Batboard

Howdy,

Just built a simple UHF repeater with a Maxtrac 50 and Radius (both 5 pin) so used the mic jack cable interface (diagram from Nand) Right now using 2 antennas with 30' vertical seperation for some testing.

Problem is, peridically the transmitter just "pickets". In other words when holding down the PTT, the transmitter just cycles in and out.

Where should I start looking?

rcit wrote: Problem is, peridically the transmitter just "pickets". In other words when holding down the PTT, the transmitter just cycles in and out.

Could you please describe this in more detail. Thank you.

rcit wrote:Howdy,

Just built a simple UHF repeater with a Maxtrac 50 and Radius (both 5 pin) so used the mic jack cable interface (diagram from Nand) Right now using 2 antennas with 30' vertical seperation for some testing.

Problem is, peridically the transmitter just "pickets". In other words when holding down the PTT, the transmitter just cycles in and out.

Where should I start looking?

How long after you start using it does it begin to do this?? What is the current RF output set at??

Frank

Hi!

If I remember correctly, these series of radios have a protection against overheating. When your cooling is not sufficient, the output power is automatically reduced and eventually the transmitter can even be shut down by the protection circuit.
Both Maxtrac and Radius are known to heat up quickly, so this might be the origin of your problem.


73!
Adam
SQ5AG - SP/F4DDP

sq5ag wrote:Hi!

If I remember correctly, these series of radios have a protection against overheating. When your cooling is not sufficient, the output power is automatically reduced and eventually the transmitter can even be shut down by the protection circuit.
Both Maxtrac and Radius are known to heat up quickly, so this might be the origin of your problem.


73!
Adam
SQ5AG - SP/F4DDP

That's what I was thinking, these are no continuous duty amps on the radios.

Frank

Current ouput is 10W and this happens right from the start (read - no heating issue I believe).

Though last night I could get a pretty good output audio...now the transmitter just cycles.

O.K I think maybe I was just blowing through the front end of the receiver. I hooked the transmitter back up to a bit of a dummy load and the problem seems to be gone.

So better isolation, or lower power, or what?

I was just gonna say that this sounds like front end noise. You may need bandpass filtering on the RX hardline to tune out whatever is coming in. More vertical seperation and better coax will also help.

Those radios DO start getting hot as soon as they begin transmitting. You will need a 4-inch fan blowing right onto the heatsink (from the back to the front seems to work best) all the time, and even then it'll still get warmer than anyone would like it to. After 30 or more minutes of continuous transmitting, the radios will start to cut back their output power, even if the heatsink is ice cold.

I ran two MaxTracs in a UHF repeater for about two years with a 6-section bandpass/reject duplexer. These provide over 100 dB of isolation. Absolutely NO desense or interaction of any kind. When I first put the repeater into operation, the transmitter was putting out 40-45 watts into a dummy load. This gave me 33 watts out of the duplexer feeding a super stationmaster antenna through 500 ft of 7/8 inch coax. Eventually I lowered the transmitter power and bought an external power amplifier which gave me 80 watts to play with, and I got about 60 out of the duplexer.

ebay occasionally has the Celwave or older Phelps Dodge 6-section duplexers for sale. I have bought three of them and had to replace the coax jumpers on two because they had used old military-style screw-together connectors and the braid had broken off, shorting out to the center conductor. Also, these were made from RG-8. The new ones are made from RG-214 and have crimp-on connectors. I bought one set new from Celwave for $250. I made the second set myself for about $60.

The use of one antenna and a duplexer is probably a better way to go, if you can get the equipment, than two antennas separated vertically.

Bob M.

Thanks Bob and others,

I will keep the power down to about 10-15 Watts, and I do have the 6 can duplexer waiting to get tuned. I'm going to try to build a case with some decent cooling to help.

It seems to be working good now...problem was blowing the reciever. More seperation was in order.

With 25 or 45 watt radios turned down to 6-7 watts, they STILL get hot very quickly. Lowering the power output doesn't seem to really help the problem - you need lots of cooling air flowing over that heatsink.

I once measured the efficiency of a 45w Maxtrac by graphing the input current times input voltage vs the output power. The efficiency was best at 40-45 watts output, yet even there the radio still draws 8-9 amps at 14.0 volts, so the efficienty is in the 30% range. The input power that doesn't generate RF ends up generating heat.

Bob M.

Check with Will. I think he makes a heat disipation system for the Maxtrac for this very purpose.

See: http://www.webimaging.com/echocomm/

Look here for my heatsink information.

http://www.webimaging.com/echocomm/maxtrac.html