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Improving VHF MaxTrac
Performance for APRS Use
In the 144-148 MHz Range
By Robert W. Meister WA1MIK
From an idea used by Scott Lichtsinn KBØNLY
Most VHF-high MaxTrac (and Radius) radios primarily cover the 146-174 MHz band. There are some that cover the 136-162 MHz band, but these are rare and were not available in all power ranges. All MaxTrac radios have their output power and deviation alignment broken up into 16 frequency bands about 2 MHz wide, that span the desired operating range, each with its own tuning values. The radio's microprocessor selects the appropriate tuning values based on the programmed transmit frequency. For example, on the 146-174 MHz range, with 28 MHz total band coverage, there are 16 tuning values 1.875 MHz apart and they start at 146.0 MHz. So a programmed frequency between 146.0 and 147.875 MHz will use the first set of tuning values, a frequency between 147.875 and 149.75 MHz will use the second set of tuning values, and so on. The table below shows these 16 tuning values for the VHF-high radios. Note that there are two quite different columns of frequencies: one for each of the two VHF-high ranges, and while they overlap, they don't share any exact frequencies. By the way, they use the 8th tuning frequency for frequency warp and overall power alignment.
This scheme works great for programmed frequencies within the valid ranges that the radio has been designed and manufactured for. The software knows these ranges and tries to limit you to valid frequencies. However, through several techniques, out-of-range frequencies below 146 MHz or above 174 MHz can still be programmed. The radio may or may not perform properly (if at all) outside the stock range. Some may go down to 145 MHz while others may go down to 140 MHz. It all depends on the tolerance of the parts in the rest of the radio and how it was assembled, tested, and aligned in the factory.
If your radio does in fact operate below 146 MHz, what tuning values will be utilized? The radio may choose some random value and it won't know what tuning values to use, so it may transmit with a power and deviation level that is completely unexpected and possibly dangerous to the radio. Since no known tuning values are used, the radio is basically operating in an uncontrolled situation. This is not a good thing. On the 12 watt 900 MHz MaxTracs, some channels end up transmitting with 0.8 watts while others end up at over 20 watts. This is why manual power control is necessary on some radios that are operating outside their designed frequency range. All MaxTracs suffer the same fate because they all use essentially the same firmware, microprocessor, algorithms, and programming software.
Scott has been configuring VHF MaxTracs for local hams to operate on the APRS frequency of 144.390 MHz and he ran into the same problem: the appropriate tuning values would be below the first row of the table above, and would therefore take on random values inside the radio. His solution was to reinitialize the radio to cover the 136-162 MHz range. This allows the 144.390 MHz transmit frequency to be covered by legal tuning values (between rows 5 and 6 of the table above), and thus be fully controllable and stable.
What You Need To Do:
There are several steps that need to be done to fully convert a 146-174 MHz radio to a 136-162 MHz model. They require several pieces of test equipment (signal generator, wattmeter, dummy load, frequency counter, deviation meter) or a decent service monitor, a digital voltmeter, and lab software (used only to blank the radio). If you don't have the appropriate test equipment, you can still get the job done well enough to live with, by following the steps below.
The tuning values in rows 1-10 of the 146-174 MHz radio can be copied into rows 7-16 of the 136-162 MHz radio. You can then place the row 1 value of the 146-174 MHz radio into rows 1-6 of the 136-162 MHz radio. At least that will give you reasonable values in the 144-148 MHz range.
There are articles elsewhere in the MaxTrac area that tell you how to blank and initialize a radio. I won't go into the details here.
It makes sense to do some quick measurements on the radio in its stock 146-174 MHz frequency range. The subject radio is a very clean, late production unit that was built in mid-1998. It has a model number of D43MJA7DA5CK. It was making 50 watts and the transmitter was about 90 Hz low in frequency. It had 9 modes programmed into it (out of a maximum of 16) and has a 16-pin accessory jack. No accessory plug came with the radio; I supplied my own with standard jumpers from pins 7 to 9 and pins 15 to 16. I used the SHIFT-NUM method for entering frequencies below 146 MHz into the stock radio.
