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.