Attn Electronics Gurus, help me repair a Sewing Machine Pfaff 7550 from the 90's

Forums › Home Workshop DIY › Attn Electronics Gurus, help me repair a Sewing Machine Pfaff 7550 from the 90's

1eStar

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#271733 25-May-2020 12:02

I need some diagnostic assistance from the electronics gurus.

I'm working on repairing my mother's retro sewing machine from the 90's a Pfaff 7550 to be exact. The problem it has at this stage is a prolonged startup, like it has a power supply issue. She tells me it was starting more quickly on a warm day.

I've opened it up and given it some power, it sits with a largely blank screen, the stepper motor for the foot control ticking back and forth until at some stage it fires up to life.

My gut feeling is dry capacitors or something to do with the power supply transformer.

Any ideas or maybe tips on testing capacitors etc?

neb

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#2491041 25-May-2020 13:34

Complete speculation without knowing more, but if it's heat-related then look for hairline cracks in the circuit board and signs of dry joints, which could have become worse over time due to thermal expansion/contraction. Also as a rule of thumb with 25-year-old electrolytics you might consider replacing them, but I wouldn't expect those symptoms if that was the problem.

If you can get a can of freezer spray try hitting various suspect areas when it's running to see if that affects anything. Alternatively, pre-warm things with a heat gun before starting it up, although that's non-targeted so will only be able to confirm it's temperature-related.

elpenguino

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#2491049 25-May-2020 13:46

A screen? Clearly this is more advanced than the Singer 6 I have in the garage.

If it has a screen it has some 'smarts' so I would suggest you try some modular checks e.g.

- is the PSU output voltage good/different when 'dead' or 'alive'

- Are the connections to the sensors/other peripherals good?

If you've got something bad in the logic board/module then that's a bit more tricky to poke around for a non-expert. But as mentioned, gently poke and prod and you might find a dry joint or something.

Another thing with old and dry electrolytic caps is that touching the end with your finger will change (increase) the value, sometimes it can be enough to change the operation of the circuit.

And of course, be careful.

Most of the posters in this thread are just like chimpanzees on MDMA, full of feelings of bonhomie, joy, and optimism. Fred99 8/4/21

neb

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#2491050 25-May-2020 13:50

elpenguino:
And of course, be careful.

Yeah, should have mentioned that one :-). He mentioned a transformer so I'm assuming a linear supply with only a tiny part exposed to 240V rather than a switchmode supply, but be careful when poking around with it while it's powered up, and remember that capacitors will store a charge for awhile after power is removed.

Gordy7

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#2491075 25-May-2020 14:13

Depends on how complicated the mechanical stuff is. I don't know anything about that particular machine.

With power off - can you turn over the motor and mechanisms by hand? Any significant stiffness? (apart from the stepper motor jogging)

Sewing machines colllect a lot of fibres.

I have blown the dust out a couple of machines with a compressor and very very sparingly used light oil on bearings, levers and cams.

Make sure to carefully remove any surplus oils as this will also collect dust.

My 2c worth.

Gordy

My first ever AM radio network connection was with a 1MHz AM crystal(OA91) radio receiver.

1eStar

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#2491265 25-May-2020 19:59

Ok thanks guys, I have a bit more info to potentially help with diagnostics.

Firstly, once it finishes the 'power up cycle', the machine is working faultlessly.

The 'power up cycle' is the stepper motor moving the needle sideways through full travel then sits in standby.

At this stage the thing I'm trying to diagnose is why it sits "buzzing, ticking" around 20Hz before it can initiate the 'power up cycle'.

The power transformer is dual voltage output, 32VAC and 5VAC, the light and motor run on 230VAC.

If I switch it off, then back on within a few seconds, it restarts immediately. If I leave it a few minutes the power up jitters "buzzing, ticking" continues for about 30seconds before it's back to normal.

The longer it's been off the longer it takes to start up.

I'm wondering whether it's a failing power supply or a capacitor that has to charge up to voltage?

Gordy7

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#2491268 25-May-2020 20:14

This any help?

https://frommycarolinahome.com/2015/06/19/pfixing-the-pfaff/

Gordy

My first ever AM radio network connection was with a 1MHz AM crystal(OA91) radio receiver.

1eStar

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#2491355 25-May-2020 20:56

Gordy7:
This any help?

https://frommycarolinahome.com/2015/06/19/pfixing-the-pfaff/

Yes I found that resource this morning, it is the correct model, but it only teaches me how to burr screw heads and load it up with oil. However no electronics knowhow gleaned. There is a guy in Pennsylvania that services them, but I'm not sure he'll dispense info too readily. He has a lot of spare parts too.

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sqishy

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#2491414 25-May-2020 22:09

Tricky as once it works it works from warm but some thoughts:

-any capacitors look bloated?

-any leaks near or around capacitors? or near transformer?

-any Zenor diodes? If these fail the capacitors near them get over voltage and play up

-any sweet sort of smell near transformer?

Gordy7

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#2491457 25-May-2020 22:29

Found a 7550 service manual online.... albeit a pretty poor copy...

A very complicated machine.... Really only for the brave to tackle :-)

Flow charts near the end showed diags to module level.... no component level tests.

In your case it seems that maybe you have to change the control board. I guess there may be power supply components on this board.

As 'squishy' says, do a visual for suspect components and replace them.

Nuke any mechanical or electrical parts and it could all be down the gurgler :-)

Gordy

My first ever AM radio network connection was with a 1MHz AM crystal(OA91) radio receiver.

neb

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#2491479 26-May-2020 00:27

For the OP, where are you located? There are still a few fix-anything electronics guys around, e.g. this place in Christchurch, which looks like a PC-only repair place but is run by this guy who seems to be able to repair anything involving electricity.

1eStar

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#2491518 26-May-2020 09:14

The machine is in Auckland. I'll have a look at the capacitors I think. Or find a service technician that would replace them.

Gordy7

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#2491583 26-May-2020 09:58

That Fixya is a good find...

Gordy

My first ever AM radio network connection was with a 1MHz AM crystal(OA91) radio receiver.

elpenguino

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#2491603 26-May-2020 10:03

It is, remember that machines produced by the same manufacturer at the same time tend to use the same parts and technology but with varying features for models.

If they used a batch of the same capacitors across their models you could be on to a clue there.

Most of the posters in this thread are just like chimpanzees on MDMA, full of feelings of bonhomie, joy, and optimism. Fred99 8/4/21

1eStar

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#2494560 29-May-2020 18:16

Ok. I've solved it. I replaced all the capacitors that i could find on both the motherboard and the secondary board behind the display. I reworked some of my dry joints, I'm not really sure which cap was the real problem, possibly the last Philips Aluminium long life Axial 470uF cap, which got a Suntan radial cap in its place.

Anyways thanks guys for the votes of confidence which got me through.

sqishy

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#2495796 31-May-2020 17:03

Nice work, a good idea if you can and have the time and a voltmeter is to check voltage across each capacitor.

Note if a zenor diode has gone faulty the voltage across the cap may be high and could future fail.