Electronics > Repair

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

<< < (7/15) > >>

max.wwwang:
Lower section after a bit tidy-up and correction.

inse:
There is no connection between P159-3 and pin 6 of the UC3844.
Otherwise mains/dc could be switched to the output (only once).
Better move the motor speed control away as it has no connection to the power supply block.
Then there is room for the transformer on the right hand side

max.wwwang:

--- Quote from: inse on December 21, 2024, 07:01:25 am ---There is no connection between P159-3 and pin 6 of the UC3844.
Otherwise mains/dc could be switched to the output (only once).
Better move the motor speed control away as it has no connection to the power supply block.

--- End quote ---

Thank you for your eagle-eye spotting. Revised again. It is now much neater, so will make more sense. I've grouped the daughter board connector pins and motor connector pins together, which are somehow separate and independent (only somehow because, if the schematic is correct, P190-5 plays a part in the power supply).

[Correction: P190-5 does not play a part in the power supply; it only controls U145, and through which controls the motor.]

Questions:

1. When both T125 and T126 are open, there will be a short circuit across the high V DC input. This makes me wonder if there is error here. (Double checked; this path exists, so short circuit can only avoid by design that T125 and T125 will never both turned on.)

2. I recall I probably put the DMM at the Ohms setting when measuring the VCC voltage to GND (physically pin 7 of UC3844 and left hand side of R19), which appeared to cause a short circuit. R37 (1k) was violent blown. Further investigation suggests that R35 (220 Ohms) and R38 (0.56 Ohm) suffered damage, and T131 is blown. It also appears T125 is gone (and of course the fuse). The rectifier bridge is intact (thanks to the fuse). What the explanation of such damage? (All known affected parts are highlighted in dotted boxes.)

inse:
T125 and T126 must not be engaged at the same time.
T125 does the PWM for speed control, T126 is probably a motor brake shorting the motor for fast stop.
If T131 is gone and takes R38 with him, the poor UC3844 is attacked by Mains/DC from two sides via R35 and R37 - no chance…

max.wwwang:

--- Quote from: inse on December 21, 2024, 08:15:50 am ---T125 and T126 must not be engaged at the same time.

--- End quote ---
That's the only possible explanation.


--- Quote from: inse on December 21, 2024, 08:15:50 am ---T125 does the PWM for speed control, T126 is probably a motor brake shorting the motor for fast stop.

--- End quote ---
That seems to make sense – when T125 is off, motor's power is off. Turning T126 on now will short P159-1/3 with R20 (0.82 Ohm), this will brake the motor actively through the IMF before it stops. Beautiful!

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod