This article details the modification of my Radio Shack HTX-10 radio for link or remote base use. This mod is also helpful for other applications, such as PSK31. The HTX-10 is a great choice for a remote base radio, as it's inexpensive and a common Ebay or hamfest find.

First, remove the upper and lower covers, and unplug the internal speaker connector.

Next, carefully drill a 1/4" hole in the rear panel. A nibbler tool may also be used if available. I recommend placing this hole on the top side of the radio, just above the solder pads for the external speaker jack. I chose this placement as there are other modifications for the HTX-10 that call for an RCA jack to be mounted on the bottom side of the rear panel.

Run 2 shielded wires and 2 unshielded wires through this hole, long enough to reach anywhere inside the radio, top or bottom of case. Leave enough wire outside the radio for your DB9 (or other) connector. The shielded wires will carry audio out, mic audio in, and ground. The unshielded wires will handle PTT and COS.

A good (free) source of shielded wire is old computer monitor video cables! Each monitor cable has about 20 feet of shielded wire, and about 30 feet of stranded hookup wire. Most monitor cables also have a ferrite core or two molded into them that might come in handy sometime.

The toughest part of this project is the COS (carrier operated squelch) modification.

Take a look at the picture of the HTX-10 main PCB. The COS connection point is close the CPU, near the number 18. A white wire is used in this picture.

NOTE: This signal cannot be directly connected to a repeater controller or PC interface.

Instead, we will use it to drive a 2N7000 or BS170 Mosfet transistor (see schematic). The COS output signal from the mosfet is low logic when the squelch is closed, and floats above ground when the squelch opens.

Next, connect a wire to mic jack pin #3 (PTT) and connect a shielded wire (center conductor) to mic pin #1 (Mic input) . Ground the shielded wire to mic jack pin #5. (Ground) .

There are several places inside the radio that you can use to wire tie your new lines. I recommend securing the lines in a few places to prevent damage to the connection points.

In this picture, you can see the COS, Mic input, ground, and PTT lines connected. Note that the lines are routed from the bottom of the radio to the top side near the mic jack.

This picture shows the connection of the Receiver audio output line. For receiver audio, I used unsquelched, unfiltered discriminator audio, which is not affected by the volume setting. This is what you want for connecting to a repeater controller or Asterisk/Allstar system. If you want squelched, filtered receiver audio, you may have to use the external speaker jack. The receiver and audio circuits are very easily loaded, and I have yet to find an easy way to pick off a muted, fixed audio from anywhere inside this radio.

Connect through a 1 uF electrolytic capacitor (+ side) to the connection point shown here. You may find that you need to adjust the value of capacitor, and possibly add a series resistor, depending on your application.

NOTE: The HTX-10 is designed to mute the speaker audio if the microphone is disconnected. The PTT switch in the microphone connects pin 2 to ground (pin 5). Mic pin 2 connects directly to the internal speaker, providing a ground signal. If you would prefer the speaker work without a mic (as I do), take this opportunity to provide a chassis ground to your internal speaker.

The last step is to connect a 2N7000 or BS170 Mosfet transistor to the COS line. This will provide an active low COS signal (0v) to your controller or PC interface with the squelch closed. This line will float above ground when the squelch opens. I like to connect this inside a DB9 connector shell (outside the radio).

Connect the BS170 Drain to your DB9 pin headed towards your controller or PC interface. I chose to connect the 1 meg resistor inside the radio (not pictured). I found that the small gauge wire was easier to solder to the SMT component than my resistor, so I installed the resistor at a convenient place near the back of the radio.

Installing the 1 meg resistor inside the radio reduces the parts count in the DB9 shell.

Connect the PTT, ground, mic audio, audio output as required by your controller. There are several common DB9 configurations for repeater connection, so I won't list any DB9 pinout here.

Motorola Mic Jack Handset Audio Modification

Motorola mobile radios are my radio of choice when it comes to RF linking, VOIP node radios (such as Allstar Link, Echolink or IRLP), APRS or Packet Radio, building an emergency repeater, or just about anywhere else a VFO isn't required. There's no reason to waste an expensive Amateur Radio when a cheap Commercial radio will do the trick!

I like the GM300 for heavy-duty applications, but I have found that the Maxtrac and Radius are also very solid units, and can be found quite a bit cheaper than the GM300.

