Maxtrac 50-54MHz (6 Meters) Amateur Radio Conversion

50-54MHz (6 Meters) Amateur Radio Conversion
You've got a Maxtrac/Radius that you want to tune up on 6m...


6m Maxtrac Modifications

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Ok, so you've managed to get some 6m frequencies programmed into your 60W 42-50 MHz Maxtrac/Radius, but you are having problems with the TX and RX locking up and working properly. Well, try the following fixes:

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Easy Fix
First, check the TX and RX VCO steering lines to make sure they swing between 2 and 8 VDC. If they don't, read on.
On the RF board under a cover are the VCOs for TX and RX. You should see a test point on the board marked SL. This is in the middle of a bunch of capacitors C109,C110,C111, C127.
This is the VCO steering line. While monitoring this point with a voltmeter, adjust L202 in the RX VCO section for around 7.8 VDC. Likewise, with the radio keyed, adjust L213 in the TX VCO section for 7.8 VDC (it helps to have a manual). The radio should work on six meters now although the sensitivity of the RX will be somewhat reduced because the front end of the radio falls off above 50 mHz.

If you are still not satisfied with the performance of the radio, you need:

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The Hard Fix
TX:

• Program the radio with some 6 Meter channels. Tweak the TX VCO on the 6 Meter channel so that you get a lock (see above). TX power will be high on the key up but will drop to about 25 Watts.
• Open the covers on the PA. There is an inverted "U" loading coil in the PA. The coil or hairpin is adjacent to the final power transistor. The coil is made of 12 ga. wire.
• Flow solder on the top and sides of the U.
• Check power out.

By experimenting with solder, power should come up to full rated power.
RX:

• There is a row of 9 coils without slugs on the rf board. Place non-ferrous brass or aluminum set screws in the cans.
  If you are unable to find Brass or Aluminum set screws all is not lost. Go to your local welding supply shop and get a piece of 1/8th inch brazing rod. Cut 9, 1/2 inch, pieces from the rod. De-burr the cut ends. You will find that the brazing rod is a very tight fit in the coil forms. To fix this, carefully, thread a 6-32 machine screw in and out of each of the coils. DO NOT bottom the screw against the circuit board as you can rip the coil right off the board. This procedure will allow a friction fit between the rod and the coil form. Now insert the rods half way into each coil. Using a pair of small pliers, while checking SINAD, move the rods in or out for best quieting. If you have a marine store handy you can probably find 1/2 inch, 6-32 brass machine screws which will work and save all of the above effort.
• Tune the cans for best RX sensitivity.

If this still doesn't work, or you want better performance, keep reading:

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The Better Fix
TX:
In working on the 6 meter conversion of a Maxtrac mobile, a few small wrinkles may appear when tuning the PA section.
When attempting to flow solder of the collector loop, the radio actually began to loose power as solder was added. The radio started out stock at 52.450MHz with 19.2W output into a Motorola R2001D service monitor.
While experimenting with tuning the hairpin loop, there was no real change in output. A new handmade loop of #12 solid copper wire, constructed with a height of 1.0" above the surface of the board was made. Then another hairpin was constructed to slide up and down on the loop, and dramatic increase in power out and a drop in collector current was observed. With the optimum location found, the second hairpin was soldered to the first, and the top part trimmed off.
The tuning process continued with the other coils by spreading the turns out and observing the output power and control voltage.
The results of the coil spread method were:

L2144 spread by an additional 2MM

L2160 spread by an additional "wire thickness" this coil is not critical

L2161 Spread by 2.4MM and observe the collector current. One should first program within the specified band limits and measure the input current while observing the output and control voltages, this coil seems to have a much larger effect on the PA tuning.

L2162 Spread by about 1.5 wire widths. Use a wooden stick to spread this coil for max output but maintaining the proper collector current.

L2141 and L2142 are very critical adjustments. Align the transmitter using RSS to properly adjust the TX power softpot at the specified frequency within the RSS, toggle the PTT and spread these coils while observing a known good wattmeter but be warned, if these coils are spread out and the power jumps way up its not real power, its a spur and will kill the pin diode CR2151 and literally burn CR2150 off the board.

With care one can get a full 60 watts from the transmitter without overloading the driver and PA transistors.
RX:
Change the value of R4 (HLB4101 board), this will be a 2.7K chip resistor and the function is bias for Q1. Change down to a 1K resistor or less.
This will not help sensitivity as it will selectivity. Pull C2 by the antenna port and replace with a 680pF. Pull add a small trim cap across C1 in parallel to tune the antenna. Also place a trimcap across C11 and add the tuning slugs from a dead 450MHz Maxtrac, except L4 and L5, these should be ferrite core trimmers.
Tuning will be the same procedures however L4 and L5 will become much sharper in response. Center these out for taste.

Last, add 2 1N34s across the antenna input to ground coupled through 22pF caps to protect the front end from overload.