Help with power board of old Bernina sewing machine - Page 1
Forum section: electronics / repair

author: ollihd
author_rank: Regular Contributor
timestamp: February 28, 2022, 09:29:49 pm
content: 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 componentsOne 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

author: fzabkar
author_rank: Super Contributor
timestamp: February 28, 2022, 09:53:12 pm
content: 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

author: ollihd
author_rank: Regular Contributor
timestamp: February 28, 2022, 10:08:19 pm
content: 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.

author: ollihd
author_rank: Regular Contributor
timestamp: February 28, 2022, 10:27:28 pm
content: 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?

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: February 28, 2022, 10:32:53 pm
content: 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?

author: ollihd
author_rank: Regular Contributor
timestamp: February 28, 2022, 10:48:34 pm
content: Hi Alex,It seems like a switch-mode power supply to me, yes. Good point about the secondary DC resistance, maybe I won't start "re-compiling" the transformer just yet.The resistance between VCC and GROUND was ~90MΩ

author: fzabkar
author_rank: Super Contributor
timestamp: February 28, 2022, 10:49:44 pm
content: A typical startup fault in this topology is a dried out Vcc capacitor (100uF 25V).Figure 25 on page 22 of the TI datasheet shows a typical application circuit.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: February 28, 2022, 10:58:14 pm
content: plus one for the VCC cap, those little buggers are usually asked to cope with a lot.

author: ollihd
author_rank: Regular Contributor
timestamp: February 28, 2022, 11:06:57 pm
content: Ok, replaced the Vcc capacitor (100uF 25V), still nothing.

author: ollihd
author_rank: Regular Contributor
timestamp: February 28, 2022, 11:57:04 pm
content: I'm thinking the next best bet would probably be to replace the UC3844.

author: ollihd
author_rank: Regular Contributor
timestamp: March 01, 2022, 01:28:54 pm
content: Changed the UC3844, still nothing....

author: ollihd
author_rank: Regular Contributor
timestamp: March 01, 2022, 09:40:05 pm
content: Oh yeah! Finally got it.Went full on brute force mode after not getting anywhere after replacing the UC3844. Decided to remove all the caps and replace them, just in case. Even though they looked "good" I know they can still be bad. They even read good, but still, wanted to go for it.And behold, one of the 1000μF 10V caps showed a tiny bit of leakage on it's other leg (see picture). Replaced them all, machine works again. Thank you all for the input.I feel super stupid for not doing this right away...

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: March 02, 2022, 01:36:20 am
content: Glad to hear it!

author: m3vuv
author_rank: Super Contributor
timestamp: March 02, 2022, 06:29:23 am
content: get or build an esr meter,look for the jdiddy five transistor jobby in designs and tech stuff,ive built 3 to the diddy design,work brill,good for in cuircit testing too!

author: richard1950
author_rank: Contributor
timestamp: August 30, 2024, 06:52:50 pm
content: Hello All from the Netherlands!I have exactly the same board and also the problem that the machine is not starting. Even worse: I seem to have a shortcut as the fuse is blown every time. I have replaced already all the capacitors and the diodes in the first stage rectifier. I am at a loss as I also cannot find a scheme of the board. Please help. Thank you very much

author: richard1950
author_rank: Contributor
timestamp: September 06, 2024, 01:17:43 pm
content: The problem is solved. See my other subject. Thanks a lot

author: RangerSteve
author_rank: Newbie
timestamp: October 15, 2024, 04:19:28 am
content: Thank you for posting your solution to this, I just fixed the board of a a Bernina 1530 that wasn't powering up (there was a short flicker on the screen on first power on, but nothing else was happening). Turned out to be the same problem - one of the 1000uF 10V caps had failed open, and the other was reading about 1400uF - both measured after being de-soldered from the board.Replaced them with two new caps and it's working like a charm - thanks again!!