Maxtrac VHF High Current Draw on Ham Freqs below 146MHz

snooozn · Post by **snooozn** » Sun Jan 20, 2002 10:32 pm

Hello -
I have two D43MJA7DA5CK Maxtrac radios. When transmitting below 146.000 MHz say 144.700 MHz
the current draw goes way up to about 14 amps at 45 to 50 watts RF out. When the same radios transmit at say 151.625 (or Ham freqs above 146.000 MHz)with 45 watts out, the current is about 10 amps which is what I would expect it to be. Anyone know how to get these radios to transmit on Ham and Business freqs at the same time without drawing 14 amps? I'm worried about the extra input power dissipation. 13.8 volts at 10 amps is 138 watts. 13.8 volts at 14 amps is 193.2 watts!! I've also heard the VHF Maratracs do the same thing?
Thanks.

I'm using a 50 ohm Bird terminated wattmeter good to 1 GHz. The radio does put out 50 watts when drawing 14 amps on the ham freqs below 146.000 and draws 10 amps when outputing 45 watts in the business band. 14 amps to get 5 more watts is too much.

The first frequency that the RSS sets the radio to when in alignment mode is 146.000MHz. The radio is set for 45 watts out 146.000 and draws about 10 amps. But as soon as you drop down to 144.xxx the output goes to 50 watts and the input current goes to 14 amps.

[ This Message was edited by: snooozn on 2002-01-22 01:03 ]

[ This Message was edited by: snooozn on 2002-01-22 23:57 ]

Nand · Post by **Nand** » Mon Jan 21, 2002 3:07 pm

Tom in D.C. · Post by **Tom in D.C.** » Mon Jan 21, 2002 7:37 pm

The final amplifier's output circuit is obviously not resonant, or anywhere NEAR
resonant, with the condition you describe.
Back when amplifiers required tuning, you would tune the final for a dip in plate current and then adjust the antenna tuning circuit for maximum current, then go back and re-dip the amplifier, then repeat the tuning of the antenna, etc.

Today, with the no-tune radios you don't have to go through all this stuff, but when you go outside the passband that the radio is designed to handle the result is all kinds of crazy things that can happen, including high plate current.

Tom, W2NJS
...in D.C.

wazzzzzzzzup · Post by **wazzzzzzzzup** » Mon Jan 21, 2002 7:43 pm

when you are looking at the current draw at 144.XXX and 151.XXX do you notice higher wattage output when higher current is drawn? i know when i use an antenna tuner on my ham radios, when i lower the SWR, it increases the current draw and as a result increases the wattage output. mabe your maxtrac finals are tuned more tward the lower part of the band, mabe in the 136-150Mhz area so you draw more power and put out more RF power.
just an idea to think about.

Monty · Post by **Monty** » Tue Jan 22, 2002 1:05 am

Hi:

As you have discovered, the Maxtrac Series
was not really Broad Banded below 146.00 Mhz
very well.

You may find all kinds of transmitt problems.

Also, you should insure your " Antenna" is
properly cut for the freq range you need to
cover....Sometimes Refelected Power can
be a problem as well.....

With a Antenna Properly set up, you should have no more than 1 watt refelected for 25-40 watts forward.

All you have to do is go to the Alighnment
Section of the RSS where all the Softpot values are located.

In VHF Maxtracs, the Soft Pot values are
pretty sensitive, so start from the low
side and work up. You also will have to
address the Dev settings as well...

From there, hex edit your freqs or if you
have the other tools, you can use that to.

some persons over look this issue when
making mods to their radios, and its not
the radios fault for sounding horrible if the
alighnment it not properly set up.

Monty

Post by **wavetar** » Tue Jan 22, 2002 3:58 am

The same problem can occur even within the frequency limits if you try & squeeze too much power out of the P.A. Current starts climbing with little or no increase in RF power past a certain point.

You didn't mention whether or not you had tried an alignment. I have found that when you go out of the rated frequencies, the current drain goes up, with no increase in RF power output compared to frequencies within the band. But, I have also found that you can lower the current drain significantly just by lowering the RF power a couple of watts.

Todd

[ This Message was edited by: wavetar on 2002-01-22 07:04 ]

Will · Post by **Will** » Wed Jan 23, 2002 10:43 am

The microprocessor system WILL NOT set the transmitt parimeters correctly for anything outside of the 146mhz. When a channel that is lower than 146mhz is dialed in by the microprocessor, the settings for the transmitter power control and deviation are not set, causing the transmitter to try to go to full current and the deviation, modulation, goes way off.

I have made some changes in Maxtrac/Radius' so they would work out of band but it is very involved and if you also have channels that are inside the proper band limits they wind up getting set lower power and the deviation goes up too far.

Nand · Post by **Nand** » Wed Jan 23, 2002 4:23 pm