[GreenKeys] TU-170 with External Loop Supply

[Harold Hallikainen]

I don't think any modification of the TU-170 is required when using an
external loop supply. Just connect the negative end of the supply to the
sleeve of J3. The positive nd of the loop supply would go through selector
magnets and keyboards to the ring of J3. Current limiting is done by the
constant current sink formed by Q6, R58, and R59.

As I recall, the voltage of the loop supply in the TU-170 is about 85 V.
If you use a higher voltage external supply, the dissipation of Q6 would
increase when in the mark condition. Also, the flyback voltage on the Q6
collector would be higher, possibly causing Q6 to fail. An external
current limiting resistor (such as 2k with a 120V supply) would limit the
dissipation of Q6, but the flyback voltage would still be a problem. I
added a zener clamp (zener and standard silicon diode in series) between
the tip and ring of J3 to limit the flyback voltage.

My TU-170 notes are at https://w6iwi.org/rtty/tu170/

Harold



-- 
FCC Rules Updated Daily at http://www.hallikainen.com
Not sent from an iPhone.