Help with Bernina 910 Board failure. - Page 1
Forum section: electronics / repair

author: richard1950
author_rank: Contributor
timestamp: September 06, 2024, 01:30:48 pm
content: Greetings from the Netherlands! I am a newby and my hobby is to repair sewing machines. The mechanical part is familiar to me. Now I have got a Bernina 910, which starts a full speed as soon as the footpedal is touched. The foot pelal is okay as I have tried another working one with the same result.The machine was given to me as one capacitor was blown with smoke and so on. It is the safety capacitor C40 from the scheme. This is of course not the reason for the high speed. It seems to me that another part has gone also. My knowledge is not sufficient to solve this. Please help me with some additional information on what to check or replace. I have already measured all electrolitic capacitors. They seem to be good.

author: MathWizard
author_rank: Super Contributor
timestamp: September 07, 2024, 01:34:18 am
content: That's an interesting circuit, and the PCB looks like a nice easy size to solder on. Just looking around I don't see a datasheet for the motor yet, but if it's just a brushed DC motor, iirc, it would go fastest when it's current is the lowest, and the load is the least, and it has the most back EMF, the voltage a motor develops in opposition to the applied voltage, that keeps the current lower than it otherwise would be, from just the winding impedance.So would the BJT T77 be turned on too little for some reason, or would C44 be shorted, or some other part in that area that's drifted or gone bad. I should try it in a sim, it will know what it does.The controller side could have problems too.So do you know the safety basics, do you have a DMM or soldering iron ? I'm not saying this would be the quick/easy way, but I could probably narrow down the problem just by removing all the electrolytic caps, and a few others. And just go over it with a DMM while unpowered. And really you could unsolder 1 side of any resistor that you still can't measure, and rule them out.Then I might apply some small DC supply voltages, and make sure nothing blows up, and then start measuring some voltages.In your case I'd say definitely check it over unpowered with a DMM 1st before plugging it in and checking voltages. It's just way safer, and some problems are easy to see with a DMM (but some stuff can look bad too, but be ok...). And usually once I know the basics of the circuit, then I'd know where I'd want to solder on a few test leads.But this circuit is fairly small, so there's not that much that can go wrong. So 1 of the 1st things I'd check is if any of the caps are shorted or near short, which might require removing some of them. And then any diodes, and resistors.There's also J12 up there going to the motor, with some diac or triac ? So yeah IDK what that motor is really.

author: richard1950
author_rank: Contributor
timestamp: September 10, 2024, 03:38:52 pm
content: Thanks a lot! It was indeed C44 which was short. It works again!

author: MathWizard
author_rank: Super Contributor
timestamp: September 11, 2024, 03:42:58 am
content: Ok nice. Did you get 1 with the same of higher voltage and temperature ratings ? In the picture of the motor I saw in on Ebay, it had 3 wires. So do you know what kind it is tho ?If I wasn't trying to decipher an LCD screen from a thermostat, I would have tried some of your circuit in a simulator. Working with motor models and SCR's/diac/triac's, that's something I need to do more of.There's all kinds of motors too, I've cleaned a few motor casing, but never really had a motor problem since I've been tinkering w/ electronics.