks.

I need some help to diagnose the unit and locate the faulty component. Could someone please help me and tell me some more test points to test this unit.

Thanks and Regards
Bucephalus

In these images i have removed three transistors and the heat sink for testing purpose only.
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