[RTTY] FSK interface circuit troubles

[Phil Sussman]

Al,

If the circuit worked using an NPN transistor, then using a PNP requires
inverted polarity on the keying line. So, I have some suggestions:

1. Verify that your FSK keying line still works by manually grounding and
ungrounding the line into your rig.

2. Go back and try the NPN circuit and see if it still inverts your keying.

3. You don't mention the type of rig you're using. Several have the ability to
invert the keying line internally, either by software or with a DIP type switch.

3a. As an alternative, if the keying (in step #2) is inverted add a second NPN
transistor to the output of the existing NPN transistor. Just add about 150K
from the collector (output) of the first transistor the base of the second with
the emitter of the second grounded. Add about 27K between b+ and the collector
of the first transistor to act as a 'pull up' resistor and connect the collector
output of the second transistor to the FSK line going to your rig. This will
invert the output of the original circuit. The values are off the top of my head
and you may need to experiment with them a bit.

Give it a try and let me know !!

de Phil - N8PS

---------

Quoting Alfred Frugoli <ke1fo at arrl.net>:

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