Pfaff transformer testing

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dsquared

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Hello,
New to the forum, and I hope I am posting in the correct location. Please advise if I've done something wrong....
Trying to fix a Pfaff Model 1171 sewing machine for a friend. Mechanically the machine appears in great condition and well cared for. When powered up, the lights work, but the motor will not turn when the pedal is pressed. Feeding the motor directly, proves it spins and the wiring is in good condition, so I am assuming the problem is in the control circuit board. Very little information is available for this older model machine. The control circuit board is not available.

When checking the board, I see there is a small step down transformer with two secondaries. (See pictures.)
The label on the transformer says:
115 V 50/60 Hz,
12V 0.3VA
10V 2VA

Preliminary testing on the board shows the transformer outputs go to two AC to DC rectifier chips. (IC number B40C800DM) The AC voltage on one chip measures 18 volts and the second measures 0 volts. I removed the transformer from the circuit and tested it. With 120VAC applied to what I think are the primary pins the output voltages are as follows:
PINS 1-2 = 4.7V
1-4 = 13.5V
1-5 = 18.1V
2-4 = 2.7V
2-5 = 3.4V
4-5 = 6.2V

In circuit, PINS 1 and 5 feed one rectifier and appear OK at 18V. PINS 2 and 4 feed the second rectifier IC and show 0 volts when in circuit.
Questions:
- Do these transformer secondary voltages seem OK considering the rating is 10V and 12V?
- If there is a short on the board downstream of the rectifier chip, could this pull the AC voltage down to zero or would the IC isolate it?
Thank you for any help the the group can offer.
Dennis
 

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Can you measure the resistances between 1-2, 2-4 & 4-5?

Also please post photos of the top and bottom of the PCB with the transformer out, so the rectifier & smoothing cap arrangement and transformer - rectifier connections etc. can be traced?

If it is a single winding through all the pins & 1-5 is correct, having other pins at the wrong voltage seems unlikely. I suspect it has two separate windings?

I found a service manual here, but apparently no board level schematics..

Service Manual - Pfaff 1171 - Sewing machine

Pfaff 1171 - Sewing machine - Service Manual -- Free Service Manuals, fast download, no limits
www.opweb.de
 
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In reply to the above questions:
- Checking resistance between all transformer pin combinations shows open circuit except between pins 1 and 5. That measures 17.1 Ohms.
- I tested the motor by feeding voltage directly to the motor leads and the motor spins normally. (Note: There is a second circuit board at the motor that takes a control signal from the main board and must use it to regulate the motor speed. Checking the two wire control signal from the main board shows no voltage. Don't know if the signal should be a voltage signal, a milliamp signal, a pulsed signal, or what. I don't think the motor board is the problem. I think it is not getting a control signal from the main board.)
- When pressed, the pedal leads show smooth variable resistance as I think there should be.
- Pictures of the circuit board are attached. Note: The burn marks on the primary side of transformer were cause by me having to remove excessive amounts of solder at the primary pins. (Perhaps from a previous repair?) There was no visible damage to the board before desoldering.
The rectifier marked "A" toward the outer edge of the board, showed 18 VAC at the AC pins. This IC is connected to pins 1 and 5 on the transformer. The rectifier "B" showed 0 volts at the AC pins connected to transformer pins 2 and 4. A photo of my crude schematic is also included.
Dennis
 

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Wow - this is fantastic!
Not only have you confirmed my suspicion, but you have found a workable replacement part.
Thanks you!

I'll order one, install it and report back. Stay tuned....
Dennis
 
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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?
 
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Success!
I bought a transformer from Newark Electronics - AVB 3.2/2/12 - that has dual 115V primaries and dual 12V secondaries. (Prox $20 delivered from the UK in 3 days!)
I should have paid more attention to the physical dimensions because it is slightly larger than the original. There's not much space inside a sewing machine housing to hide a transformer, but by relocating a capacitor I got it to fit in the space available. The pin out was quite different and I could not mount it to the board, but I stuffed it in and ran jumper wires from point to point. If I had to do it over, I'd get the smaller AVB 2.3/2/12 as rjenkinsgb suggested.
The end result was it works great!
Thanks you all for your help and steering me down the path.
Regards,
Dennis
 
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