maxtrac as a very low power exciter

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W2IBC
Posts: 19
Joined: Fri Jun 14, 2013 4:29 pm
What radios do you own?: Maxtrac,gm300,msf5000

maxtrac as a very low power exciter

Post by W2IBC »

Planning a 440 machine. Thinking for the time being using a maxtrac for RX and another maxtrac for and exciter.
I can get ahold of some GE Master II PA's pretty cheap. Now the GE's IIRC use about 250mw in the exciters to drive the PA

Will it be possible to get a maxtrac down that low? and if I can what strange problems may show up?
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Bill_G
Posts: 3087
Joined: Thu Sep 17, 2009 5:00 am

Re: maxtrac as a very low power exciter

Post by Bill_G »

According to the Maxtrac UHF manual, the RF board puts out +17dbm (50mW). So, you will need the next low power section of the rfpa to drive your GE amps. With the proper matching, I imagine it's possible to get the controlled stage to reliably drive your amps.
W2IBC
Posts: 19
Joined: Fri Jun 14, 2013 4:29 pm
What radios do you own?: Maxtrac,gm300,msf5000

Re: maxtrac as a very low power exciter

Post by W2IBC »

Hmm, the joys of having to build a repeater with a super small budget eh?

I got a extra maxtrac PA laying over here. might have to give it a shot see if we can produce stable output at 250mW

course still wondering if its even worth the trouble.
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KI4M
Posts: 158
Joined: Mon May 01, 2006 8:08 pm

Re: maxtrac as a very low power exciter

Post by KI4M »

I would suggest checking the spectral purity of the radio you will be using for the transmitter. I would check it at full rated power, half power, and then at your desired power. I think you will find that some Maxtracs will become spurious when ran at less than half rated output power. If that is the case the easy work around is attenuation and run it at a power level it is happy at on the spec an. to meet the input requirements for your amplifier.

Also you might look for Micor or MSR-2000 amplifiers. The Micor amplifiers will require an external lpf on them as the way they are from the factory they use an external "antenna network" that has a circulator/lpf all in one nice little compact piece. The beauty of those amplifiers is they are 50 ohm stages. You simply figure out which stage input most closely matches the output of your Maxtrac and drive it from there. I recall you have to pull the DC off but it's been years since I did one of those. I bet if you search around you will find a Micor compastation with all the parts you need free for the taking. That way you could have a nice 15V unregulated supply for the amplifier and the antenna network mentioned above as well.
W2IBC
Posts: 19
Joined: Fri Jun 14, 2013 4:29 pm
What radios do you own?: Maxtrac,gm300,msf5000

Re: maxtrac as a very low power exciter

Post by W2IBC »

the trac going spurious is something that bothers me.
Big issue for us with running the maxtrac at full. is duty cycle.. we can keep the machine keyed up for long periods (vhf machine can be keyed for hours strait.) so I was thinking MAYBE it could handle on just the driver.. and using an amp to get the big output.

Course instead of micky mousing around, I would much rather just find a UHF MSR-2000 cheap locally how ever no such luck on that.
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KI4M
Posts: 158
Joined: Mon May 01, 2006 8:08 pm

Re: maxtrac as a very low power exciter

Post by KI4M »

You can cut the Maxtrac down to 3 dB under full power, remove the plastic covers over the top/bottom (leave all metal shielding in place) and place a fan on the heatsink to blow all the time. Those crazy radios get hot as heck at full power or 3 dB down etc. I would give it a try and see what happens. I am not a fan of MSR-2000 amplifiers unless you can find them on a continuous duty heatsink. The only UHF stations I have ever seen had the ICS heatsink on them. Even with forced air cooling they get very hot. The harmonic filter substrate in them is prone to separation as well if they get hot so keep that in mind. The Micor amplifiers aren't that prone to failure in my experience.

I found a MSF-5K CXB last week here for free and snagged it up. If you know anyone in the commercial two way business ask them what they have in the back of the shop leftover from narrowband conversion. You might be surprised what you can get for either cheap $$ wise or for free for hauling it off. Never hurts to ask and I have seen a bunch of nice stuff go to the dumpster because no one was there to haul it off.
W2IBC
Posts: 19
Joined: Fri Jun 14, 2013 4:29 pm
What radios do you own?: Maxtrac,gm300,msf5000

Re: maxtrac as a very low power exciter

Post by W2IBC »

well looking around I nabbed a GE Mastr II 110w UHF machine pretty cheap. with the GE power supply. and some moto t1504a uhf duplexers. wasn't free but worth the price I paid IMO. (shipping is just what killed me)
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