Need Help to Recover +5VSB unit of ATX PSU ....

[Strange Problem]

Hi,
All the single Parts of this +5VSB unit is OK . But this SMPS is not turning on . In the Red circle of the Transformer will be 1000V but it is showing 320V . Why there 1000V is not coming there ? Why the SMPS is not turning on ? What's the problem there ?? I have attached the schematic below .

Please help me to figure this out .....

Thanks

 

[skogsjanne]

With 0,4V on the base of T1 it's off and it's perfectly normal that you have 320V where you expect 1kV.

Since T1 has a maximum VCE of 400V I doubt that the circuit is designed for 1kV on the collector of T1.

 

[Strange Problem]

Most of the ATX PSU uses 2SC3150 or 2SC3866 or 2SC5027 transistors for +5VSB Primary Section . Look at those three transistors data sheet - they all have Vce is 800V . But if you work in ATX PSU then you will see that Collector is Actually crossing 1000V, may be it is 1500V to 2000V !!! I think this volt is generated by the Transformer not by the Transistor . Here you can use any transistor which Vce is greater than 320V and which can deliver more than 2A current for +5VSB section . I think it is such kind of design .

Actually - This PSU had 2SC3150 transistor which has burnt . I have Replaced it with MJE13005 . I have repaired many +5VSB unit with this MJE13005 , actually where was 2SC3150 or 2SC3866 or 2SC5027 transistor . And this MJE13005 collector also cross more than 1000V !!! So, I think this volt is generated by the transformer . You can see it easily in your mobile charger . 13001 or 13002 or 13003 collector is crossing more than 1000V !!! I have no choice of using 13005 because in my market - it is impossible to find 2SC3150 to 5027 . With 13005 +5VSB unit works absolutely Fine . And I have Repaired many Power Supply with this 13005 .

You said Base is 0.4V and it is normal to turn off the Transistor . But in some good Power supply 0.2V to 0.4V is enough to turn on the +5VSB SMPS unit . And some PSU I have seen base is in -1V to -2V . In both kind or PSU 13005 works perfectly . I have increased the Base voltage to 0.7V . The SMPS was turned on and I was getting more than 1000V but after 5 minutes 13005 and C945 both burnt due to excessive heat !! That means a good PSU need 0.2V to 0.4V to turn on .

 

[skogsjanne]

OK, you know more than I about this.

 

[Strange Problem]

OK, you know more than I about this.

No, that is not . I just shared my practical knowledge with you as you can help me properly to solve the problem - not more than that . May be you are getting angry !! I could not express myself due to my poor English - sorry for that .

 

[skogsjanne]

No, I really mean it. You have practical experience of this, I have not. Nothing wrong with your english. At least as far as I can determine
Yes, the high voltage is generated by the inductance of the transformer when the transistor switches off. But 1kV sound very high, especially if the transiastor max VCE is 400V.

 

[Strange Problem]

Thanks for your helpful tips . My own power supply has a 2SC5027 transistor in +5VSB unit which is now connected with my PC . Let me few hours to check with it . I will inform the result few hours later .

Thanks

 

[Strange Problem]

No dude , It does not work . :sad: The same Result with 2SC5027.:sad: Then I thought that - it is the problem of Inductance of Transformer . So, I changed the Transformer also . But nothing work .

What should I do now ??

 

[skogsjanne]

Sorry but I'm completely clueless. I think your knowledge of ATX PSUs is much better than mine.
Personally I am chocked, ok maybe not exactly chocked but disturbed, by how crude the standby part of an ATX PSU is. No wonder the standby power measured at the plug is about 10W.

I wish you luck with repairing the power supplies.

 

[mtwieg]

It sounds very bizarre that you are expecting 1000V or more across a transistor rated for 300-400V. The transistor should not be able to withstand that voltage.

Do you know what the turns ratio for the transformer is? How are you measuring the voltage on that node? The problem may be that the transformer is dead, or the output is short circuited. Or T1 is dead or not being driven properly.