[GreenKeys] M19 Selector Magnet - magic smoke?

[Ralph Mowery]

As discussed many times the old Model 15 series of printers really need
around 100 volts and up with a series resistor of 1000 to 3000 ohms
depending on the voltage and a few other things to get a 60 ma loop.  It
only takes about 12 volts to work the selector magnets, but the machine will
not print correctly due to the time it takes the coils to pull in.

I played around with about 50 volts and that was still not enough to give
copy.

Hoff and his st-5 and st-6 used a bipolar power transistor  without a heat
sink.  The transistor dissipates very little power if fully on or off.  I
used one of the small transistors similar in size of the plastic 2n2222 that
was rated for high voltage in a copy of the ST-6 I built and it ran for many
years, often for hours at a time  copying the pictures off 20 meters that
were sent mainly on Saturday and Sunday. 

With the correct value of resistor to give 60 ma of current it is difficult
to cause problems.  Resistors seldom reduce value, especially the wire wound
power types.  

Ralph ku4pt
 




-----Original Message-----
From: greenkeys-bounces at mailman.qth.net
[mailto:greenkeys-bounces at mailman.qth.net] On Behalf Of Harold Hallikainen
Sent: Monday, August 02, 2021 2:46 PM
To: greenkeys at mailman.qth.net
Subject: Re: [GreenKeys] M19 Selector Magnet - magic smoke?

Back in high school (a little over 50 years ago), I discovered T=L/R. I
had a couple mile long teletype loop to a friend's house. I could get 60
mA with about 12V, but the printout was garbage. With the low loop
resistance, it took too long for the magnetic field to build up in the
selector magnets. So, I went to a high voltage supply with series
resistor. The high R (10k or so) made the time constant short enough to
give reliable copy.

As shown at https://w6iwi.org/rtty/, I have the series resistor mounted on
the 152950 loop supply. I did add a fuse holder on the AC line side of the
transformer. But, I think it would be quite rare for the resistor to fail
to a short (it is a large wire wound resistor), so I don't think a loop
fuse is required.

The TU-170, however, uses a bipolar transistor in an active current sink
circuit (
https://w6iwi.org/rtty/tu170/Flesher%20TU-170%20Terminal%20Unit.pdf#page=18
). A common failure of a BJT is an emitter to collector short, so a fuse
here makes sense (and there is one, the 0.125A fuse at terminal U).

I have a bunch of scope photos at https://w6iwi.org/rtty/tu170/ showing
the varying effects of the selector magnet inductance based on the
armature position. The inductance is a lot higher when the armature is
against the selector magnet pole pieces. And, for a holding magnet, it is
always against the pole pieces on a mark to space transition.

Harold