Ten Tec Scout/Argo-556 and DSP Audio Filters
DSP audio filters are a great tool for making signals easier to copy
on the noisy HF bands, but they aren't always easy to connect to your
station rig. This is a description of a circuit that lets you interface
some common DSP filters to the Ten Tec Scout-555 and Argo-556 radios.
Many times the pitch of the station that you're working on CW isn't
exactly the same as your sidetone, and the narrower settings of DSP
filters will keep you from hearing the sidetone. I use a Timewave
DSP-59+, and it needs it's PTT input keyed to ground to let you hear
your own sidetone when sending. The JPS NIR-10 also uses a
keyed-to-ground input for the same bypass function. The DSP-59+
doesn't actually switch the audio around the filter, it changes the
algorithm of the microprocessor to make the filter wider when the
PTTI connection is grounded.
I haven't tried an MFJ-784B DSP unit, but the '784B manual
indicates that it's T/R line (on the TO RADIO connector) should be
grounded to bypass all filter functions for monitoring the sidetone.
When using my Kenwood TS-450s everything was working just fine
because it has a +12vdc signal that is high whenever the radio is
transmitting. It's available on the REMOTE accessory connector on
the rear panel, and I use it to turn on a 4N25 optocoupler. The
output side of the 4N25 is used to key the DSP PTT input to ground.
This keeps stray RF from getting back into the rig because there is
no DC connection to the +12v line from the DSP filter.
Optocouplers are handy for providing electrical isolation between
two circuits, but allowing signals to be sent through them. They are
usually packaged like a transistor with 4 or 6 'legs', or in a 6-pin
DIP. Inside there is an LED and a photo-transistor; when the LED is
turned on, the transistor will also turn on. They typically provide
500v to 1500v of electrical isolation between the input and output.
Last year I acquired an Argo 556, the QRP version of the Ten Tec
Scout. I couldn't use the DSP filter very well with the Argo because
it doesn't have an interface with control inputs and outputs. I had
to keep switching the filter into bypass manually, much too awkward
for CW contesting.
However, the Ten Tec manual describes how to use the 'T' signal in
the radio to key an amplifier; just what's needed for keying the DSP
unit. They show a circuit on page 3-2 of the Scout/Argo manual that
uses a transistor, but I like using the optocouplers whenever
possible. I'd hate to mess up that 'T' signal since it controls the
radio's transmit timing (it's 'high' whenever the radio is
Since the Argo 556 doesn't have the 50w PA stage in it, there is a
blank panel at the rear instead of the heatsink that a Scout has. I
added an RCA connector to the panel for the new output. The manual
suggests running a small cable through the metal rivet in the rear
panel of the Scout if you don't want to drill a new hole in it's
crowded rear panel.
I mounted the 4N25 right to the PC board by soldering the LED
cathode (pin 2) to the copper ground foil of the LL Driver board.
There are a number of places that it can be soldered to along the
back edge of the board. The other leg of the 4N25 LED (pin 1)
connects to one end of a 2.2K, 1/4 watt resistor; the other end
of the resistor is soldered to a small pad on the LL Driver board
to the rear of the connector labeled '56'. Scouts have a white
wire from this pad to the 50w PA, it's unused in the Argo. That
is the 12v 'T' signal; the 2.2k resistor limits the current
through the LED when 'T' is high (+12v).
Now we have the anode of the LED in the optocoupler going to the
'T' signal through a resistor, and the cathode of the LED going to
ground. Next, the leads of the photo-transistor in the 'opto' are
connected to the output connector. We only need to connect the
collector (pin 5) and emitter (pin 4) of the photo-transistor. I
used some very small coax to connect the two legs of the 'opto' to
the RCA jack on the rear panel, the collector going to the center
of the RCA.
The 4N25 optocouplers can turn on up to 30v at 50ma, and that is
more than enough for keying the PTT of the JPS and Timewave DSP
filters. 4N25, 4N35, or 4N37 DIP IC's are available at Tech America
stores (mailorder 1-800-877-0072) or DigiKey (1-800-DIGI-KEY).
To see the circuits that this connects to, look at the Scout
schematic (fig 3-23 in the manual); find the LL Driver in the top
left-hand corner. Along the bottom edge of the 'box' are two
signals that go down to the RF PA box at the bottom of the page,
+13.8V and T. That's the 'T' pad that the 2.2k resistor connects to.
This small circuit has made my Argo 556 compatible with the DSP
audio filters, and it's a real pleasure to use the combination on
the crowded CW bands.
73 -- Glenn AE0Q/V31RY