Hardware Insights Forum

Hi,
I have got another two PSU for repairing . One of the PSU Primary 5VSB Section has gone . This Primary 5VSB Section has designed with 2N60 Mosfet . It's 2N60 itself and some other components has gone . I have replaced them with same value except Zener Diode, One Resistor and one Small Transistor . I have attached the Schematic of this Primary 5VSB Section below . The Red marked and ??? Signed R7 Resistor value could not be read . Because it was totally burn . And I could not read the Zener value properly . Because it's some text was vanished from it's body . But most of the PSU for Mosfet design uses 12V to 18V Zener in Primary 5VSB section . This is why I have attached a 15V Zener there . And I could not get 2SC1384 in my market . This is why I have replaced it with 2SC2655 . 2SC2655 is nearest value of 2SC1384 .

I have tested the Resistor R7 with different value - such as - 4Ω, 100Ω, 220Ω, 330Ω, 1K, 220K, 560K, 820K and changed Zener value with 7.2V, 12V, 14V, 18V but every time my 100W Bulb in Series line goes up !!! Then What will be the Proper Resistor and Zener Value ??

One more thing - When I am using 1MΩ Resistor or not any Resistor for R7, then 100W Bulb in Series line is not going up . But Drain (D) is not creating 1000V, Just showing 320V only . This means there is no short circuit in this section, Only When I am attaching Resistor in R7 , then it is going to short circuit . The Problem in there - May be Zener or may be with R7 Resistor .

What will I do now ?? Please watch this Diagram and please tell me what's error going on and Where and What have I to change ..........

Is the zenner diode before the transformer or after? I can't see it in the schematic. I'll have to see if I can find a diagram of another 2-transistor 5vsb circuit to find the value of the resistor. The problem is, though, that repairs to those 2-transistor 5vsb circuits often result in fireworks. You may be able to mod it using a DM311 PWM chip, though. Again, I'll let you know when I can find a schematic. Oh, and do you know what model the Power supply is?

Thanks c_hegge for your reply . As it is in High voltage primary section, so Zener must before the transformer . In the Diagram you are watching transformer's windings - It is primary side of the transformer .


Hahahaha ........ please don't ask the Model ....... Because you know the model number ......... hahahaha .......

Here it is a Mosfet design PSU . But it is different from mine ..... ! Please have a look .

I believe that R7 in your diagram corresponds to R39 (a 4K7 resistor) in the other diagram (it has to be that or R50). I'm still not sure which zenner the failed one is. Is it ZD1 (in your diagram)?

La la la la ......... I think - I got my girls . But where is the Zener Dude ?? Will 2N60 work without Zener ?? Is this diagram OK ?? Please have a look this diagram ............. and please tell me will I go for it ??

My best guess would be 18v for the zenner based on other 2-transistor 5vsb circuits. However, repairs to two-transistor 5vsb circuits often end in fireworks. You'd be best off modding the PSU with a DM311 chip, which will require you to design an add-on PCB and remove most of the components from the existing circuit. (or just replace the whole PSU).

Whether or not the zenner is needed will depend on the design of the PSU. I'd say that if it was there originally in your PSU, it's there for a reason.

But the last diagram work for me ....... !! It is totally similar to my 5VSB unit except Zener Diode . Belive it or not - Just Remove the Zener and added a 72Ω for R7 and wow the power supply is running perfectly .... !!! By watching the PCB, it seems to be virgin power supply . It does not run for a second . Because it's manufacturer added zener there by mistaken . Actually Zener position will be totally blank . So, when just given power to PSU - right that moment PSU BOOOOMMMM !!!

I do not understand China . When there is not need any zener then why they create place for Zener on the PCB . I totally do not understand that !! Can you belive it - how LOL china is ....... again orginal packaging has the ERROR!!!

I was afraid that might happen. Can we see a picture of the carnaige?

The PSU is probably toast now.

Hahahaha ........ No dude .... This is not carnage . Which diagram I have upload last - that is absolutely correct . The PCB itself and We were wrong !! I can not show you the picture because I have already delivered it to customer .