| Electronics > Repair |
| BERNINA power supply board (Switch Mode) issue diagnosis and beyond (fixed) |
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| max.wwwang:
Another project fixing an elusive issue of a power supply board. Photo of both sides attached (back side flipped). What's known: There are two sections of the circuity on the board, separated by a broad line on the frond side, which I believe are electrically isolated. AC mains comes in from the socket at the bottom right corner. There are three output sockets in the upper section of the board: P162 (green, 2 pin), P 161 (black, 2 pin), and P163 (red, 5 pin). The 4 pin green socket (P160) is input (into a couple of opto couplers), which we can ignore for now. The output voltage of the three sockets are: P162: 6V DC (pin 2) P161: 28V DC (pin 1) P163: 5V DC (pins 1/3), 28V DC (pin 4) Symptom: The voltage of 6V DC from P162 presents at some time, not other times. The circuit might be plain to the experienced who might be able to pinpoint the culprit at the first glance. All input is welcome, including the working of this board, topology of the transformers U146 (top left) and U145 (smaller, bottom right). What's been done: Caps on the affected rail C88(1000uF 50V) and on the one across the PWM controller IC (UC3844) VCC/GND (100uF 25V) are replaced with used similar ones (for test, no new replacement handy) but no cure. [Worse than that, probably due to error while probing, further damages occurred beyond that. Detail to follow.] Questions: 1. C86 (1000uF 50V) and C87(100uF, 35V) are in parallel, side by side, on the 28V DC rail. Why there need to be two caps, and of different voltage ratings (not simply one 1100uF, 50V) – for filtering different frequencies of ripples (even so, why different V ratings)? 2. From what I think is a fairly simple single layer board and the clear photos of both sided, side by side, is anyone able to tell the working principles of this SWPS? And likely weak spots probably common to this topology? 3. More to come. |
| u666sa:
List voltages of all pins for UC3844 |
| inse:
Bernina is a brand of sewing machines, afaik. The 6V is missing while the other voltages are present - did I get this right? There is not much circuitry to check in between the connector P162 and the transformer: test the diode FES8DT, the small inductor L142 and the transformer winding. Answer 1: Maybe they calculated that 1000uF was not sufficient, 1100uF not available, so they placed two caps. Voltage rating maybe due to price and availability. The smaller transformer/coil is connected to P159 which may (only assumption) be the motor connector. The motor is not mains isolated and has some speed control circuit via ST13005 |
| floobydust:
I notice it uses a discrete 4 channel opto-isolator made up of IR LED's and phototransistors. They would be sensistive to ambient light and need a cover. D110/T132, D107/T127, T129/D109, D108/T128. They remind me of Lite-On LTR301, LTR302. |
| max.wwwang:
Things have become worse than just that, thought this original issue still needs to be figured out and fixed. When probing, I suspect there was a short circuit between the supply UC3844 (VCC/GND) blowing R37 and probably D115 (possibly more but hopefully all can be replaced). Fuse is also blown. So unfortunately before these damaged components are replaced, I'm unable to power it on again and measure voltages. |
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