Elna electronic sewing machine control

Thread Starter

tinkerman

Joined Jul 22, 2012
151
Can someone perhaps help to identify a transistor?? in the control circuit. I have a circuit worked out, correctly I hope, but the single transistor on the board which controls the current to the motor has NO MARKINGS!! The part is perfectly functional but one capacitor on the board had failed causing the motor to run continually without the foot control depressed. Got everything back to normal but would like to go further with the design.

Would upload a schematic but can't see how to do it. :(
 

Thread Starter

tinkerman

Joined Jul 22, 2012
151
See the 'Upload file' next to Post Reply.
Max.
Can someone perhaps help to identify a transistor?? in the control circuit. I have a circuit worked out, correctly I hope, but the single transistor on the board which controls the current to the motor has NO MARKINGS!! The part is perfectly functional but one capacitor on the board had failed causing the motor to run continually without the foot control depressed. Got everything back to normal but would like to go further with the design.

Would upload a schematic but can't see how to do it. :(
Thanks Max. Here is my sketch. Hopefully it is correct. Q1 is NOT and NPN transistor it seems from diode tests with a multimeter. It had a red dot on the top of the can. That's all. :( I can provide photos of the board top and foil side if that helps.

Thanks
 

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Thread Starter

tinkerman

Joined Jul 22, 2012
151
I don’t think triac. Needs a diac as I understand it. SCR never thought of that. There is a diode D1 so motor received only positive pulses. This machine is 1968 vintage.
 

MisterBill2

Joined Jan 23, 2018
5,733
Yes, the device could be an SCR, a bit more likely in 1968. but an NPN transistor would also be able to work. and my guess is that the capacitor that failed was C1, and that it failed with either high leakage or a short circuit.
 

Thread Starter

tinkerman

Joined Jul 22, 2012
151
Yes, the device could be an SCR, a bit more likely in 1968. but an NPN transistor would also be able to work. and my guess is that the capacitor that failed was C1, and that it failed with either high leakage or a short circuit.
No turned out it was the electrolytic C2 that had failed. 20uF 16V. First thing I tried was an 1A NPN transistor but it heated almost immediately. So then I changed capacitors. I've never seen a SCR circuit before without a diode in the gate. Didn't cross my mind. I think I'll bread board a circuit like it with a SCR and see how that turns out.
 

pfofit

Joined Nov 29, 2006
57
Is R6, R11, R12, R13, C12, and the zener all connected together?
Is the emitter connected to the common line and L1 or just to L1?
Could you put dots on the intersecting lines of your diagram. I like dots. :)
You have a T2 designation at the top and bottom.
What is the voltage/ waveform across C2 without touching the foot pedal?
Can you post a pic of the board?
Is there a label on the motor?
thanks
 

MisterBill2

Joined Jan 23, 2018
5,733
More likely a Triac, a SCR is uni-directional.
Max.
Yes, an SCR is indeed unidirectional, and looking at the circuit, if it is correct, there is also a unidirectional diode just upstream from the device. I am not certain why it is placed there, but that is what I see.
And if the part that failed was that capacitor C2, then yes, an SCR is likely. Sort of a different type of circuit, but nothing unbelievable.
And now, after seeing the waveform, it is more likely an SCR, although a triac would explain the waveform, if D1 is not really a diode.
And D2, shown as a zener diode, seems to be serving the same purpose as a diac would serve.
 
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MaxHeadRoom

Joined Jul 18, 2013
20,400
The only problem is if a SCR then you are not getting full wave AC to the Universal motor.
If SCR I would expect a bridge before it in order to switch each half cycle and run the motor on DC this way.
Logically the circuit shown would be an Triac.
Max.
 
I know that this is an old thread, but since this is the only Internet search hit for a schematic of this device, I thought it might be useful to add my findings to this thread in case it helps future searchers.

I think that the original poster's schematic has a few errors or else the circuit I have is different. I am attaching the schematic of the device that I have which has the same model number (447300). The differences are:

- There are two screw terminal connections labelled T2 - I assign T1 to the bottom one.
- D1 is out of place - it connects T2 to R12 and foot control switches common.
- R8 and anode of Q1 (collector in original) connect to R1 and the first foot control contact.
- R13 and R7 values are swapped.
- R10 is 10K not 1K.
- C2 is 22uF not 20uF.

I have attached a schematic of my reverse engineering results with component annotations made to match the schematic from the original poster. I have also attached images of the PCB.

From my waveform observations, the circuit is only switching positive half cycles as predicted, but with the speed selector in the "+" position, the full speed switch bypasses the circuit to give the full AC waveform.

Andy
 

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