SMPS Vcc is not generating

Thread Starter

emreanilterzioglu

Joined Aug 4, 2020
6
Hi guys,
I am working on 12V 30A smps that has only 1V output. I haven't got board's schematic but I found one same like mine( Schematic Link).
  1. I measured high voltage capacitors (C2 and C3 on the schematic.) and bridge rectifier. They are working.
  2. Also checked power switching transistor on board they seem fine. (Q1 and Q4 on the schematic.)
  3. Checked PWM transistors on board. They also fine. (Q2 and Q3 on the schematic.)
  4. I measured PWM IC's Vcc pin(pin12) and it is 2.4V.
I think, there could be a problem with D12 and D11 but not checked( because they are located under transformer).
Also on this link author says this circuit has the self-starting capability. After I read it, I become suspicious of C10 also.
I attached pictures both front and back.
Please help me with this issue guys.
Thanks.
 

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Dodgydave

Joined Jun 22, 2012
9,322
Q1 and Q4 are the self oscillating pair and the components around them with the transformer and C10 is used as a DC blocker with the Half supply at one end. When they're working the TL494 takes over.
 

Thread Starter

emreanilterzioglu

Joined Aug 4, 2020
6
Q1 and Q4 are the self oscillating pair and the components around them with the transformer and C10 is used as a DC blocker with the Half supply at one end. When they're working the TL494 takes over.
Thanks for the reply @Dodgydave,
I checked Q1 and Q4 both working. Not sure about C10 I measured 110V Dc between on its pins. I also checked components around self-oscillating pair for the short circuit, they are fine but I will test them one by one off-circuit and let you know soon. What else can cause this situation?
 

AlbertHall

Joined Jun 4, 2014
10,258
Is the thermal switch that operates the fan accessible?
If it is try shorting the switch terminals and see what the fan does (if anything).
 
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