I'm installing a GM300 mobile radio into my racecar using a car wiring harness and am having trouble getting mic audio to work. I used to have a GP300 with the small dual-mini jack adapter and that worked *flawlessly*.
The radio keys up and the audio is very faint on the receiving end. I've benchtested the unit with a standard RJ45/8 pin connector PTT mic and it works great. However, each external mic I connect to the RJ45 jack on pin 4&5 (gnd/mic in) -OR- on the rear accessory connector pin 2 & 7 (which will be the permanent connector) gives extremely low audio output; they are both the same. I have a suspicion that I have an impedance imbalance on my mic input. adding a resistor to the mics I have seemed to improve the audio level but only slightly.
any suggestions???? Help! I have a race in a week and want to use the new radio!
Jeff
GM300 in Racecar w/ helmet mic/harness - NO MIC AUDIO
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GM300 in Racecar w/ helmet mic/harness - NO MIC AUDIO
Jeff Stevenson
Bushwacker Racing http://www.bushwacker-racing.com
Porsche Racing Club http://www.porscheracingclub.com
Bushwacker Racing http://www.bushwacker-racing.com
Porsche Racing Club http://www.porscheracingclub.com
Is the harness from Racing Radios or Racing Electronics?
The GP300 harness uses the PTT in series with the audio on these harnesses. To use one of these harnesses on the GM series, the mic audio and PTT must be seperated. If using a dynamic mic in the helmet, you MAY have to use an impedance matching transformer with the 600 ohm side to the mic and the 6K to 10K-ish side to the radio's mic input. Ususally, a small box from Radio Shack will house this transformer. I feed audio into the RJ45s and mount the impedance network about 6" from the RJ45. I also fill the box with epoxy once all is tested. Using Motorola RSS, you can adjust the microphone gain to get more punch out of the radio.
The GM/Radius is a great radio for racing. It takes heat AND vibration well. You may want to de-tune it's output to 5 to 10 watts, too. For the antenna, I use the Antenex Phantom (yes- I know it's less aerodydynamic, but I assure you it won't slow you down!) on sedans and a portable antenna with BNC connector mounted to a BNC bulkhead feeder on open-wheeled cars. The HT antenna works well on 'glass bodies since it needs no groundplane at it's base. All assuming you are on UHF or 800. I haven't seen a car yet on VHF.
I think all NASCAR teams should go to lowband. Imagine a stock car going 190 MPH with a ball-mounted 7' whip. Talk about rubbin' !
By the way, what sanction or class are you racing in and with what?
I do alot of radio work for the local SCCA and Pro 2000 guys in the area. I'm currently supercharging and intercooling my 87 928S4 to have some serious fun....and a little at Mid-Ohio!
The GP300 harness uses the PTT in series with the audio on these harnesses. To use one of these harnesses on the GM series, the mic audio and PTT must be seperated. If using a dynamic mic in the helmet, you MAY have to use an impedance matching transformer with the 600 ohm side to the mic and the 6K to 10K-ish side to the radio's mic input. Ususally, a small box from Radio Shack will house this transformer. I feed audio into the RJ45s and mount the impedance network about 6" from the RJ45. I also fill the box with epoxy once all is tested. Using Motorola RSS, you can adjust the microphone gain to get more punch out of the radio.
The GM/Radius is a great radio for racing. It takes heat AND vibration well. You may want to de-tune it's output to 5 to 10 watts, too. For the antenna, I use the Antenex Phantom (yes- I know it's less aerodydynamic, but I assure you it won't slow you down!) on sedans and a portable antenna with BNC connector mounted to a BNC bulkhead feeder on open-wheeled cars. The HT antenna works well on 'glass bodies since it needs no groundplane at it's base. All assuming you are on UHF or 800. I haven't seen a car yet on VHF.
I think all NASCAR teams should go to lowband. Imagine a stock car going 190 MPH with a ball-mounted 7' whip. Talk about rubbin' !
By the way, what sanction or class are you racing in and with what?
I do alot of radio work for the local SCCA and Pro 2000 guys in the area. I'm currently supercharging and intercooling my 87 928S4 to have some serious fun....and a little at Mid-Ohio!
Jim,
thanks for the info! the harness actally is from a place called findsoneline.com - and the ptt isn't wired in series - it's a separate wire (already fixed that problem). I think the real problem is in the mic - since I was using a GP300 before and it worked great, the GM300 doesnt have the preamp in the radio - it's in the mic. So... I'm going to buy an old GM300 mic and take out the preamp board and install it inline w/ the mic. should solve the problem.
The ant we use is a std uhf and I put a groundplane circle under the cabon fiber roof. seems to work well enough.
The club - Porsche Racing Club. I started it and I'm the president (so I get to make the rules!). http://www.porscheracingclub.com
The car - started life as a stock 79 sc and it's far from that now. I run in PCA GT3R now. http://www.bushwacker-racing.com has the whole story
thanks for the help!
Jeff
thanks for the info! the harness actally is from a place called findsoneline.com - and the ptt isn't wired in series - it's a separate wire (already fixed that problem). I think the real problem is in the mic - since I was using a GP300 before and it worked great, the GM300 doesnt have the preamp in the radio - it's in the mic. So... I'm going to buy an old GM300 mic and take out the preamp board and install it inline w/ the mic. should solve the problem.
The ant we use is a std uhf and I put a groundplane circle under the cabon fiber roof. seems to work well enough.
The club - Porsche Racing Club. I started it and I'm the president (so I get to make the rules!). http://www.porscheracingclub.com
The car - started life as a stock 79 sc and it's far from that now. I run in PCA GT3R now. http://www.bushwacker-racing.com has the whole story
thanks for the help!
Jeff
Jeff Stevenson
Bushwacker Racing http://www.bushwacker-racing.com
Porsche Racing Club http://www.porscheracingclub.com
Bushwacker Racing http://www.bushwacker-racing.com
Porsche Racing Club http://www.porscheracingclub.com