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Follow-up and Update to Replacement Volume Controls for MaxTrac, Radius, and GM300 Mobile Radios By Robert W. Meister WA1MIK

This article is a continuation of my previous article that describes in detail how to replace the volume control on Motorola MaxTrac, Radius, or GM300 mobile radios.

The original Motorola part (labeled "Orig." in the photo below) has not been available since around 2006. At that time Motorola could supply a similar part (labeled "Mot." in the photo below) with slightly closer spacing between the switch and the pot. They redesigned the VOL/MIC circuit board to accept these newer parts. Even those became unavailable in 2016. Fortunately for me, I had bought several prior to that but recently I had to use two and started looking for a suitable replacement.

I searched a popular auction site for "gm300 volume control" and found a Chinese seller with an auction for five pots (labeled "New" in the photo below) that claimed to fit the GM300 as well as several other radios. For $15 and free shipping, I purchased a set to see how they'd do. I got them in less than two weeks in August 2017. The photo below shows all three pots.

The first thing I noticed was that the switch on the back of the control only had two leads (SPST). The original switch had three leads but only the outer two were utilized (SPST). The new Motorola replacement switch had three leads (SPDT) and a jumper was needed to make the switch work in the radio. Then I noticed the pot was marked B10K on the back, which told me the pot's value was 10,000 ohms, not the 2,000 ohms of the originals. The seller conveniently omitted that key piece of information. I didn't know if this would be an issue or not. The radios have a 10 ohm 1/4 watt fixed resistor in series with the ground end of the volume control pot to limit the minimum volume to something audible; this provides a 10/2,010 ratio. The new pot's 10,000 ohm value would provide a 10/10,010 ratio, meaning the minimum volume would be considerably less. I also determined that the "B" in the part number indicated a linear taper, not an audio taper that the Motorola parts had. This just means the volume will get a lot louder much sooner in the rotation than it did with the Motorola pots.

I took several physical and electrical measurements of the various pots; these have been summarized in the table below. The audio levels were made with a 400 Hz tone at 3.0 kHz deviation and an RF level of -80dBm. Afterwards, I removed a new Motorola pot from a radio that I had just installed a week ago and installed one of the new Chinese pots and took additional measurements with the stock 10 ohm 1/4 watt fixed resistor. I ended up replacing that with a 51 ohm 1/4 watt fixed resistor to increase the minimum volume to the same level as with the Motorola pots, now a 51/10,051 ratio, which for all intents and purposes, is the same as the original 10/2,010 ratio.

The replacement procedure is exactly the same as described in the previous article EXCEPT you do not need to add a jumper from the middle to left switch terminal, because the new switch has no middle terminal and the circuit board didn't use it anyway. It was only necessary for the "Mot." control that had an SPDT switch.

Measurement	Orig.	Mot.	New
Sw/Pot Spacing	8mm	7mm	7mm
Switch Type	SPST	SPDT	SPST
Switch Leads	3	3	2
Pot Resistance	2k	2k	10k
Pot Taper	Audio	Audio	Linear
Top of pot Level	1,600mV	1,600mV	1,634mV
Pot Arm Min. Level	8.00mV	8.00mV	1.634mV
Min. Ratio: 10 ohm	1:201	1:201	1:1001
Min. Ratio: 50 ohm	- - -	- - -	1:201

The "Top of pot Level" increased slightly because the pot resistance increased from 2k to 10k. The 10 ohm 1/4 watt fixed resistor is located right next to the volume control; it can be seen in the photos in the other article. You can also short out this resistor if you want the minimum volume to be reduced to nothing.

Contact Information:

The author can be reached at: his-callsign [ at ] comcast [ dot ] net.

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This page originally posted 19-Aug-2017.

Article text, photos, and layout Copyright © 2017 by Robert W. Meister WA1MIK

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.