Power board of an old Bernina sewing machine

[richard1950]

Hello All from the Netherlands!

I have a Bernina 1090 with a power board L1230 240V 0789E which is mentioned in the topic 137 by ollihd. It has a slightly different layout.  I have 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 have also replaced the Power Mosfet (2) and the Silicon NPN power transistor.  I am at a loss as I also cannot find a scheme of the board. Please help. Thank you very much

Don't mind the lose end of one resistor (R11) this was for testing purposes to find the shortcut.

By the way: The resistance between the power input is 4-6 mega ohms (so I don't understand why the fuse is blown)

I have added the schematics of a almost simular board. Perhaps this could help.

[tooki]

Blurry, low-resolution images make it very difficult to provide help. Take clear photos, front and back, using bright, soft lighting that avoids shadows. And then don’t shrink them to 640x480, do something reasonable like 1200x900 to 2400x1800.

We can’t identify threads by ID, and even if we could, it’s unlikely that “137” is correct, in that this forum has nearly 200,000 threads, and 137 would have to be extremely old. Just post the URL if you want to refer to another thread!

When you say “all capacitors”, do you mean just the electrolytic caps, or all the film and ceramic caps as well?

I assume that by “shortcut”, you mean “short circuit” (often abbreviated to just “short”)? (A shortcut is an easier or faster way to get somewhere.)

[richard1950]

[ Specified attachment is not available ]Yes you are absolutely right. So I will add better pictures. It is indeed a short. Or too much current for the fuse. And indeed I have replaced all the electrolitic capacitors and the X2 ones.

[richard1950]

some more pictures:

[tooki]

Thanks, much better! Still a lot of shadows, but practically everything is legible now. And at least at first glance, nothing obvious stands out.

Does it blow the fuse if you power it up with no load connected?

Have you tested the other diodes on the board to see if they've gone short?

[richard1950]

Yes I have tested without and other connection to the board except power. No diodes have short. I am really at a loss as between the power leads there is no short, but as soon as I switch on the fuse blows. So somewhere too much current

[tooki]

Well, one thing to check, since it's easy to verify, is whether it's the right kind of fuse! Not only the value, but whether it's a fast or slow blow fuse. If it's a fast blow fuse by mistake (even if the correct value), maybe there's a high inrush current blowing the fuse.

[richard1950]

I have ordered a slow fuse, but still it looks different from the original one. I will try to get a higher amperage fuse also slow version. see the attached picture for a simular board with some explanation

[tooki]

I didn't ask what you ordered, I asked what you used already. Is it possible that you put in a fast blow fuse by mistake?

The board says 0.8AT, so 0.8A slow blow. It's OK for the fuse to look different; there are many styles of slow blow fuses. Don't use a higher value, as that risks allowing more damage to happen to other parts.

[tooki]

By the way, here's the service manual.

[tooki]

And the one to the functionally identical Bernina 1130, on which the 1090 is apparently based (according to the 1090's service manual).

[richard1950]

Thanks for the information. I have decided to connect the board to a low voltage transformer (16v). Still the fuse is blown. After replacing it with a stronger fuse I can now check the short by switching for a split second and see what happens. I took out one by one components to see if this is the problem. I know now that the problem is in the high voltage part. The low voltage part seems to be ok. At least after taking out transformer U146 there is still a short.  I took out Diodes, the U145 transformer C67, D104, T126, D105, C65, R58 (wire only) and still no result. It could be one of the transistors T123, T124, or T150. That I will try next. But this are really one of my last opportunities.

[richard1950]

I got the print working again! Thanks to the assistance of Gerard Boom, a friend from the local Repair Cafe who managed to find the problem within an hour. Two rectifier diodes which I replaced were again faulty and the diode D117 had a shortcut.

After installing the print works. It seems only that one of the sensors is either broken or not correctly placed at the machine is making strange movements. But that I will sort out hopefully.

Thanks for the assistance!

[tooki]

Thanks for taking the time to provide an update. (People often neglect to do this, so it's much appreciated.)

How did he diagnose the problem? In particular, I'm curious as to how he found a diode that failed short, when you stated earlier that you tested all the diodes and they were fine.

[richard1950]

In the end I removed the main transformer between the high voltage and the low voltage circuit. My friend started again from the beginning by testing the main rectifier first. At that moment he found the two faulty diodes. He tested, like me, with low voltage. Now the high voltage circuit was working again he looked for the low voltage circuit and found the main diode faulty by measuring in place.