[GreenKeys] Model 15

[Tom Hunter]

At 75 years old I can't remember what I had for breakfast this morning. But I remember 50 years ago pretty clearly. I am repeating a messaage I sent to AA6RK in the hope it might help some new Teletype users.

Tom   N3CRK
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Bob,

WB2TTY jogged my memory on something you need to know. The 15's have felt clutches. There is an important one on the mainshaft. They look like 2 shiny steel disks with a round felt clamped between them. I seem to remember that they are an inch or so in diameter. Those felts must be saturated with oil at all times or they will soon be ruined.

The trick is too pry the disks slighty apart carefully with a screwdriver. Try not to scratch them.  then you can easily get oil alongside the felt. I use genuine Teletype oil. Ask the guys on greenkeys what to substitute. Never use an oil with additives like 3-in-1 oil.

Next tip: when you first tun on the 15 with no loop connected - it will "run open." It will pound away like it wants to print but there will be nothing to print.

The selector magnets have a great deal of reactance, the current (probably 60 mils) can take an appreciable time to build up. The cure is to use a constant-current supply.

The easiest way to get constant current is to start with a high voltage and drop it through a resistor.  120 to 170 VDC is common. Use Mr. Georg Ohm's law to figure the correct dropping resistor for .060 Amp Now subtract the resistance of the selector magnets to get the actual value of the resistor. With Mr. Ohm's law again figure the power dissapated by the resistor. It sould be in the order of 5 to 10 watts - definately a power resistor. At least part of that resistance should a wire-wound pot, to set the current right at 60 mils.

When you get a loop connected , the selector magnet will pull in  and the machine will quiet down. At this point, if the keyboard is also in the loop, you should be able to type. If everything you type comes out correctly then your are in business.  Otherwise - back to trouble shooting. You must decide if the keyboard or printer is at fault. I have done this so many times, troubleshooting is second nature to me. If you get stuck, send me an email and I will try to walk you through it.

Tom   N3CRK