The printer loop has flyback capture onto a storage cap, along with a switching voltage boost option to generate the high voltage and quick switching times required by the high inductance selector magnet loops of the Model 14 and 15 era machines.
The keyboard loop on this era of machine has to simply sense switch closures. Over a short loop (like we have in our labs), and given contacts in good condition, 15v is plenty to reliably test closure of the keyboard distributor contacts. A higher voltage does not “clean” the contacts, but instead, results in an arc that can briefly conduct through a layer of oxidation or oil film. The conduction, however, might be only for the instant of the arc (as you see in Eric’s video). In this case, the bit is still sensed incorrectly. The arc can also clear enough oxidation or oil away that then that spot continues to make low resistance contact while mechanically closed. In this case, the bit is sensed correctly. The problem with “cleaning” with high voltage is that each arc creates more damage to the contact surface and also creates its own burn residue that, in turn, raises contact resistance.
I’d continue to work on the contacts with paper tape or a business card whetted in contact cleaner. Move the paper back and forth while you apply a bit of pressure to close the contacts against the paper and each other. The keyboard contacts on my Model 15 took quite a bit of work, but I was eventually able to get them under 1 ohm.
On a “modern” model 33, the distributor is a carbon wiper with at least 40 ohms of resistance, and even there, just 12 volts in keyboard loop current is reliable for reading the keyboard electronically.
Mike
Thanks all, makes sense. I am using deramps adapters, and I am fairly certain that if I can remove the deep layer of gunk/oxides/whatever that formed over many decades of shed storage on these contacts that were open, it will work great on the low voltage circuit.
I say that because the start bit contact works beautifully. It is the other 5 contacts that were left open to the air (were mechanically open when stored badly) that I am having trouble with.
I bet a good shot of high voltage would help, I have a high voltage supply out in the shed I may bring in. I expected the contact cleaner to chemically remove the problematic materials when combined with my mechanical cleaning, but apparently not.
-Eric
It was around 2009:
I remember a discussion years ago where a number of folks said high voltage was important for the contacts. The design voltage was much higher than 15v. You might try a loop source that delivers higher voltage and 60ma. That might work better with your contacts.
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