Maxtrac - Continuous Duty Rating?

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W6JK
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Maxtrac - Continuous Duty Rating?

Post by W6JK »

Is there any concensus as to how much power a Maxtrac repeater can safely run, whether D27 or D37?

Jeff
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Post by kcbooboo »

I used to run D34 and D44 MaxTracs on a UHF repeater. I tried rated power, half rated power, and about 1/3rd rated power. They get hot, very hot, no matter what amount of power they're making. A fan moving lots of air over the heatsink helps a lot. I eventually ended up with a D34 (25w radio) running about 7 watts into an outboard power amp. The radio stays cool, but the algorithm inside still thinks it should be getting hot after 45-60 minutes of continuous transmitting. Some radios shut down sooner. This is with the transmit timeout set to 000. You can't set the power too low because the transmitters start putting out spurious emissions.

The 900 MHz MaxTracs use an RF power amp module that's supposed to be good for 18-20 watts. The 30w radio follows this with a single transistor to bump the power by 3-6dB. All the components are well mounted to the heatsink, but the radios are still not rated for continuous duty. I wouldn't run the radio at anything higher than their rated power: 12 or 30 watts. By the way, the D37 is rated for 30 watts on 896-902 MHz but only 20 watts on 935-941 MHz, so for amateur repeater use, you don't want to go over about 20 watts.

I had a D27 on my 900 MHz repeater and when set to 12 watts with a fan blowing on it, it stays comfortable. Your mileage will vary. Even at that power setting, the radios draw about 4.5 amps at 14 volts, so you have to get rid of an awful lot of heat (about 45-50 watts-worth). I just switched to a 300 watt continuous-duty Nucleus transmitter, and that has plenty of air flow and heatsink fins and does NOT get hot, especially when running at 130 watts.

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Post by flecom »

if anything why not just disconnect the PA in the maxtrac and run the radio straight into an external pa that is rated for continuous duty?
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Post by RESCUE161 »

That's what I've been wondering.
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W6JK
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Power control

Post by W6JK »

Bob, did you use the hardware power control mod on those radios, or did you find it unnecessary?

As to the alternate PA question, I don't know what they're doing with the unloved Nuclii out here, but I'd be happy to haul one away from somewhere and put it to use.

The actual Plan B is to run an 800 MHz Micor, already obtained. However, it's clear that the Maxtracs will be on the air much sooner!

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Post by kcbooboo »

Yes, I had to use manual power control. The one I was given would put out 19-20 watts on frequencies in the 927-928 MHz range. In fact, the one I'm using was the basis for an article about manual power control over on repeater-builder.com.

The MaxTracs are a quick and dirty way to get something on the air, and with a big (4-3/4 inch) fan keeping it cool, it seems to run quite nicely.

Now that I've increased my transmitter power by a factor of 10, the repeater has a mouth that's bigger than its ears, even with a preamp on the receiver. There's just too much other stuff around 902 MHz that kills weak signals. A GTX portable (about 3 watts) might be noisy getting into the repeater, but it hears the repeater solidly.

I also have some TOKO 915 MHz filters that replace the ones in the 800 MHz MaxTracs, should you want to convert one to be a 900 MHz receiver. No reason to waste a 900 MHz MaxTrac unless you want to run HearClear on the repeater. These are a lot cheaper to find, and they work great, even though they're wide-band (as compared to a 900 MHz radio).

Bob M.
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11K0

Post by W6JK »

I do want to experiment with HearClear. In fact, if one of these radios has problems, I'm thinking about trying its module in the Micor instead of trying to fix it.

I'm thinking the 800 radios are better suited to user use anyway. The band is supposedly filling up out here, and the coordinator requires us to observe the 11K0 emission standard, so it's desirable to have a receiver appropriate to that standard. This way, users running excess deviation will be more likely to fix it. I'm sure things are looser elsewhere, but this is the world I'm working in.
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Post by W6JK »

kcbooboo wrote:Yes, I had to use manual power control.
Does that apply to the UHF radio also? I was asking a broader question.

'JK
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Post by kcbooboo »

The MaxTrac/Radius I'm using on UHF controls its output power properly. It just thinks the radio is getting hot after 45 minutes of continuous transmit, and cuts the power back until it reaches zero.

The manual power control was only necessary for 900 MHz radios (so far).

Bob M.
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Post by W6JK »

OK, that's what I thought. Since there are no relays to protect, I figure I can use a short carrier tail and let the transmitter drop between user transmissions. It's not what people are used to, but it should fool the radio. That. and a fan to keep things cool, ought to do it.
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Post by kcbooboo »

Yup, that's about all you can do for now, until someone figures out how to disable the shutdown algorithm inside the radio.

Once it thinks it's hot enough and shuts off the output power, you can get maybe a minute of transmitting after the PTT line shuts off (i.e. the repeater controller releases it). Several seconds of dead time gets you minutes of transmitting time. It's still a royal P.I.T.A. but until you get something better, all you can do is live with it.

Eventually I obtained an MSF5000 station and turned it into a repeater. No more timeouts, plus it's rated for continuous duty cycle. I added a fan to it just to stir up the air.

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Post by flecom »

OR

you could just disconnect the PA on the maxtrac and run the 10mW exciter power into something like a MASTR III PA that uses a 10mW drive anyway for 110w out, and that is rated for continious duty
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Post by W6JK »

That's a nice way to make lots o' watts, but it doesn't keep the Maxtrac from shutting down when it thinks it must be too hot.
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Post by flecom »

W6JK wrote:That's a nice way to make lots o' watts, but it doesn't keep the Maxtrac from shutting down when it thinks it must be too hot.
ah so the radio just thinks its getting too hot... hrmm, i wonder how effective it would be to just make a circuit to unkey the radio for 1/100th of second every min or if something like that would be enough to trick it into being happy?

how long does it take for the radio to think its over heating?
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Post by W6JK »

Bob sez 45 minutes. I haven't had it happen here because I'm using a short carrier tail. The repeater usually drops carrier between user transmissions.
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Post by kcbooboo »

45-60 minutes of continuous transmitting. My repeaters have a courtesy beep 1 second after loss of input carrier, and a transmitter hang time of 5 seconds after that, so with a good technical QSO going on during drive time, we can get 45 minutes of transmitter ON-time.

I did have a fan blowing on the power amp, and the radio was running at around 7 watts output and was ice cold.

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Post by flecom »

i would get rid of the hang time, 5 seconds is reallllly long, and i dont think there are any realys to wear out in the maxtrac anyway so i dont think it would hurt anything to drop it...
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