Hantek Oscilloscope PSU board faulty - help understanding the circuit

[punkieys17]

Hi everyone. My Hantek DSO5102P stated acting up and eventually failed to switch on. The PSU was not putting any power out. Thought it was going to be an easy repair but eventually had to draw out the circuit myself to help. I just cannot understand the circuit. Across teminals 1 to 3 on the transformer I get 15vAC....with the DC lines from the bridge rectifier at 330v (mains is 240AC). I get no voltages on any other of the transformer terminals. Ive replaced all the electrolytic capacitors and measured the SMD decoupling capacitors and none show shorts. There are no voltages at all on any of the outputs. I have replaced all the semis in desperation ....Im confused that on one side terminals 1&3 and 2&4 are connected.... does the circuit created an ac circuit across one side of the transformer via terminals 2&4.

I admit to being stumped.

I attach a JPEG of the circuit diagram         

[fzabkar]

The resolution of your circuit is too low.

[punkieys17]

Large resolution picture posted

[fzabkar]

The MOSFET chopper (Q1) is pulsed by IC N2. The secondary voltage developed by transformer winding 1-3 powers the IC via D6. It seems to me that this area of the circuit is drawn incorrectly.

The datasheet indicates that Vdd should be 15V or greater.

CR6850T, Chip-Rail, current mode PWM controller, DIP-8:
https://avtronic.eu/medias/cr6850t/cr6850t.pdf (see pages 2 and 13)

[wasedadoc]

R9, R10 and the unlabelled capacitor above C15 are the startup circuit which provides initial power to the IC until the 1-3 winding and R12, D6 come into operation. D6 is drawn reversed to what it should be.

There should not be a direct connection between 7 and 8 of the transformer.

D7 is drawn the wrong way around to get -8 volts.

R171 is certainly not 1000 ohms.

Re-examine how N2 and N3 are actually connected and correct your diagram.  N2 does not appear to have a proper ground connection in your drawing.

The Y1 capacitor at the bottom looks all wrong.  Should not be between the FET drain and the zero volt line on the output side.  I could expect it to be between the negative end of the 400 volt capacitor and the zero volt line on the output side.  Or maybe from the FET drain to the negative end of the 400 volt capacitor.

In future please use png format, not jpg, for schematics and other "clean" line drawings.

[EPTech]

Hi, the supply may be in hiccup mode. This may be caused by a cc on the secondary or the switch ic not getting a steady working voltage. Try replacing the capacitor that lives between pins 2 and 8 of N2.

Happy hunting.