Scout DSP Audio Filter Mod

From the Unofficial Ten Tec Pages

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 transmitting).

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

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