@Quad, you might want to replace that dented 1000uF capacitor?
@Sun, it appears that you are changing the base drive on the U4 transistor, which looks like some sort of feedback for the MB358 op amp. i think there is more to control of the transformer in the middle section, e.g. what is the part number of ic U1?
Would you be able to read the markings on Q1 and D4(?), the ones that are attached to the heatsinks?
Can you double check the marking on U1—it is not coming up as any part? i use a silver sharpie pen to help bring out the digits, but white chalk or most anything light colored might help identify that component. It controls Q1 to switch the primary side of the transformer, and that is what you need to do to lower the output voltage.
i have mostly traced out the board, but i can’t tell some of the connections for U2. Would you be able to check where these go?
Does the top of R31 go to pin 6?
Does the bottom of the “106” resistor go to pin 7?
i need to verify all the connections to pins 2, 6 and 7, and the bottom of R30—there are hidden traces under the chip so you will need a meter to ring out where they go.
The U1 chip may be an FAN6604 Flyback Converter PWM Controller.
The transformer appears to have a primary and 2 secondary windings; one secondary is the power winding for the DC Output and the other is an auxillary winding to create a low level supply voltage for U1.
The 6604 requires 17V Vdd to start and has an UVLO that will turn it off if Vdd drops below 10V.
The duty cycle of the transformer (and thus the Output DC voltage) is controlled by several sources: the U4 chip and the U2 op amp pulsing the U3 opto coupler; and the current sense resistors (R4-R7) and U3 opto feedback into U1.
Here are the “?” areas since i can’t ring out the board over the Thanksgiving Staycation Holiday. Stuck at home, fun little project.
The U4 TL431 is being used as a single-side comparator looking at the Output DC voltage.
There is no current sensing on the Output DC, only on the transformer primary winding current, so some additional BMS or Li cell charging IC might need to be added to use this as a CC-CV charger.