Attached is the waveform, and the decoder receiving the R in slow mo.

-Eric

On Sun, Aug 6, 2023, 12:33 PM Eric Moore <mooreericnyc@gmail.com> wrote:
I went at it again and am now getting a lovely waveform. I did use some 3000 grit sandpaper, but very lightly just to break up the surface a bit with a generous application of contact cleaner

I dialed in the timing, and got a perfect R on the decoder in loopback! I am also getting lots of extraneous characters, so going to disable loopback while I work on the printer, but still, progress.

Next, my vertical drive for the printer is not moving the full cycle vertically, again. It is a symptom of a bad bend around the code arms this printer suffered, and mis-alignment on the vertical printer cam arm.

I am afraid I may need to disassemble for the umpteenth time to adjust the printer, again, again, again...


-Eric



On Sun, Aug 6, 2023, 9:47 AM Mike Douglas <deramp5113@yahoo.com> wrote:
Note that the loop adapter Eric is using has separate printer and keyboard loops and Eric has those loops wired separately on his Model 14.

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

On Aug 6, 2023, at 8:44 AM, Eric Moore <mooreericnyc@gmail.com> wrote:


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




On Sun, Aug 6, 2023 at 8:33 AM Paul Heller <paul0926@comcast.net> wrote:
It was around 2009:




On Aug 6, 2023, at 7:31 AM, Paul Heller <paul0926@comcast.net> wrote:

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. 


On Aug 6, 2023, at 7:27 AM, Jeffrey Golas <jeffg@junknet.net> wrote:


Dont use sandpaper! The MA is important as it does affect the waveforms. The machine wants to see 60ma whether it's 120vdc or 15vdc. Set the scope to be wide enough to get an entire character if you havent already, it almost looks like the pic may just be showing the stop bit (33ms?).

Dial in the loop first, verify its 60ma at idle.

Jeff

On Aug 6, 2023, at 9:05 AM, Eric Moore <mooreericnyc@gmail.com> wrote:


https://deramp.com/downloads/teletype/Model%2015/loop%20interface%20board/Schematic%20Rev%20A.pdf

Verified the printer loop at 60ma. The keyboard loop runs at 15V, not sure on the ma, but not sure it matters much as it just opens and closes without driving any coils or similar.

I am fairly certain my issue is the contacts somehow. I see voltage (and continuity) up to those contacts, but despite cleaning fairly extensively with contact cleaner, they are still too high resistance. I am going to try some sandpaper today and see what is what...

-Eric

On Sun, Aug 6, 2023, 12:10 AM Gerry Block <gblock@sbcglobal.net> wrote:
typo:  "60 ma"

Sent from my iPhone

On Aug 5, 2023, at 10:07 PM, Gerry Block <gblock@sbcglobal.net> wrote:

eric

what does your loop schematic look like?  sketch it out and enclose photo. 
what is the loop power supply voltage. 
what is the series resistance to make the 60ms. 
are you sure it is running at 60 ms?
where is the scope probe grounded and where is the loop supply grounded?
i doubt it is the contacts.  

gerry

Sent from my iPhone

On Aug 5, 2023, at 5:49 PM, Eric Moore <mooreericnyc@gmail.com> wrote:


I have cleaned with contact cleaner and paper tape pretty extensively, but can always go another round.

I do not think that is a good waveform, but similarly, no idea.

-Eric

On Sat, Aug 5, 2023, 7:23 PM Jeffrey Golas <jeffg@junknet.net> wrote:
Correct me if Im wrong but could just be "dirty"; as the cam rubs against the switch youre seeing noise like a dirty volume pot.

Id use some alcohol and a business card and scrub the contacts (plus Im not sure thats even a terrible signal for a 1940s device. Going machine to machine I dont think that tiny bit of leakage woulda mattered.

Again, correct if wrong, Im here learning too.

Jeff KC3GJX

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