I measured the sensitivity, in dBm, for 20dB quieting (no crackles in the signal) and the DC current draw (transmitting at 50 watts into a dummy load) in DC amperes. The RF power amplifier and receiver front end are not rated for operation below 146 MHz.
|Stock||50 Watts||Stock||50 Watts||Adj VCO||25 Watts|
|146-174 MHz||136-162 MHz||136-162 MHz|
|Sens.||TX DCA||Sens.||TX DCA||Sens.||TX DCA|
Operation of the stock radio below 146 MHz was not good. The receiver picked up local interference (birdies) on 144 and 140 MHz, even with a dummy load attached. (The reference oscillator in the MaxTrac runs at 14.4 MHz, so the 10th harmonic is an expected signal. The 140 MHz signal was probably the 14th harmonic of my 10 MHz GPS-disciplined oscillator running in my shack.) It heard nothing at 138 or 136 MHz because the VCO was unlocked. The transmitter output varied between 10 and 25 watts, drawing 9-11 amps. I could measure the frequency all the way down to 136 MHz and it was right where it should have been, even though the transmit VCO was operating way below its legitimate range.
While I had the radio apart, I also measured and recorded the VCO Steering Line Voltage (SLV) of the original radio and after the conversion. I anticipated I'd need to adjust the two VCOs after conversion, so those readings are shown as well.
|146-174 MHz||136-162 MHz||136-162 MHz|
|RX SLV||TX SLV||RX SLV||TX SLV||RX SLV||TX SLV|
The VCOs are officially usable with an SLV of 1.8V to 7.8V but they will work in the range of about 1.2 to 8.4V. They are not considered locked with a voltage below 0.7V or above 9.0V, so the stock 146-174 MHz radio would not receive below 140 MHz. The receive VCO ran out of mechanical range; the values in the "Adj" columns in the table above were taken with both of the tuning slugs completely removed from their coils.
What I Did To The Radio:
The radio was blanked, initialized as a 32-channel, 25-watt radio in the 136-162 MHz range, and mostly aligned with separate pieces of test equipment. I did not bother aligning or measuring the transmit deviation since there's nothing frequency-dependent in that circuitry, however you should align the transmit deviation if you convert your radio. According to the service manual and the MDF file in the MaxTrac RSS, only the 25-watt VHF radio (D33) is available in the 136-162 MHz range. Even so, I managed to adjust the radio's output to 50 watts for comparison purposes. I completely removed the metal tuning slugs from the VCO coils to get the tuning voltages as high as possible. I programmed in the appropriate test frequencies and measured the sensitivity and output power in the radio's new range and added the results to the table above. I encountered problems trying to get 50 watts out of the radio; this is evident in the recorded data. I readjusted the transmitter output power and aligned the PA at 25 watts and the radio operated properly in the 136-162 MHz range.
There were no software-related issues. The various options chosen during initialization allowed programming of any frequency between 136 and 162 MHz.
The radio lost some sensitivity below 146 MHz, due to the tuning of the receiver front-end coils. Ferrous metal screws could be added to these coils to improve the sensitivity at the lower frequencies (brass screws will raise the operating frequency). It would not receive at 138 MHz or below, because this was too low for the VCO; the only fix for this is to change components or add a small value padding capacitor to the VCO circuit.
The radio model parameters wanted it to be initialized as a 25-watt model, but I initially attempted to force it to run at 50 watts. I'm sure the radio would have been capable of this much power, but the microprocessor sensed excessive current during transmit (it knew I had initialized it as a 25-watt radio and here I was trying to get 50 watts out of it) so it cut back on the output power. It was not willing to make this much power below 146 MHz. I subsequently adjusted the radio for 25 watts and it worked a lot better in the 136-162 MHz range.
I considered hex-editing the MDF file to try to initialize the radio to run at the 45-watt power level, but I decided it wasn't worth the effort. You can push the 25-watt radio up to 30-35 watts and it will still be stable there. The radio will operate very nicely for APRS purposes.
Acknowledgements and Credits:
MaxTrac, Radius, RSS, and a whole slew of other terms are trademarks of Motorola, Inc.
Specifications and circuit information came from the MaxTrac Detailed Service Manual, no longer in print.
Thanks go to Scott KB0NLY for sharing some of his conversion tips with us.
The author can be contacted at: his-callsign [ at ] comcast [ dot ] net.
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This page originally posted on Sunday 01-Aug-2010
Article text, artistic layout, and hand-coded HTML © Copyright 2010 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.