Electronics > Repair

BERNINA power supply board (Switch Mode) issue diagnosis and beyond (fixed)

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max.wwwang:
Can you analyse the working around transformer U145?

inse:
U145 generates the drive signal for the highside switch T125 as it needs to be above the Mains/DC level.

max.wwwang:

--- Quote from: inse on December 21, 2024, 08:39:11 am ---U145 generates the drive signal for the highside switch T125 as it needs to be above the Mains/DC level.

--- End quote ---

Yes. So correction is made to my post above – P190-5 does not play a part in the power supply; it only feeds in U145, which controls the motor. This part is separate from power supply for the upper section.

Though I'm still very keen to understand how exactly U145 controls the motor.

Revised again based on improved understanding.

max.wwwang:
Correction: T125 is probably ok. Previous thought it was likely gone was because when measuring across B/E with the DMM diode mode, both ways seemed shorted. Now I understand this is the correct behaviour because these two poles are across not only R22 (88 Ohms) but also – and more importantly – the secondary winding of U145. This will make T125 shorted between B/E both ways.

Reorganising the schematic makes it further away from the centre of disaster. It looks like if I replace these parts, (hopefully) at least I can get back to where I was. To be safe, I will also order replacement UC3844.

This also means that, the motor is driven through U145 by input from P190-5, which much be a pulsing signal, and it's always only when the potential of the RHS of R22 is higher than LHS. This will turn on T125 and supply power (DC) to the motor. This is never a stable or sustained status, and will be an alternating on/off process. The nature of this pulse will determine the speed of the motor.

max.wwwang:
Some questions asked previously remain unsolved, such as those related to the wiring of the U146 windings. One more question –

How is the input DC voltage on pin 3 of I150 (MC78L12ACP) achieved (presumably around 25V)?

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