Most of the articles you read about Maxtracs and Radius radios will strongly suggest that you only buy radios with the 16 pin accessory connector. While I agree that the 16 pin connector is great, the 5-pin radios can be had MUCH cheaper, and are easily modified to bring COR/COS out the front mic jack.

Once COR/COS is brought out to the mic jack, you will then have every connection available that you need for RF linking, or even repeater connection. Note: If you require discriminator audio, another modification is required , or you can just look for a 16-pin radio for the receiver.

This arrangement allows you to use two inexpensive Motorolas as an emergency repeater, using only a simple cable, or you can connect a hardware repeater controller if you wish.

Occasionally, you will find radios that have a very low handset audio level. Handset audio is a fixed-level muted, de-emphasised receiver audio that is present on pin 8 (Motorola numbering). It is unclear to me whether certain logic board models have different handset audio levels, or perhaps a bad series resistor?

If you experience this issue, you can get intimate with the SMT components on the logic board, or skip that altogether and perform this simple modification.

I have found a simple way to obtain a suitable fixed-level receiver audio from a Maxtrac or Radius.

The first step is to remove the entire front panel from the radio. Next, remove the volume knob and the PCB with the volume knob and mic jack.

Locate the orange/white wire that connects to pin 4 on the plastic connector that connects to the radio's logic board. Carefully pull the wire from the connector. Note that pin 4 is beside the empty pin 3.

Refer to the picture to see the location to connect your new series resistor. I chose 180 ohms, as the level closely matched my other 5 pin and 16 pin radios. Once you decide on the value of your series resistor, carefully solder to this point. I like to use a piece of larger wire insulation to slide over the solder joint and a drop of hot melt glue to secure the other end of the resistor.

This is a picture of the completed faceplate. Note that the handset audio still connects to mic jack pin 8 (Motorola numbering). Note that standard 568A/568B numbering is opposite of Motorola.

Retrofitting shack power supplies to include Anderson Powerpole connectors

In late 2007, I stumbled upon a great article by Robert Schulz KC6UDS detailing his clever modification of an Astron SS-30 power supply. The original supply has only one pair of set screws for connecting radios and accessories, and it's a real pain to remove and replace wires with these stock connectors.

Robert noticed that his Astron had two knockouts on the rear panel that appear to be for an optional second set of set screw connectors. He found that four sets of Anderson Powerpole connectors could be installed in this location. I modified my own SS-30 in early 2008 and am very glad that I did. The Powerpole connectors are rated at 30A per pair, so they are more than adequate for my purposes.

After performing the mod on my SS-30, I looked at the other power supply in my shack, a Daiwa PS-304 linear power supply. It originally featured two sets of spring-clips in addition to the 5-way binding posts. The spring clips were rated for a maximum of 6A and always seemed to be a source of intermittent connections, etc.

The more I looked at the PS-304, the more I was certain that I could replace the lousy spring terminals with many sets of Powerpoles! Applying the same techniques used in the SS-30 mod, I found that six sets of Powerpoles would fit in my PS-304!

Before you begin modifying your PS-304, I suggest you read the KC6UDS Astron SS-30 Modification Article

I needed to use some sheet aluminum as spacers. Any home improvement or hobby store should have a good selection of sheet metal for a couple of dollars. As you can see in the picture, I used one spacer above the Powerpoles to take up the extra vertical space between the Powerpoles and PS-304 cabinet.

I also used spacers between the Powerpoles. although I found that there was not enough room horizontally for a spacer between each set of Powerpoles, so I fit the spacers as needed to keep the connectors straight, and also fill any remaining space in the opening. I tried to cut the spacers carefully, so that they fit very tightly in the cabinet. I do not want these metal pieces coming loose inside my power supply!

Once the connectors are fit tightly, apply liberal amounts of glue inside the cabinet to secure all of the Powerpoles to the supply. Next, use a zip tie (or two) wrapped around all 6 pairs of Powerpoles to keep everything secured together.

After allowing the glue to dry overnight, I set to work installing the jumpers from the binding posts to the Powerpole connectors. As specified in the SS-30 article, I'd suggest using the largest wire you can comfortably use. I found that 12 AWG was about the largest gauge wire that could be crammed in the PS-304. In the interest of abundant safety, I rated my new Powerpoles at 15A per pair.

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Last Updated August 3, 2022 ©1997-2022 Kyle Yoksh