[RTTY] Resolution to FSK interface circuit troubles

[Alfred Frugoli]

Hey everyone.

Thanks for all your input and ideas.  In the end none of them were directly
the right answer, but many of them got me thinking.

So here's what ended up working.  I can't change the FSK polarity on the
TS-940 - no menu options (this is a 25 year old rig) and no dip switches
that I'm aware of.  So, I needed a circuit that would do the inversion for
me.

I'm no technical wizard, but it does not seem like the PNP circuit at the
bottom of http://www.aa5au.com/rttyinterface.html will work.  Providing
current to the emitter does not seem like it will have any effect on the
resistance between the collector and base.

I ended up building a circuit where the base is connected to the signal
source (pin 3 on the DB9), the emitter is connected to the signal ground
(pin 5 on the DB9) and the collector goes to the rig.  Based on my
rudimentery understanding of transistors it seems to me that providing some
current to the base will cause the resistance between the collector and the
emitter to reduce thus grounding the RTTY circuit and causing the frequency
to shift.  This theory seems to be played out by the fact that the circuit I
built works..

Anyhow, I put a writeup of my saga, including a schematic of the circuit
that worked for me on my blog at
http://ke1fo.wordpress.com/2008/12/11/the-saga-of-the-serial-to-fsk-interface/.
 Please take a look, and any comments you have would be appreciated.

Thanks.

73 de Al, KE1FO

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


On Tue, Dec 9, 2008 at 10:16 PM, Alfred Frugoli <ke1fo at arrl.net> wrote:

> 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.
>