Electronics > Repair

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

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tautech:

--- Quote from: max.wwwang on December 21, 2024, 11:30:29 pm ---
--- Quote from: tautech on December 21, 2024, 08:39:20 pm ---So very often use of a SMPS controller IC very closely mirrors App notes and/or datasheets.
They should be your first port of call when troubleshooting SMPS.

Typical failure points are the controller IC Vcc cap (high ESR and/or diminished uF capacity), dropper resistors from HVDC and sometimes a flyback diode.

--- End quote ---
Thank you sir. Very true, the circuits using the chips are typically very similar, or even identical to the examples in the corresponding app notes. This is demonstrated even in my limited experience.

I will take note of your nice summary of the common failures of these things. Probably you said the same in one of my previous repair projects asking for help here. And I vividly remember in one previous project, another member, seeing through the problem of the circuit I was working on, was almost screaming out --- just replace that silly cap!  :-DD And it turned out he was exactly right.

My problem is, probably due to my relatively low frequency of exposing to these projects, things learned at one point easily fades away from memory. This is something I feel very frustrated about.  :palm:

--- End quote ---
;D

While I don't do many repairs these days I do take the time to inspect broken/damaged appliances and try to diagnose them and you soon get a feel for which parts of common SMPS are under stress and it stays with you constantly haunting......

Much can be diagnosed simply and quickly;
Won't go or in a tick mode:
Is UVLO exceeded ? > check dropper resistors which are necessary for SMPS to start.
Is the IC Vcc cap sound, I normally pull them anyways and tests generally suggest replacement is necessary.
Flyback diode of course....
IIRC yours has a zener which is another thing to check....

IMO often the wrong ESR cap is used as they should be Low ESR to absorb charge from the flyback which is pulsing @ 15+kHz.
The LV output caps are normally pretty easy, either deformed and/or running hot.....which if you are brave can be checked with a finger.

A good amount of datasheet study and the experience gained has served me well.  :)

max.wwwang:

--- Quote from: tautech on December 22, 2024, 12:22:26 am ---While I don't do many repairs these days I do take the time to inspect broken/damaged appliances and try to diagnose them and you soon get a feel for which parts of common SMPS are under stress and it stays with you constantly haunting......

Much can be diagnosed simply and quickly;
Won't go or in a tick mode:
Is UVLO exceeded ? > check dropper resistors which are necessary for SMPS to start.
Is the IC Vcc cap sound, I normally pull them anyways and tests generally suggest replacement is necessary.
Flyback diode of course....
IIRC yours has a zener which is another thing to check....

IMO often the wrong ESR cap is used as they should be Low ESR to absorb charge from the flyback which is pulsing @ 15+kHz.
The LV output caps are normally pretty easy, either deformed and/or running hot.....which if you are brave can be checked with a finger.

A good amount of datasheet study and the experience gained has served me well.  :)

--- End quote ---
It has more than one Zeners. What's the best way to check them apart from DMM diode mode? There does not seem to be a way to determine with DMM their voltage spec.

What's your gut feeling about its original symptom of only not having stable 6V output (but the rest was all good)?

max.wwwang:
Combined schematic tidied further based on the app notes, and it is probably close to figuring out the winding wiring, with one exception that a closed loop winding at the primary side does not make sense to me.

Parts ordered from the mighty Ali. Fingers crossed.

tautech:

--- Quote from: max.wwwang on December 22, 2024, 03:19:46 am ---
--- Quote from: tautech on December 22, 2024, 12:22:26 am ---While I don't do many repairs these days I do take the time to inspect broken/damaged appliances and try to diagnose them and you soon get a feel for which parts of common SMPS are under stress and it stays with you constantly haunting......

Much can be diagnosed simply and quickly;
Won't go or in a tick mode:
Is UVLO exceeded ? > check dropper resistors which are necessary for SMPS to start.
Is the IC Vcc cap sound, I normally pull them anyways and tests generally suggest replacement is necessary.
Flyback diode of course....
IIRC yours has a zener which is another thing to check....

IMO often the wrong ESR cap is used as they should be Low ESR to absorb charge from the flyback which is pulsing @ 15+kHz.
The LV output caps are normally pretty easy, either deformed and/or running hot.....which if you are brave can be checked with a finger.

A good amount of datasheet study and the experience gained has served me well.  :)

--- End quote ---
It has more than one Zeners. What's the best way to check them apart from DMM diode mode? There does not seem to be a way to determine with DMM their voltage spec.
--- End quote ---
Variable PSU and a current limiting resistor normally finds reverse breakdown voltage with little fuss.


--- Quote ---What's your gut feeling about its original symptom of only not having stable 6V output (but the rest was all good)?
--- End quote ---
Connectivity issue somewhere or something intermittent with the diode or its snubber.
There's really not much to check in a standalone winding on the secondary side.

If nothing is obvious get out the magnifying glass.

A little story about a MIG welder I repaired years ago after it had taken a fall.....
Seemed simple enuf in that a small PCB mount transformer that provided control supply was only mounted by its through hole pins and in the fall the transformer had yanked on one and broken the fine copper to the primary.

Poor construction not having a zip tie around it so to not rely on just the pins.  |O
Anyways, seemed an easy fix as RS had the exact transformer so it was briefly tested and sent back to the customer.
A week later it was back and on further inspection a few cracked solder joints were found and repaired on the SMD part of the PCB.
All was well one would think but another week later it was back !  :palm:
After spending ages looking for cracks none were found then a EE buddy suggested I flex the PCB and listen for clicking.
Sure enuf clickity clack all day long.
I ended up hand reflowing every component on that PCB and never saw it again.  :phew:

tautech:
Stick this in your datasheet records:
https://www.ti.com/lit/ds/symlink/uc3844.pdf

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