Batboard

I have searched and have a good idea that's what's happening, but I need some confirmation. M44 ham band, running at 30 watts. After about 30 minutes of constant transmit, the audio starts popping and drops off. The radio is still transmitting and my power meter says 100 watts. After another 20 seconds it stops transmitting. I took the temp of the pa and got a reading of 155 degrees F. I do have a small 6" desk fan blowing right on the pa. Once it cools off it's good to go for another 30 minutes.

With all do respect, please don't tell me that gm300s shouldn't be used like this. I know I am asking a lot from the radio and overworking it. I just need to know if this is typical overheat behavior.

Opinions wanted.

OK... There's a bit of a disconnect here that's not quite clear. For one, you say that the power meter says 100 watts at one point. Is there an external PA that this is driving?
Second of all... GM300s shouldn't be used like this. Sorry, but, you have to expect that it's going to blow up on you if you drop a brick on the PTT switch for a half-an-hour. The heatsink on this radio is in NO WAY suitable for continuous duty transmission! If you absolutely must use this GM300 at continuous duty, you're going to have to mount its PA transistor to a massive heatsink, which will require heavy modification to the radio. A "small 6' desk fan" won't cut it.
If this radio is not connected to an external PA, and you're seeing 100 watts out of it, either your power meter is screwed up, or the power control circuitry in the radio is overheating and causing the power amplifier to run away in power output.

If you want to start a good campfire, this is a good way to do it If you want to communicate via the radio, this *probably* isn't the best way to do it.

No, that is not typical. Usually they go down in flames as the solder melts at the tabs on the finals. In my opinion, you have been very lucky.

Well now I am backing the power down. I started at 78, which is where it was when I got it. 73 got me to 30 watts, but the longer the ptt is pressed the higher the watts go. Very strange, they are supposed to back down. Anyway, took it down to 40 which is 5 watts and it is still raising although slower, possibly because less heat.

Does this make sense to anyone?

w1dth wrote:I have searched and have a good idea that's what's happening, but I need some confirmation. M44 ham band, running at 30 watts. After about 30 minutes of constant transmit, the audio starts popping and drops off. The radio is still transmitting and my power meter says 100 watts. After another 20 seconds it stops transmitting. I took the temp of the pa and got a reading of 155 degrees F. I do have a small 6" desk fan blowing right on the pa. Once it cools off it's good to go for another 30 minutes.

With all do respect, please don't tell me that gm300s shouldn't be used like this. I know I am asking a lot from the radio and overworking it. I just need to know if this is typical overheat behavior.

Opinions wanted.

You have me some what confused by saying your running 30 watts and then you say the power meter reads 100 watts. The GM300 in no way can produce 100 watts.

The second point I would like to make is that the radio was designed to be in transmit for only 5% of the time. It does not have any way the required heat sink design to operate for the duration your telling us about. You might get away with longer transmissions by reducing the output of the GM300 to maybe 20 watts. But in reducing the power output, you still have the issue with the driver stage not having enough heat sinking. You can't get to that area of the radio with an external fan.

If your looking to make this type of long duration operation of the transmitter, you had better look into locating a continuous duty transmitter. Some of the used base stations coming on the used market are rated for this type of operation. You may want to look for one.

Jim

Thanks for the input guys. Yeah I am confused too. I have the power down to 20 watts on the initial key-up now, but the longer you press ptt, the higher the output gets. It seems to settle at 45 watts now after 3 minutes. What can make the radio do this? Why wouldn't it be backing down once the thermistor sees the heat? Either my meter is screwed or my radio is screwed.

This is the fan I have blowing on it. Right behind the PA. The grill of the fan is touching the PA. I lied... it's 4".
http://www.buy.com/prod/holmes-hnf0410a ... 44834.html

Do you have model numbers or names for the base stations coming on the market that would be suitable?

I thought about moving the PA transistor to a computer CPU cooler. Some of those are rated to dissipate 100 watts. Anyone do that? Would that work?

The GM300s and Maxtracs were designed to automatically back the tx power down as the heat sink temperature goes up, so it sounds like there's definitely a problem with either temperature sensing at the heatsink or the power backoff circuitry is not functioning. When you start trying to use a mobile radio rated for 5-10% transmit time for continuous duty all bets on performance meeting specs or reliability are off.

