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Maxtrac GM300 repair

4X5SD

Newbie
Joined
Feb 14, 2020
Messages
4
Hello,
A friend of mine gave me an old GM300 together with the magnetic mount car antenna which I placed on the metal railings of the balcony. The antenna is a fiberglass NW2001 (Diamond?) for 144/430 MHz. It cannot be tuned anyway so I did not check the SWR. Initialy, it was working fine. However, during a conversation on 145,550 MHz today, I realized that the radio stopped to transmit and receive. The heat sink of the power amplifier was quite warm. I measured the current draw during TX, it is about 7 Amperes. Does it mean that the transistors or the switching diodes of the power amplifier are dead? Is there anything that I can check?
Thanks!
Anton.
 

cmdrwill

Member
Joined
Mar 30, 2005
Messages
3,457
Location
So Cali
It means that you exceeded the duty cycle for the transmitter/PA. Some radios can be recovered, the radio is old enough to have very poor transistor to heat sink heat transfer. The thermal paste drys out.

Maxtrac and Radius radio transmitters are only rated at 10 percent duty cycle.

magnetic mount car antenna which I placed on the metal railings
And maybe not enough ground plane and high reflected transmitter power which overheats the PA.
 

wgbecks

Member
Joined
Jan 17, 2005
Messages
96
Location
Porterfield, Wisconsin
A couple of points. There may not be anything wrong with the radio. The Radius series that the GM300 is a family member have a relatively low duty cycle rating. That is the PA heatsink gets very hot on transmit and they will fold way back on power output once the heatsink get hot. Let it completely cool off and retest. You should also attempt to determine if the antenna is providing a VSWR as well because this can reduce key down time and damage the output stage.

Now, you should be aware that there is a know quirk with Maxtrac's that may or may not exist with the GM300 when operating the transmitter below 146 MHz. The Maxtrac's at least go bonkers with power control circuit when operated down in the 145 MHz segment of the band and typically case RF output far in excess of the rated power that will definitely cause the heatsink to overheat very rapidly.

I have successfully used GM300 on UHF for remote receiver links and such with the addition of a muffin fan continuously blowing cool are on the marginal heatsink at the rear of the radio. These radio's are non designed for long winded QSO's!
 

4X5SD

Newbie
Joined
Feb 14, 2020
Messages
4
And maybe not enough ground plane and high reflected transmitter power which overheats the PA.
Yes, I think there was not enough ground plane.
Let it completely cool off and retest.
I tried to test the radio again. It did not recovered. The weird thing is the lack or RX. If the power transistor is damaged, should it affect the RX function?
 

wgbecks

Member
Joined
Jan 17, 2005
Messages
96
Location
Porterfield, Wisconsin
Generally, no. That is unless one of the PIN diodes used to switch between transmit and receive branches was damaged. You really need
to get a VHF rated wattmeter and perhaps a good dummy load to verify if the transmitter is dead. Might be that the antenna is junk and that's why no receive.
 

4X5SD

Newbie
Joined
Feb 14, 2020
Messages
4
Generally, no. That is unless one of the PIN diodes used to switch between transmit and receive branches was damaged. You really need
to get a VHF rated wattmeter and perhaps a good dummy load to verify if the transmitter is dead. Might be that the antenna is junk and that's why no receive.
Yes, it was an antenna. The radio works with another antenna, installed on my car. The problem is solved. Thank you!
 

MTS2000des

Member
Joined
Jul 12, 2008
Messages
3,079
Location
Cobb County, GA Stadium Crime Zone
The shop I worked for in the 1990s made quite a bit off replacing PA's on the GM300, Radios Mxxx and MaxTrac radios. We stocked PA modules fo the then popular VHF and UHF versions in use by the various taxi companies at the time we replaced so many. The taxi drivers would use them on a talkaround channel to gasbag back and forth, back when cellphones were luxury items and airtime was per minute billing.

These radios are truly designed for 5/5/90 duty cycle. I realize they are cheap/free and thus a popular radio for ham use, but without enhanced cooling such as a high CFM fan blowing directly on the heat sink, they won't last long in ham duty cycle.
 

Golay

Member
Joined
Apr 28, 2016
Messages
357
Location
Nankin Township, Michigan
This may not work for you.
Many years ago now, we ran GM300's on power supplies.
Every once in awhile, the ones that were out in the mill would just quit. Like you turned the power off.
We would take them apart, blowing all the dust and grime and what not out of them, and it would revive them.
Just one of many suggestions.
 

jim202

Member
Joined
Mar 7, 2002
Messages
2,588
Location
New Orleans region
It has been a long time since I have programmed a Maxtrac GM300. I believe there is a TOT (Time Out Timer) in the software that can be turned on. I do that with all my radios I use because I tend to be long winded. The radio, once the TOT is set, will shut off the TX after the set time and give a tone out of the speaker until you release the PTT. It also helps if your mobile and end up sitting on the mic.

Addition of moving air across the heat sink is a good move for us long winded hams.
 

MTS2000des

Member
Joined
Jul 12, 2008
Messages
3,079
Location
Cobb County, GA Stadium Crime Zone
I believe there is a TOT (Time Out Timer) in the software that can be turned on. I do that with all my radios I use because I tend to be long winded.
Yes, in the Maxtrac/GM RSS packages, one can set a time out timer. A golden rule that should be followed on all radios. And not just "resetting" the timer for another long keydown...aside from defeating the purpose of maintaining the radios' rated duty cycle, it is also rude on repeaters (yes this happens around here).
 
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