[GreenKeys] Model 28 Drive Voltage

[Bob Camp]

Hi

Well obviously I have an odd set of parts sitting around here. The 
skintight 28 I got new in the box came with the magic vacuum tube keyer 
in a box that mounts under the machine. As I was poking inside my 28 
KSR it sure seems to have the same thing mounted on a panel in the back 
of the machine. Once I found it in two out of three machine (didn't 
check the third one) I more or less assumed it was a standard part of a 
model 28.

The circuit is interesting in that it seems to be a constant current 
driver for the magnets. It puts about 90 volts across series connected 
magnets. That doesn't seem like a lot of voltage. The other interesting 
thing is that the tube gets into the act so the tolerance on the loop 
voltage is at best about 5 or 10 percent.

The obvious advantage of a constant current driver being that the time 
is shorter than with an equivalent resistor based loop with the same 
voltage on it. I used to use transistor based constant current drivers 
on model 15's and 19's just because they made things a little easier. 
You can hook all kinds of stuff up in series and not worry about it. I 
didn't realize that they dated back to the tube era.

Also listed in the documentation is a really weird transistor keying 
circuit for the selector magnets. It puts around 35 volts on a pair of 
parallel connected magnets. Rather than being a classic constant 
current circuit they used a one shot timer to boost the current at turn 
on. They are only running about 40 mills in the "60 ma" magnets once 
they get them turned on ....

All that still comes back to what the minimum drive voltage for the 
tube based circuit is.

One observation based on the circuits that Teletype used - it seems 
like you can drive the magnets with about 40 volts per selector coil if 
you use a constant current driver. It's also pretty clear that Teletype 
didn't worry a whole lot about 1% tolerance on the loop current.

Another observation - on a model 15 the selector magnets didn't have 
much of anything across them. On the circuits I just looked at there is 
a 0.1 uf on the tube circuit and a bunch of 0.022's on the transistor 
circuit.

Bottom line would seem to be that with about $3 in parts (zener and two 
mosfets)  with a 50 or 60 volt supply you could do a constant current 
loop for parallel connected selector magnets. As far as I can tell it 
would run at least as well as the "official" circuits. It would key 
with something like a 3 volt signal. You could drive it off a laptop 
RS-232 port.

If that's all a bit confusing it's because I'm really not sure where 
this is all going ....

	Take Care!

		Bob Camp
		KB8TQ





On May 24, 2004, at 9:31 AM, Gary Chatters wrote:

>
>
> On Sun, 23 May 2004, Bob Camp wrote:
>
>> Hi
>>
>> Each of the 28's I have sitting here have a cute little box in them.
>> The box has a high voltage supply and a vacuum tube to switch the
>> magnets. I had more or less assumed that it was a standard part of a 
>> 28
>> ....
>>
>
>
> A few years ago I acquired a LESU chassis that had several of those 
> 6005
> keyer circuits, but have never seen one installed in a model 28.
>
>
> Gary
>
>
> _______________________________________________
> GreenKeys mailing list
> GreenKeys at mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/greenkeys
>