UHF GM300 In Repeater Service - Blown Finals

railtrailbiker · Post by **railtrailbiker** » Mon Oct 02, 2006 2:49 pm

I've been using high power UHF GM300's for several years to drive an after-market PA in a repeater set up. I've always reduced the radios' power output well below their rated 40 watts; typcially in the 20-25 watt range. I also have a small box fan directed on the heat sink 24/7. Despite the power reduction and constant cooling, I've blown the finals on 3 radios in 15 years.

Today, a tech told me that regardless of the fan, I shouldn't have the radio set to any higher than 15 watts if it's used in repeater service.

Comments? Should I cut back to 15 watts, or have I just had a string of bad luck with finals?

Thanks.

Post by **wavetar** » Mon Oct 02, 2006 2:56 pm

Depending on the duty cycle of the repeater, even cutting it down to 15 may not eliminate the problem. Mobiles are generally only made to work with no higher than approx 25% duty cycle. Even most repeaters are made for a 50% duty cycle at full power. You may find the GM300 will generate just as much heat at 15 watts as it does at 25, as the finals are quite inefficient. Won't hurt to try, just expect to replace them every so many years if the duty cycle is high.

Todd

Bruce1807 · Post by **Bruce1807** » Mon Oct 02, 2006 2:56 pm

5 years for a set of finals?
sound good to me for a radio that was never really designed for a repeater,
That said reducing your power out will certainly put less stress on the finals

MotoMax300 · Post by **MotoMax300** » Mon Oct 02, 2006 2:57 pm

I'd use 15watt maxtrac/gm300's .... I wouldnt try to lower the gm300/maxtrac 45watt that low, just unstable in my opionion.

jim · Post by **jim** » Mon Oct 02, 2006 6:33 pm

Keep both case halves removed.
Place 5" muffin fan to blow directly on the PC board in conjunstion with your existing heat sink fan.

Just like a computer, the processor (ie; final) has a fan on it and the main case (ie; radio body) has a fan to cool it.

What's the duplexer tuning like? Is there any reflected power back to the radio?

Vercomm · Post by **Vercomm** » Mon Oct 02, 2006 6:46 pm

I have used that same setup, but I use Glenayre Power Amps which only require 4watts input. I have never had a GM300 blow. You're PA probably won't let you go that low. As the previous posts have said lower power as much as you can and keep the fans going!! Also a good idea to check your filter system to make sure you don't have reflected power!

Good Luck!

jackhackett · Post by **jackhackett** » Mon Oct 02, 2006 10:48 pm

I've rarely seen blown finals in any of the radius series used in a repeater, usually the solder joints get overheated and resoldering them brings them back up to snuff.

There was some discussion here a while back about using a tuner between the tx radio and duplexer (or in your case PA) to keep the final cooler, so that's something you could look into. Another thing you could try is see if you can get the heat sink from a GR1225, it's got bigger fins and I believe will fit the GM300 PA board.

If I was going to build a repeater with a radius I think what I would do is build a custom heatsink, mount the PA board on a piece of copper stock and then mount that to some finned heat sink material with a fan, maybe an old Slot-1 CPU heatsink (and of course use some good thermal compound).
You could build a box around the PA board with copper clad board for shielding and mounting.

The copper stock should absorb and spread the heat from the final faster than the cast aluminum stock heatsink. I think that's the main problem with the Radius PAs, they don't get the heat from the transistor to the cooling fins fast enough, so no amount of fans are going to help, if you've ever felt one you'll see the transistor gets pretty hot way before the fins even get warm.

I've never had the opportunity to try it, so of course it's all theoretical, could be an interesting project though.

tvsjr · Post by **tvsjr** » Mon Oct 02, 2006 11:28 pm

Heat's your enemy. You need a few things... spectrum analyzer to watch the output spectral quality, an ammeter to look at current draw, and a wattmeter/dummy load. Key the radio with RSS and back the power down slowly. You should find some point where the power keeps dropping but the current doesn't. As long as you aren't seeing nasty spurs, that's where you leave the power.

That being said, using a mobile radio is a Bad Idea. I've taken part in several hunts for transmitters (usually paging) generating nasty spurs and wiping out public safety communications... more than one has been a PA being driven by a Maxtrac/GM300, including one particularly nasty one which was sweeping from about 220 all the way to 931 when the transmitter would key. Obviously, this can get you in trouble with your friends at the FCC.

You should consider grabbing something designed for continuous duty... MTR2000, Quantar, or maybe even an evilbad Kenwood TKR-750/850. The TKR can be had for $1K, is synthesized, has a great front end, and is rated for continuous duty ops at 25 watts (no issue backing power down... mine runs at 5 watts quite well to drive an outboard 150-watt PA). One of these units would be the Right Way to do things.

railtrailbiker · Post by **railtrailbiker** » Tue Oct 03, 2006 7:11 am

Thanks for all of the suggestions thus far.

2 questions:

1) Does anyone know approximately how much it will cost to replace the finals?

2) Isn't there an option in the GM1225 for a heavy duty final if you plan to use the radio in a GR1225 repeater set up? I'm wondering if there is a similar heavy duty final available for the GM300. Any idea?

Thanks.

stay-con · Post by **stay-con** » Tue Oct 03, 2006 7:50 am

jackhackett wrote:I've rarely seen blown finals in any of the radius series used in a repeater, usually the solder joints get overheated and resoldering them brings them back up to snuff.

Using 2% silver solder improves their survivablity considerablly.

Jeff

Post by **wavetar** » Tue Oct 03, 2006 8:26 am

railtrailbiker wrote:
2) Isn't there an option in the GM1225 for a heavy duty final if you plan to use the radio in a GR1225 repeater set up? I'm wondering if there is a similar heavy duty final available for the GM300. Any idea?

Thanks.

Not really...the GR1225 final is very similar to the GM300, but the heatsink is larger. Combine that with the fan available in the housing options, and Motorola states it's good for 50% duty cycle. Even then, as jackhackett pointed out, the solder will dry out & cause problems after time. Silver solder as suggested by stay-con does help out immensly in that regard.

There is a "100 watt" option for the GR1225, but that just means you get the 10-watt GR1225, and it connects to a 100-watt external amplifier. Been there, done that...I'd much rather go with an MTR2000 in those cases.

Todd

Will · Post by **Will** » Tue Oct 03, 2006 12:35 pm

Many years ago we changed the heatsink casting to the one used on the R1225 hi power radio. The larger heatsink will work with the GM300 power amp board.

Another 'trick' we have used is to bypass the antenna switch diodes on the TX.

When using the 'afterburner' power amplifier, use a 1 to 10 watt LPI Maxtrac/Radius power amplifier/heatsink assy. These run nicely at 2 to 10 watts output.

jackhackett · Post by **jackhackett** » Tue Oct 03, 2006 5:55 pm

[quote="wavetar"]
Not really...the GR1225 final is very similar to the GM300, but the heatsink is larger. Combine that with the fan available in the housing options, and Motorola states it's good for 50% duty cycle. Even then, as jackhackett pointed out, the solder will dry out & cause problems after time. Silver solder as suggested by stay-con does help out immensly in that regard.[/quote]

I know of one GR1225 that's being used in much heavier service than it should be (24/7 with a high duty cycle) and about once a year it quits, I open it up and resolder the transistor and it works for another year, I think i've done it 3 or 4 times now. Only parts I've had to replace were the ceramic caps once. Those transistors are tough. (and yes, I use silver solder)