Motorola GM300 as repeater revisited

[Josh]

People have discussed repeatedly on how the GM300 makes a bad repeater transmitter due to the small heat-sink, etc.

My question is, has anyone attempted to replace the highly inefficient power transistor with another one that is more efficient?

Instead of passing half the amperage as heat, a more efficient power transistor would consume less power getting the radio's rated power and also distribute less heat.

If anyone has done this, tried it, or has insight and part numbers for a better replacement part, let me know. I would love to try it on UHF.

The best specs I see are like 65% drain efficiency.... isn't that what the GM300 already delivers?

-Josh

[HLA]

i guess it just depends on it's environment and conditions because i used a gm300 for transmit and recieve for years with no problems. last year i swapped them out with some cdm's so i could convert it to narrow band but they still work great.

[Josh]

i guess it just depends on it's environment and conditions because i used a gm300 for transmit and recieve for years with no problems. last year i swapped them out with some cdm's so i could convert it to narrow band but they still work great.

I've owned and used GM300 radios for about 6 years or so now, and know they're good for intermittent duty tx as base or mobile units, but here I'm speaking specifically about repeater duty, where the radio acts as a transmitter and therefore TX's for quite a while during a conversation.

-Josh

[Grog]

Just a hunch, but I bet he is talking about a repeater as well. Hence the plural usage......

last year i swapped them out with some cdm's so i could convert it to narrow band but they still work great.

[Josh]

Well anyhow, the radio I'm going to use I will have to tune down to 25 watts, and have a temperature controlled fan (4.5") sucking air away from it. I guess we'll see how that goes.

It seems like the MTR2000 has an awesomely efficient PA, since somewhere I remember reading that the 110 watt model runs off of like 13 amps, and the 40 11amps, I may have to recheck that though.

-Josh

[Al]

The draw on an MTR PA may be 13 amps, but remember that's at 28 VDC vice 13.6 VDC for the mobile radios. The efficiency on class C PAs all run fairly close, and the mobiles operate class C just like the MTR PA. If I remember, Will has had some success using larger heat sinks on GM300 type radios, allowing them to be used for longer transmit times without heat destroying the PA transistor or the builtin thermistor forcing power cutback.

[wavetar]

From the factory, the 45-watt version at full power was rated for a 20% duty cycle. This is because it's a mobile radio shoved into a repeater housing with no modifications whatsoever. I have seen Will post that he's used the much larger GR1225 heatsink in combination with a fan with good results on higher duty cycles.

According to the specs straight from Motorola, cutting the power back on the high powered units to 25-watts allowed for a 100% duty cycle...I think most of us would have to disagree with that.

Todd