Control Pedal Issue

[marcoghislanzoni]

Dear All,

I just bought a used PFAFF 1222 which is in very good shape. As I tested it extensively after brining it home, it turns out it has an issue to the control pedal.
As soon as the pedal is pressed even lightly, the machine runs immediately at full speed. There is really no (or minimal) speed progression control through the pedal. By pressing it all the way down and then slowly coming back, you can feel the motor revs going down a bit but it is really minimal. I would expect the motor to slow down significantly when the pedal is almost completely released.

By reading all the posts above, I went to check the resistance between the two pins from the pedal (while disconnected) and found the following values:

1.8 M at rest (pedal fully released)
0.593 k first level
0.493 k second level
0.424 k third level
0.336 k fourth level
0.237k fifth level
0.137k sixth level
0 (zero) when fully pressed

1.8 M is the internal resistance of the anti-disturbance filter (combine capacitor and resistor) and this is ok. The other values however seem way too low. I was expecting, from previous posts, something starting from 20k and going down. Since the lower the resistance, the faster the motor spins, no surprise it runs so fast with those value!

To make it a little more manageable I acted on the "N" potentiometer on the control board and regulated the maximum speed, Now the machine runs a bit slower, still there is still a minimal speed progression control from the pedal.

Any hints to what could be wrong with the pedal? Should I assume the resistor bank (black cilinder) has developed some issues over time lowering the resistance values (hence it needs to be replaced) or something else could be causing the issue?

I have attached few pictures of the pedal circuitry (sorry if the images are rotated, didn't figure out how to get them to show up properly on this forum).

Thanks in advance for your recommendations!

Cheers,
Marco.

**broken link removed** **broken link removed**

 

[AnalogKid]

What does the vendor say about the correct resistance values? Also, are you sure that the pedal is switching resistor, and not capacitors? Of course a schematic would be nice...

ak

 

[gophert]

The blue wires and the solder joints connecting them look new. I am guessing something happened there. I would focus on a bad capacitor (likely the one with the new blue wires attached). If it is a triac controlled speed controller with an RC delay on the gate, then a bad capacitor (zero capacitance) will cause the triac to trip on immediately no matter how much resistance is controlling current to the gate. I would try changing both the flat capacitor (with the blue wires) and the round cylinder-like capacitor. If you bother to order one, order both because the parts will be way cheaper than the shipping costs.

You should really get a schematic or simply buy a used speed controller to go with it to make sure the previous owner put everything back the way it was originally. If not, you are just hoping the previous owner was on the right track rather than just hacking together some repair. Parts could be missing and things could be connected incorrectly. Impossible to tell from a picture and a tube full of wires that hide most of the circuitry.

 

[strantor]

Try disconnecting the capacitor and re-take the measurements.

 

[alec_t]

1.8 M is the internal resistance of the anti-disturbance filter (combine capacitor and resistor) and this is ok.

If that measurement is for the R and C in series then the C seems to be leaky. I'd follow Gopher's suggestion.

 

[MaxHeadRoom78]

Unfortunately I see the service manual does not give any details of the switch regardless of having an Electrical section.
Max.

 

[Les Jones]

I think the tubular part may just contain resistors that are shorted out as the contacts close one after the other when the foot pedal is pressed. I think that the bank of resistors is just in series with the motor. The wires going to the tubular part look like they have heat resistant insulation on them which would fit in with the tubular part getting hot in operation. The resistance values measured do not fit in with this theory as they are too high.
Marco, could you trace out the schematic of the foot pedal and read the values written on the capacitors. We will just have to try to guess what is inside the tube. One other thought. Is the belt on the motor so it has some mechanical load on it ? If there was no load on the motor it may not slow down as much as expected.

Les.

 

[strantor]

Unfortunately I see the service manual does not give any details of the switch regardless of having an Electrical section.
Max.

Service manual? care to share?

 

[strantor]

I think the tubular part may just contain resistors that are shorted out as the contacts close one after the other when the foot pedal is pressed. I think that the bank of resistors is just in series with the motor. The wires going to the tubular part look like they have heat resistant insulation on them which would fit in with the tubular part getting hot in operation. The resistance values measured do not fit in with this theory as they are too high.
Marco, could you trace out the schematic of the foot pedal and read the values written on the capacitors. We will just have to try to guess what is inside the tube. One other thought. Is the belt on the motor so it has some mechanical load on it ? If there was no load on the motor it may not slow down as much as expected.

