This radio connected to a 25 amp power supply and running high power on Cobra 200gtl.. When transmitting,on high power, the needle moves properly and output power is good for a few seconds then within 2 seconds the power drops off rapidly , needle in meter then does slowly drop away under ssb modulation till it does not react at all. On low power the transmitter is ok ....Any solutions??
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Re: Cobra 200gtl High Power Transmit problem??
Depending on the type of power supply, radio frequency (RF) energy from the transmitter may be 'fooling" it into thinking that you're overloading it. Check to see if there's any discernible dimming of the meter lamp when this problem occurs, if so, the supply is the culprit.
Otherwise, its probable that the final amplifier transistors are overheating, either they are 'leaky' or not properly attached to the heat sink.
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It sounds like the final transistor may be shorted/burned out. If you don't know how to locate or test the transistor you'll have to take it in to a tech. These transistors are usually quite cheap but as to the labour cost it depends on your tech. Usually what causes this damage is a bad antennae match or a bad antennae itself, even a shorted antennae cable. After you've repaired the radio check the antennae and cable first before connecting it up again, then check your match with an SWR meter. Good luck
Check your antenna. Make sure it is getting a proper ground. If mag mount; make sure base of antenna and where you put it are both super clean. Check where whip goes into coil or base. Make sure all allen screws are tight and whip is not either all the way down to the base or up to the top. If antenna is mounted; make sure that it has a good ground. You might have to remove some paint around the base to get a good connection. DO NOT TRANSMIT WITH A HIGH SWR. YOU CAN BURN OUT THE EXPENSIVE POWER OUTPUT TRANSISTOR. Hope it helps.
Was the radio working before the high power transitor mod? If so, it appears there is more to do than just replacing the transistor. Check www.worldwidedx.com under" cobra 29 high power mod with 2sc1969 transitor." There are instructions for the complete mod. Hope this helps
Without more in depth detail you have had your receive damaged by a high power radio transmitting near you. I suspect a blown diode or tr7 the first RF amp transistor has failed.
Now it you are describing a low volume problem. Like the volume of your radio not increasing when you plug it in that can be a broken with in mic cord, bad switch in handset or solder connection at mic plug. It will still transmit.
hi More than likely your pa transistor has blown have you checked it with a power meter if so r you getting the red tx led when the ptt is pressed if so then your problem lies with the rf transistor
replacing rf transistor
TX means transmit. There are 3 settings for the meter. The top one is to measure your TX/RX that's transmit and receive. CAL is to calibrate the meter to check your SWR.
Reasons your needle may not be moving much when you key while on TX/RX. The dynamic is probably all the way down. That is your mic gain. If you turn that up when you talk the needle will swing more. If the radio is stock and not modded, you can turn this all the way up and leave it.
Also there could be something wrong with the mic. If people can hear you then its not the mic.
If people can hear you fine when you transmit, it may just be a bad meter. Not a big problem, you will just need an external meter to make sure you swr is not high.this is important because if it is over 1.5 you run the risk of blowing the final transistor.
Check your antenna. Make sure it is getting a proper ground. Check where whip goes into coil or base. Make sure all allen screws are tight and whip is not either all the way down to the base or up to the top. If antenna is mounted; make sure that it has a good ground. You might have to remove some paint around the base to get a good connection. DO NOT TRANSMIT WITH A HIGH SWR. YOU CAN BURN OUT THE EXPENSIVE POWER OUTPUT TRANSISTOR. Hope it helps.
I had a similar problem on my 706 MK1. The problem was corroded connections on the white power connector at the rear, When on tx the voltage drop across this connector was great enough to switch the radio off. try cleaning/re-seating this connector
Hello Darren, Yes it sounds like the mic is wired bass ackwards,,! A cobra 25 mic wiring is
PIN #1- Shield or ground wire
PIN#2- Audio or Transmit modulation
PIN #3- Transmit or Dead Key
PIN #4- Receive Audio.
When your keying the mic, the RX wires are being connected to make the RX work,,,
*** Let me know how you get along with it. Either buy a new mic wired for your radio or you or someone comfortable with soldering needs to resolder the correct wires back on the mic plug,,,Like I said, let me know and have a great day,, Mechanic is back quiet!