Please help fixing this power supply 110vac to 24vdc 5vdc

[imcool]

hello,
this printer internal power supply is dead,I cant figure out with my limited knowledge that why the lower output part of circuit is not getting any DCV. I have measured voltages across transformer and I found 24vdc but nothing on the other side of transformer, please have a look at pictures, any help is kindly appreciated. thanks.

Measuring VAC shows on 0 VAC across any two selected pins on yellow transformer. only in DC mode is showing 24VDC as shown in picture.














U1 is following chip


Thank you

[R_J]

Re: Please help fixing this power supply 110vac to 24vdc 5vdc

What is the voltage ACROSS the 150µf/400v capacitor in the primary, It needs to be at least 160vdc, if not check that the fuse is not open, and then check the bridge rectidier.

[budm]

Re: Please help fixing this power supply 110vac to 24vdc 5vdc

what is the voltage across the 150µf/400v capacitor in the primary, it needs to be at least 160vdc, if not check that the fuse is not open, and then check the bridge rectidier.

+1
Measure between the two pins of the caps, be real careful not to touch anything in the hot side with your body or hands since you are making measurement in the HOT DEADLY side of the circuit! You should at least plug it into GFCI outlet.

[imcool]

Re: Please help fixing this power supply 110vac to 24vdc 5vdc

hello, i measured more voltages, please have a look..

[R_J]

Re: Please help fixing this power supply 110vac to 24vdc 5vdc

What is the number on U1 ? To get full voltage on the main cap, the triac needs to be turned on, but to do that you need some voltage in the secondary to operate the 5 pin optocoupler. I suspect the U1 needs to be operating in a low power burst mode and pulsing the transformer just enough to get some secondary voltage. I suspect the 100µf cap, 11 volts is the ic's vcc voltage and may be too low, this is why the ic's number would help.

[stj]

Re: Please help fixing this power supply 110vac to 24vdc 5vdc

it looks like you have to turn the triac on to power the rectifier!

[R_J]

Re: Please help fixing this power supply 110vac to 24vdc 5vdc

Just a note but that 1002 is a dual diode, not a mosfet and that blue "varistor" is just a capacitor connected between hot and cold ground.
There is a low ohm (3watt?) resistor between the transformer and the primary mosfet, check that it is not open, it is the source resistor for the mosfet (connects mosfet source to hot ground)

[Per Hansson]

Re: Please help fixing this power supply 110vac to 24vdc 5vdc

imcool: There is the text written "Pon/off" on the output connector, this needs to be activated because otherwise the power supply is in sleep mode with outputs disabled.
Tracing this yellow wires PCB connections back you reach the 5-pin optocoupler, the soldering on it looks bad on the leg closest to the PCB edge.

[R_J]

Re: Please help fixing this power supply 110vac to 24vdc 5vdc

Usually on these type of power supplies, in standby mode, the primary gets a few pulses, enough to produce somewhere around 5~8 volts on the +24 volt line, this will be enough voltage to supply the led side of the optocoupler. The secondary can not be totaly turned off.

[imcool]

Re: Please help fixing this power supply 110vac to 24vdc 5vdc

What is the number on U1 ? To get full voltage on the main cap, the triac needs to be turned on, but to do that you need some voltage in the secondary to operate the 5 pin optocoupler. I suspect the U1 needs to be operating in a low power burst mode and pulsing the transformer just enough to get some secondary voltage. I suspect the 100µf cap, 11 volts is the ic's vcc voltage and may be too low, this is why the ic's number would help.

[imcool]

Re: Please help fixing this power supply 110vac to 24vdc 5vdc

it looks like you have to turn the triac on to power the rectifier!

imcool: There is the text written "Pon/off" on the output connector, this needs to be activated because otherwise the power supply is in sleep mode with outputs disabled.
Tracing this yellow wires PCB connections back you reach the 5-pin optocoupler, the soldering on it looks bad on the leg closest to the PCB edge.

what to do to activate it?

[imcool]

Re: Please help fixing this power supply 110vac to 24vdc 5vdc

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