Les.

I suspect it's a multitap wirewound ceramic resistor and it's in series with the field of a separately excited DC motor.
The "high" (0-600ohm) resistances seem maybe appropriate for field current, but not armature current.
That's my SWAG; curious to see the motor, or the service manual

 

[granddad]

This link any use, had to fix sewing machines in the past can be a
https://www.electro-tech-online.com/threads/is-this-a-three-leg-capacitor.94810/page-3
https://elektrotanya.com/?q=hu/content/pfaff-1212-varrogep-elektronika-alkatresz-azonositasa

 

[marcoghislanzoni]

Dear All,

thank you very much for your replies! I tried this morning to bypass all RF filter capacitors and connect directly to the output of the resistance bank. No differences, same exact behavior when it comes to speed control. Hence I don't believe the RF filters are the culprit here. By the way, if you scroll up in this same thread you will find the schematic for the control board. According to it the pedal should act as a 20k potentiometer, while in my case it starts from 600 ohm down. Way too low!

I am starting to consider the option that the foot control I got with my machine is in fact from a different model, where the required resistance value range to control the motor speed was in fact much lower than for the Pfaff 1222. It is likely that the old pedal failed (e.g. mechanically or on the resistance bank) and someone adapted a different one, putting back in place the old RF filters (hence the new blue wires), but not considering that the resistance bank was in fact on different range. With those values of resistance the pedal acts mostly like an on/off switch, with very minimal speed control (in fact it is always working at the very end of the original 20k range).

UPDATE:
By some extensive research on Google, I found this thread (sorry in German, please use Google translator) which is related to a Pfaff 96 model.
https://www.mikrocontroller.net/topic/301892
The OP mentions issues with the foot control and lists the resistance values he measured in his original pedal:
590Ohm, 492Ohm, 423Ohm, 333Ohm, 238Ohm, 139Ohm, 0 Ohm
This is in fact what I have in my pedal! Definitely my pedal (at least the resistance bank) does not belong to the Pfaff 1222 but rather to a different model, very likely the Pfaff 96.

Now I see two possibilities:
1) Look for an original pedal and switch it in (mine could very likely work with a Pfaff 96 --> Ebay!)
2) Modify/replace the resistance bank in my pedal to work with a range of resistances which is compatible with the Pfaff 1222.

Number 2 sounds like a far more interesting project to me!

I think I can "reproduce" the original resistance bank with more modern components, but first I need to understand how it is supposed to work. My first idea is that, as the pedal is pressed, more and more resistances contained in the bank are added in parallel resulting in a linearly decreasing total resistance.

By counting the bending metal strips, there are basically 8 states of the pedal, including the totally depressed one (1.8M ohm = fully open) and the totally pressed one (0 ohm = fully closed). The first step after fully open should be 20k (motor rotates very slowly) and then linearly down to 0k (motor rotates at its fastest speed). This should be the sequence of resistances the motor controller sees:

20k --> 16.6k --> 13.3k --> 10k --> 6.6k --> 3.3k --> 0k

Now, to obtain that linear sequence by cumulatively adding more and more parallel resistances, this should be the content of the resistance bank:

20k --> 96k --> 67k --> 41k --> 20k--> 6.6k --> 0k

Reverting to standard resistor values:

20k --> 100k --> 68k --> 43k --> 20k --> 6.8k --> 0k
[EDIT: there was an error in the original post, I wrote 1.0M instead of 100k. Corrected. Thanks: alec_t!]

so that:

20k // 100k = 16.7k
16.7k // 68k = 13.4k
... on so on.

Does this make sense? Given the rating of the original component, I would go with high W resistors. Thoughts?

Cheers,
Marco.

 

[alec_t]

Makes sense, except perhaps that 1.0M should be 100k?

 

[marcoghislanzoni]

Makes sense, except perhaps that 1.0M should be 100k?

You are totally right, how did I write 1.0M?!?
100k indeed. I have corrected myself above, with thanks to you for spotting the error!