The TX power compensation may not be working since it's programmed in the ham bands, I had the same problem when programming radios in the T-band. The radio would run full boat power out about 60 watts for a UHF M44

the shop I worked at in the 90's made a ton of money on Maxtrac/GM300's used by a local taxi company. We stocked VHF and UHF PA's we replaced them so often. These guys were too cheap to get cellphones and liked to chat on them for long periods. Long keydowns of more than 60 seconds result in PA slabs so hot you could burn yourself on the heat sink.

These radios BARELY have enough heat sinking for the 5-5-90 duty cycle. Ham jabber duty cycle is quite a bit more. It would be better to get a high power Spectra and back it down to about 60 percent power (much more robust heat sink). The Maxtrac doesn't do too well when you reduce the drive power down too far.

The problem you describe is indeed a sign of overtemp.

Well I figured out my watt meter is screwed. The readings I've been fuming you guys are wrongbut I imagine you know that all ready. I have a new 1 ordered and it should be here today.

Hello:
The GM300 mobile if its not a T bander was manufactured to cover 438 to 470 mhz .
So if the corect model number it would be designed to run in the ham band.
But as many have already said = Yes the GM300 did have a known problem with long tx times causing very very blistering hot PAs resulting in melting the solder on the finals fins and components sliding thus shorting out.
I cant say i have had the GM300 pa fail for good after that but they did require unsoldering and resoldering with a higher melting temp solder to get them up and running again.
Sometime back i remember it being mentioned that the finned pa from a gr1225 or r1225 could be used to help better cool the gm300 in this situation but it needed the rx portion added if being used for both rx and tx .
Dont recall if you could just simply swap the GM300 pa electronics into the GR1225 / r1225 bigger extended finned pa or not now .
Anyone know if it can be done ?
But from your post of the 30 min key up time id really doubt it would help here.
But you could consider an ASTRON SL15 R power supply as it has a built in cooling fan to help out the heat issue better.
Your in a pretty tough spot with the useage you describe thats for sure.
Might even be better if you put on a 10 watt pa and go that route since pre driver temps will be much lower .
Might even solve your troubles along with a ASTRON SL15 R power supply for better cooling.
Jusy my thoughts and only my thoughts as others may have an even better solve.
Good Luck
Satelite

I've got to ask... Are you running Newsline on your GM300? What in the hell else could cause a 30 minute keydown??

d119 wrote:I've got to ask... Are you running Newsline on your GM300? What in the hell else could cause a 30 minute keydown??

A drunk ham on 75 meters AM.

Ever listen to those Hameys talk, they give Seinfield (the show about nothing) a run for the money LOL. They can be long winded

70cm FM linked repeater. I am a LI boy living in PA and really miss LI. I keep my repeater here in PA linked to LI so I can carry on conversations. The transmitter stays keyed the whole time anyone is talking. It's not uncommon to have 1-2 hour long conversations in a round table fashion with several people without the repeater dropping at all in that time frame. I have removed the PA board from the stock heat sink and am in the process of hooking up a computer CPU fan to the transistor. This CPU fan and heat sink can dissipate about 100 watts of heat. If it all works it will be the first 100% duty cycle GM300. Best thing is the radio cost me $65 and the heat sink cost me nothing. If it works it will be the cheapest repeater transmit radio ever! Muhahahaha!!!

you'd do better off with a Spectra high power rolled back or a Syntor X. The GM300 PA will die. No matter how much cooling you can throw at it short of a cryogenic freezer, the rig is too feeble to handle it. Good luck, let us all know how it works out.

It's done. cost me nothing. It is an experiment. If it works... yay for me. If not, then I will find another way. Either way, radios are just a hobby for me so it is more like a challenge to see if I can pull it off. I get bored with out tinkering with stuff. Thanks for the input guys, we'll see how it works out in a couple of days.

w1dth wrote: If it works it will be the cheapest repeater transmit radio ever! Muhahahaha!!!

I've seen some REAL crap hooked up as a TX unit...an ol' Midland 70 was the worst I can think of right now.

This reminds me of the thread years ago where guys were talking about watercooling Spectras. Wish I could find it, but board search fails me every time I go looking.

You know, what you could do is get a 2-10W LPI PA and use it to drive a real continuous duty PA. UHF MSF5000 PAs (and power supplies) are cheap and plentiful on eBay, and I'm sure you could find someone here to swap your 40W PA with their "useless" 10W PA.

His 40 watt PA will be toast for sure by then......