| Electronics > Repair |
| Help with power board of old Bernina sewing machine |
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| ollihd:
Hi All, I have this old Bernina sewing machine, the Bernina 1530 to be exact. It has been in the family for years and now it found it's way to my home. Upon trying to turn the sewing machine on, nothing happens. I started inspecting the machine and quickly found out that the power board has to be the culprit. I cannot find any schematics for the board or the transformer on it so I'm kind of lost here at the moment. So far I have tested: - Machine is getting 230VAC (no broken cord etc) - Main fuse for 230 is fine - Checked all the capacitors (nothing wrong there, haven't tested off circuit thought) - Power MOSFETs seem to be in working order - The NPN Power Transistor seems to be working - No broken traces or bad solder joints - No obvious "burn" of any components One thing I saw while looking at the board through my thermal camera was that there are two resistors heating up really fast right after power up (the ones in the red boxes in the attached document). Also checked them for voltage and they gave out ~160VDC. They are basically the only two components giving out a reading that is higher than room temperature. It also seems that the secondary side of the transformer is not getting any voltage. Any suggestions on how to proceed are greatly appreciated! Will PayPal/Venmo/whatever you a beer at least if you can help me solve this :) P.S. Document attached with the main components identified and the resistors in question marked. - Olli from Finland |
| fzabkar:
Those two resistors appear to be the startup resistors for the UC3844 PWM controller. Check the Vcc pin of UC3844. Test for shorted outputs on the secondary side. https://www.ti.com/lit/gpn/uc2845 |
| ollihd:
Hi fzabkar, Thanks for the quick reply. Reading between the UC3844 VCC(7) and GROUND(5) pin read 1VDC. No shorted outputs on the secondary side. |
| ollihd:
Re-checked the secondary outputs and all of them show continuity (0 Ohms) against one another... so must be shorted. Damn. I have no idea on the specifications of that transformer. Is opening it, measuring the winding and trying to "re-compile" it an option? |
| Alex_Baker:
Is is a switchmode power supply? Those transformers usually have a very low DC resistance on the secondary, but please correct me if I am wrong. Did you measure the resistance between VCC and ground on the IC? |
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