 

[gophert]

You are totally right, how did I write 1.0M?!?
100k indeed. I have corrected myself above, with thanks to you for spotting the error!

I would check out a store selling/repairing sewing machines to see if you can find the correct replacement. You need a pedal from an 1100 or 1200 series machine, it will be much easier to start with a proper rheostat than trying to hack an AC power supply.

Also, what is the model number of the power supply that you have?

From a company selling an upgraded pedal for those machines...
==============================================

Upgrade Your Old Worn Foot Control

This newly designed replacement Pfaff foot control and power cord fits the Pfaff 1222, 1221 and other 1100 & 1200 series models that have the square foot control/power connection plug. Regain precision speed control with your classic Pfaff sewing machine. This Pfaff foot control only works with the later made 1200/1100 series models, please see the photo to tell if your machine has the older or newer style receptacle.
https://www.thecolorfulworldofsewing.com/Pfaff-Sewing-Machine-Foot-Control-For-1222-series.html​

===============================================

 

[marcoghislanzoni]

I would check out a store selling/repairing sewing machines to see if you can find the correct replacement. You need a pedal from an 1100 or 1200 series machine, it will be much easier to start with a proper rheostat than trying to hack an AC power supply.

Also, what is the model number of the power supply that you have?

From a company selling an upgraded pedal for those machines...
==============================================

Upgrade Your Old Worn Foot Control

This newly designed replacement Pfaff foot control and power cord fits the Pfaff 1222, 1221 and other 1100 & 1200 series models that have the square foot control/power connection plug. Regain precision speed control with your classic Pfaff sewing machine. This Pfaff foot control only works with the later made 1200/1100 series models, please see the photo to tell if your machine has the older or newer style receptacle.
https://www.thecolorfulworldofsewing.com/Pfaff-Sewing-Machine-Foot-Control-For-1222-series.html​

===============================================

Thanks for the link gophert! I have just identified a suitable replacement on eBay - Europe and I may make an offer to buy it.
Still the idea of modifying to existing pedal to adapt it back to the machine sounds like much more fun than simply doing the above...
Will keep everyone posted. In case I get the original pedal I will open it and post some pictures for future reference. I am curious to see how similar/different it is from the one I currently have.

Cheers,
Marco.

 

[Les Jones]

Hi Marco,
In my post #7 I said "The resistance values measured do not fit in with this theory as they are too high" I now realise I misread the values. I tought the first value was 593K not 593 ohms (As 0.593K is) So the theory about the high power resitors is still a valid one. I think the wrong foot control has been fitted and the capacitors would fit in with it being used as a resistance in series with the motor. I think your idea of replacing the tapped high power resister with higher value one is a good one. You will not need the capacitors in the modified version. I suspect the contacts will close one after the other (You will have to check this when all the wires have been removed from the switch.) I get slightly differnt results to you in my calculations but the values will probably have to be adjusted as the phase control system will not be linear.

593 100% 20.0K
3.4K (3.3K)
493 83% 16.6K
2.2K (2.2K)
424 72% 14.4K
3.0K (2.7K or 3.3K)
336 57% 11.4K
3.4K (3.3K)
237 40% 8.0K
3.4K (3.3K)
137 23% 4.6K
4.6K (4.7K)
0 0% 0.0K

These resitors will probably only need to be 1/4 watt power rating. The values in the fourth and fith column are the values that will go across each set of contacts so they will be shorted out as the contact closes. The first contact that closes will have no resistor across it the second will have the 3.4 K (At the top of the table) across it and so on down the table.

Edit.
I have just read your post #15 I think the correct pedal for your machine will just be a 20 K potentiometer.

Les.

 

[marcoghislanzoni]

Dear All,
here a quick update. I have opted for ordering the right pedal for my machine since I found it on eBay for a very decent price. It is labeled as model: AT 0021.
As soon as I get it, I plan to open it and check what is inside, especially the value/range of the potentiometer controlling the motor speed.
I will post my findings back to this thread for future references.
I still have the "old" pedal anyway, so if time permits I may still try to adapt it as spare part (or sell it back on eBay!)
Cheers,
Marco.

 

[KeepItSimpleStupid]

Aside: Could the pot have been replaced with an audio taper rather than linear?