Electronics > Repair
BERNINA power supply board (Switch Mode) issue diagnosis and beyond (fixed)
inse:
If the other voltages work again, you only need to concentrate on the marked mesh.
Are you sure the intermittent light is caused by the power supply, not the connector, switch, socket or lamp?
What kind of lamp is it, 6V halogen?
max.wwwang:
--- Quote from: inse on December 19, 2024, 05:38:44 am ---If the other voltages work again, you only need to concentrate on the marked mesh.
Are you sure the intermittent light is caused by the power supply, not the connector, switch, socket or lamp?
What kind of lamp is it, 6V halogen?
--- End quote ---
Thanks. Yes, my question was exactly, if all other output voltage rails work just fine, if we can be sure the problem must be in the upper section. Sounds like your answer is yes.
Yes, I checked; It's not the problem of the bulb, wiring, or switch. When the light is not on, there is no positive voltage across the two pins of the socket. The bulb is traditional incandescent 6V.
By the way, I'm close to finishing the circuit schematic – a bit messy atm as you might imagine. Hopefully the affected parts are limited. And anyway, I'm going to replace all of the electrolytical caps, except those which I cannot find replacement for.
inse:
I wouldn‘t replace the capacitors unless there was failure evidence.
In case it turns out, a failure in the transformer winding is causing the intermittent problem (what I suspect), maybe a workaround was the only option: replace the incandescent bulb by a 24V LED lamp and feed it from the 28V rail.
max.wwwang:
--- Quote from: inse on December 20, 2024, 07:20:38 am ---In case it turns out, a failure in the transformer winding is causing the intermittent problem (what I suspect), maybe a circumvention was the only option: replace the incandescent bulb by a 24V LED lamp and feed it from the 28V rail.
--- End quote ---
That's a really credible and solid point. I hope I thought of this earlier. Now I can only hope I can get it back at least to its previous status, with an intermittent 6V output but the rest is working just fine.
Facing the current state, now I have another specific question.
I have now identified power (N-channel) MOSFET T131 (IRFBC30) as bad and needs replacement. Its Source pin is connected to ground via a 2W or 3W resistor (R38), which has band green/blue/grey or siler/gold. I'm not sure if it's 0.56 Ohm (silver) or 650 M Ohm (grey), because I'm not sure if the 3rd band is silver or grey.
Given the radical difference, which one clearly does not make sense?
My gut feeling is that, given the MOSFET as large current device, 650 M Ohm does not make sense. (It must be 0.56 Ohm.)
Sorama:
Beter check first for dry solder joints.
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