[GreenKeys] More on Model 15 Wiring

[John, W9DDD]

Good morning Paul,

I have a 28 with one coil reading 1,500 ohms that prints just fine in 
local loop.  I don't know if the bad one is helping the good one.  They 
are running in series once I discovered the problem.

I have a spare coil (actually I have a several) which is on the to do 
list for replacement.  Someone here told me how to do the replacement 
without a major disassembly but it required and ope end 3/16" wrench 
which was missing from my 60's vintage ignition wrench set.  Try finding 
one of those in this era of electronic ignitions and metric parts. 
Finally found a reasonably priced set with one in it on the 'zon.

I would drop your loop current to 20-30 mA until you can locate a 
replacement.

I don't know how lucky you might be repairing the bad coil.  I tried 
re-soldering the terminals on my 28 coil, which didn't help.  Any other 
tricks requiring removing it.  I may conduct an autopsy after I get it 
replaced.

John, W9DDD

On 10/6/2020 10:49 AM, ad7i wrote:
> Thanks to all who responded to my request for info about Model 15 wiring.
> 
> Jerry Murphy and Nick England suggested that the power relay is likely 
> related to TWX service and that the relay may have been activated by 48 
> VDC or 120 AC with a suitable current limiting resistor.  My M15 is a 
> Bell System TTY with a faceplate that looks like TXW. See photo below.  
> So that may explain the inclusion of the power relay.  This TTY came to 
> me with a wire strap across the relay contacts, so the motor runs 
> whenever AC power is supplied to the TTY.
> 
> m15 twx plate.jpg
> John Koster reminded me about the selector magnet switch that is 
> sometimes included with Model 15s.  My M15 does have this switch, a 
> switch I had totally overlooked when I was measuring the selector magnet 
> resistance.  This switch is used to put the two magnets into a parallel 
> or series configuration.  A photo of a schematic snipped (pasted below) 
> shows the switching circuit.
> 
> m15-magnets.jpg
> 
> 
> Unfortunately, on my M15, when the switch is in the series position, I 
> measure 5K ohms between terminals 45 and 46, which means that something 
> is open (other than the resistor).  I was hoping it was a switch 
> problem, but investigating further, it looks like the inner coil is 
> open.  I was originally planning to leave the problem alone, since the 
> M15 prints fine.  But in thinking more about it, given that power is 
> proportional to the square of current, and with one of the two parallel 
> coils being open, that means that the working coil carries twice the 
> normal current and thus is dissipating 4x the normal power.  Not having 
> experience with TTYs, I'd expect that coils go open either because of 
> excessive current (heat) or that the magnet's internal wire breaks where 
> it connects to the terminal because of vibration when the wire it too 
> taut.  Who knows how long the inner magnet has been open on this M15, 
> but I'm thinking that I should either repair or replace the inner 
> coil.   Do others agree that it needs to be repaired?  Or do you think I 
> can safely leave it "as is"?
> 
> thanks,
> 
> Paul, ad7i
> 
> 
> 
>