Replacing PCB mounted transformer

[abunnyuk]

Hi all.

I'm an IT engineer and whilst I can repair/replace the odd component, I'm no electrical engineer so apologies if I use the wrong language.

My partner has an 1981 Berlina sewing machine on which the PCB mounted transformer (literally) went pop.

The details of the original transformer are elusive but appear to be 240v input and a 9.7v output at 0.5va. The original was brown with 2 input pins and (what I believe are) 2 output pins, the latter of which lead me to be believe it was single output as dual out transformers I found online had 4 pins. I may be wrong.

I thought I'd found a suitable replacement part but it's not as wide as the original part, it's blue, and one of the output pins is not in the expected place compare to the PCB, but the PCB is very old and standards move on.

The original transformer (aside from a lovely crack) helpfully has zero identifiable markings on it. It did, however, have a custom plastic cover (presumably made by Bernina themselves) which has "240/9.7V - 0.5VA" written on it which is what I used when searching for a replacement part.

Here are various photos:

• PCB top minus the transformer:

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• Old and new transformers side-by-side (input pins on right):

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• PCB bottom and old transformer:

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• PCb bottom and new transformer. If the output pin in top-left was higher then would align with the hole on the PCB although it would still not be wide enough.

​

• Writing on the plastic transformer cover

​

I would very much appreciate help with the following:

1. Is my assumption that it's a single output correct? The PCB has 2 female terminals which 2 ribbons then connect to parts inside the sewing machine, so I'm wondering if that means it has 2 outputs.
2. If it needs 2 outputs then would there be more output pins on the transformer?
3. Are the size and colour differences down to new standards or do I simply have the wrong component?
4. If the new transformer specs are correct, do I simply need to find a way to extend the output pins to the PCB, such as drilling new holes, securing the transformer in place with glue, and soldering wires to the original holes?

Many thanks in advance,
Bunny

 

[Bluejets]

1. yes, single output.

2. correct, there would be at least 3 perhaps 4.

3. transformers of a given rating are usually the same physical size so I'd say offhand your replacement may not be suitable compared with the original.
However, transformer may be built using more modern materials which may make it physically smaller.
The important figure is the voltage and the VA rating.

4. correct.

9.7v is an odd size and if correct would be unusual.
Where did you find the voltage and va ratings..??
If 9.7 then your replacement 9v should be close enough.
As they say, suck it and see.

 

[shrtrnd]

I would try to obtain the same transformer as the original, which you apparently couldn't find.
I think some of us are a little hesitant to offer advice because we'd like to see any information printed on the original transformer.
Then compare that to the replacement you bought.
The transformer will often have two power input leads, and two output leads, …. the transformer, if it is a transformer, and not a DC to DC converter, may be just a transformer, or that housing may also have a bridge rectifier inside of it to output a DC voltage(?)
I see someone has posted a reply to you before I finished my post. Maybe they have some advice for you.
MY input, is, I'd sure like to know what the original transformer had printed on it, or if it was destroyed, the exact model number of your 1981 Berlina sewing machine, to try to find that information somewhere on-line.
There are ways to mount what you bought, but I wouldn't do that until I was sure a had an equivalent replacement part.
I have to tell you: 'transformers' rarely go 'pop' by themselves. My opinion is that your partner has some other electrical fault somewhere in her machine that caused the transformer housing to experience a current surge that 'popped' the housing.
Just replacing the part you bought may result in another 'pop' This is another reason I think you should post the model number of her Berlina, so that someone here can try to find a schematic of the entire circuit your printed circuit board goes to.

 

[abunnyuk]

I would try to obtain the same transformer as the original, which you apparently couldn't find.
I think some of us are a little hesitant to offer advice because we'd like to see any information printed on the original transformer.
Then compare that to the replacement you bought.
The transformer will have two power input leads, and two output leads, …. the transformer, if it is a transformer, and not a DC to DC converter, may be just a transformer, or it may have a bridge rectifier inside of it to output a DC voltage(?)
I see someone has posted a reply to you before I finished my post. Maybe they have some advice for you.
MY input, is, I'd sure like to know what the original transformer had printed on it, or if it was destroyed, the exact model number of
your 1981 Berlina sewing machine, to try to find that information somewhere on-line.
There are ways to mount what you bought, but I wouldn't do that until I was sure a had an equivalent replacement part.

This is a very good point and I've added the writing that's on plastic cover that was snuggly sat on top of the transformer. The transformer itself had zero writing on it, which is less than helpful.

I've updated the original post with a photo of the cover.

Sadly all attempts to find details on the original part have proven fruitless and the service manual simply calls the PCB the "transformer circuit board" and provides no actual detail of the components.

 

[abunnyuk]

1. yes, single output.

2. correct, there would be at least 3 perhaps 4.

3. transformers of a given rating are usually the same physical size so I'd say offhand your replacement may not be suitable compared with the original.
However, transformer may be built using more modern materials which may make it physically smaller.
The important figure is the voltage and the VA rating.

4. correct.

9.7v is an odd size and if correct would be unusual.
Where did you find the voltage and va ratings..??
If 9.7 then your replacement 9v should be close enough.
As they say, suck it and see.

Thank you for answering the questions I had. As the specs of the new part look correct, my thoughts were along the lines of adapting the board to fit the new component but wanted to triple-check with people who know what they're talking about first.

I'll wait a little while for other people to reply and to see if any other insights become available before biting the bullet.

 

[Bluejets]

I have to tell you: 'transformers' rarely go 'pop' by themselves

Have to disagree with that comment.
They do burn out, especially the primary and being fully encased, would make for more spectacular cook-up.

A slightly higher va rating wouldn't hurt.
Even though it may be marked as 9v, the output will always be higher and the larger VA will sustain this level to match your unusual 9v7.

 

[abunnyuk]

I tested the replacement transformer by soldering the male end of breadboard wires to the PCB, connecting the female ends to the corresponding transformer pins... and it worked perfectly!

Tomorrow I'll make it permanent by drilling new holes and soldering wires underneath.

Thank you to everyone for your help. Very much appreciated and I've learned a lot.

 

[Bluejets]

In the words of Young Mr.Grace (are you being served).........

You've done very well.