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Low Power MaxTrac 900

This forum is for the discussions targeted at converting various models of Motorola equipment to operate in the 900MHz Amateur Band.

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Low Power MaxTrac 900

Postby Jonathan KC8RYW » Tue Jul 22, 2008 10:21 pm

I'm looking to make the MaxTrac 900 12W into something more reliable with lower power (without worrying about a PA burning up) for a repeater transmitter.

The VHF and UHF LPI Maxtracs just lack the last section of the PA.

But the Maxtrac 900 PA has the last three sections together in the same RF hybrid module.

The MaxTrac 900 RF board outputs about 200 mW according to the service manual.

How about just bypassing the RF module (MHW820-3???) at the output of Q3001 (MRF599??) on the 15W PA board?

What about just bypassing Q3001 and feeding the RF direct from the RF board to the RF module (5180110E03)?
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Re: Low Power MaxTrac 900

Postby kcbooboo » Thu Jul 24, 2008 2:30 am

The Spectra PA is very similar to the MaxTrac PA in that they use a driver transistor to feed an RF brick. The Spectra service manual says that just about any point you'd use to troubleshoot, is 50 ohms. They tell you what to disconnect and measure to check each stage.

I don't think that bypassing the driver transistor will make the RF brick put out lower power. It's a class-C device and needs some minimum amount of drive before it will produce any power.

Probably the simplest way would be to disconnect the RF brick and use the driver output to feed your external PA. Adjusting the power level may be a problem, because the radios normally fiddle with a control voltage in the RF brick to do that.

I don't know what mechanism the 2-4w MaxTracs use to control RF power in the PA. I've never looked into it.

Bob M.

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