Output Voltage is High ........ What to do ??

[Galib]

Hi,
One power supply's Fan is twitching for one/two second and then shutdown . I have shorted the DTC of TL494 with Ground and then Power supply is running . Then Measure the Output voltage . But the Output voltage is showing high . The 12V is showing 26V and 3.3V is showing 9V . But there is no 5V, 5V is showing 0V !!! . But all the Output Rectifiers are OK .

One more thing - when I shorting DTC of TL494 with Ground, Three PIN of Switching transformer is also showing high - it is almost 15V . But a good power supply switching voltage would not exceed more than 4V .

Guys what is the Problem could be there . Why 12V and 3.3V is showing high but no 5V output ?? How can I fix it ??

[shovenose]

Well, before you finish working on the power supply once you know the solution, I would recommend changing the secondary capacitors, because they are being subjected to extremely high voltages.
But I don't know what the actual problem is. I'm sure c_hegge will be able to figure it out.

[Galib]

Thanks shovenose for your reply . I have repaired that power supply . Actually It is a brand new power supply and it did not run for a single minute since opening from it's packet !! Can you belive this - Orginal packaging has the problem !!! Bastard CHINA . It problem was Main Transformer EE-35 . May be it's windings problem . Multimeter is showing Transformer is OK but it does not give the proper voltage . I have changed it with a old one . Wow ... It is working perfectly !!! And all the voltage are absolutely OK and it is running fine .

This time I am 100% Successful . I have repaired 5 out of 5 which I have shown in the Picture of previous post . I have just delivered them .

[c_hegge]

I have been away for the weekend, so sorry about the delay in replying, but yes, the transformer would have been my first suggestion. As shovenose said, replace the secondary caps. the ones on the 12v rail are probably only rated up to 16v. if they have been given anything above that, they will be fried, even if they don't look bulged.

[Galib]

Thanks c_hegge for your reply . I have a power supply which I could not repair it due to it's 5VSB Transformer has gone . This 5VSB Transformer is EE-19C Core . Can you help me to re-winding this EE-19C Transformer . What SWG Wire will I use for Primary and Secondary side and How many will be it's Truns in each side ??

[c_hegge]

I haven't had to rapair transformers before, since it's more trouble than it's worth, although I have successfully scavenged them from other PSUs. I'd recommend replacing the PSU or transformer.

If you really want to rewind it, however, you will have to find out how many turns there are yourself, since the amount of turns will vary from PSU to PSU, although I *might* be able to find out if you can supply the whole part number on the top.

[Galib]

Here it is ...............

[attachment=388]
[attachment=389]

[Galib]

I have opened the 5VSB transformer . Transformer Re-winding not only difficult, it is totally impossible !!! Because Carbon core attached with it's body with orginal ALTECO SUPER-GLUE . It is impossible to open these carbon core without breaking them !!!

However, The Internal Diagram of 5VSB Transformer is -

[c_hegge]

I suspected that may have been the case, so I was never really keen on the idea of re-winding it. Your best bet would be to scavenge a 5vsb transformer from another dead PSU.