Batboard

Please forgive me if this is posted in the wrong section, but I couldn't find a section to put it in that seems to fit the need.

I have a 45 watt maxtrac 300 (which I believe was originally a radius 45w). I think it may have been blanked and reinitialized as a Maxtrac300 High Sig. I plan on using it as an echolink link to a repeater. I wanted to restrict the output power to around 10 watts. The repeater I need to hit is fairly close. I went through the PA settings, where you can calibrate the output power across 16 frequency settings, and set it for as close to 10w as I could at all 16 points.

Last night I had the idea of checking current draw at 10 watts output power. I was curious what I could get away with as far as a power supply goes, and wanted to see how many amps the radio was drawing on TX. To my surprise, it was drawing 7.2 peak and 7.0 stable amps at TX on 146.175. This is within the bandsplit of the radio as it is a 146-175 bandsplit rig. So, I did some further testing and found that as the frequency moved up, all the way to 174, the current drain dropped. At 174mhz and 10w RF output, its drawing around 3.5 amps.

This seems odd to me, given a 25w maxtrac I have puts out 3.5 amps at 14w RF output. Is there an adjustment I can check? I was thinking possibly the VCO adjustment may be out of whack (previously modified) and need bringing back into "sanity" so to speak. I'm kind of at a loss here, but I want to make sure its operating efficiently and not generating extra current draw that isn't doing anything useful. Thanks in advance, and I'm sorry if I got this in the wrong place, but I figured "Legacy radio discussion" would be a good place to start.

This is a peculiar anomaly to the Maxtrac/Radius (GM300) style radios. I can't explain why it does this, but I have seen the same thing when working with GM300's. Additionally, I have been told these radios may tend to get a bit more "dirty" as you throttle back the power.

Andy

Andy Corbin wrote:This is a peculiar anomaly to the Maxtrac/Radius (GM300) style radios. I can't explain why it does this, but I have seen the same thing when working with GM300's. Additionally, I have been told these radios may tend to get a bit more "dirty" as you throttle back the power.

Andy

When using solid state PA stages in a transmitter, they tend to get dirty as you lower the output power. This is just the way most of the solid state amps work. As a result of this, it's a good idea to put the radio on a spectrum analyzer and see if it has become a comb generator. If you look up the specs for the model radio you have, it will show a low power and a high power setting. It is best not to go below the low power setting the radio is made for.

Also bear in mind that the duty cycle of the packet operation could exceed the rating of the radio. This will depend on activity and what your doing for sending messages. Even while receiving messages, your transmitter will send a burst of info saying it received the packet OK. This high duty will heat up both the heat sink and the driver transistor. You can always put a fan blowing on the TX heat sink, but the driver transistor will probably need a heat sink added to it. I believe the driver is out in the open on those radios and can have a slip on metal heat sink placed on it to help cool it. You might have to tweak the tuning of the radio to account for the changes you have made.

Don't forget to look at the TX signal on a spectrum analyzer when your all done. In some cases, the hash and off frequency signals can be larger than your on frequency signal.

To get back to one of your questions, the VHF TX power needed for a 25 watt transmitter is 9.5 amps. For a 45 watt VHF model, it is 15 amps. Your power supply needs to be above this current rating so you don't overload the power supply during transmitting.

I would not try to lower the transmitter output below what the low power setting that comes from the factory.

Note the 40/45 watt models have a lower limit of 20-25 watts. Below that they can be unstable/spurious. The 25-watt models had a lower limit of 10 watts.

You're operating the radio outside the design spec, so things like high current draw can be expected.