[RTTY] FSK interface circuit troubles

[Alfred Frugoli]

Hello folks,

I'm trying to build up the interface listed at the bottom of
http://www.aa5au.com/rttyinterface.html#PNP_Transistor_FSK_Circuit using a
PNP transistor.  I'm using generic PNP transistors from Radio Shack (the
package says they're 2n3906 but there are a mix of various numbers in the
package) and a 1k resistor.

I'm building up the circuit for use with Software FSK in Writelog (using
RTTYwrite not MMTTY).  I originally built the NPN circuit at the top of the
page and got inverted keying.  Then I built up the PNP circuit and all was
well.  I then went to build up a 2nd identical interface, and it did not
work.  I took apart the case of the first interface to make sure they were
wired identically.  I put it in line without the case to make sure it
worked, and it did not.  Since that point I have not been able to build a
working interface, and the original that did work, no longer does.

I have tested the RTTY port on the rig, and all is well there - when I
ground the center pin it shifts tones.  I've tested the Serial port, and I
get either +9v or -9vdc between pin 3 and signal ground depening on the
state of the serial port.  I've tried a different USB/Serial converter, and
no joy there, so I'm assuming I've done something wrong in the circuit.
I've tested my transistors prior to inserting them in the circuit and they
seem to be good both before being in the circuit and after not working (i.e.
nothing is zapping them when they get connected to the rig and serial
port).  I've "reversed" the transistor (i.e. swapping emitter and collector
leads just in case the transistor is miswired/mislabeled).  I've tried
connecting the base back to the "ground" connection of the RTTY port on the
rig rather than to pin 5 of the serial port (signal ground).

Everything I've tried has resulted in the rig not shifting tones.  The
USB/Serial converter has an LED to show activity, and when I send I can see
activity on the prort, but the rig just stays on a single tone and does not
shift at all.

I don't know what else to try, except maybe a lower value resistor, but that
doesn't seem to make sense since the circuit worked once.

Thoughts?

73 de Al, KE1FO

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Visit my amateur radio contesting blog at ke1fo.wordpress.com.