Mini Magnum Radio not transmitting after Final Replaced
My buddy had a loose coax on his wife's little wil antenna and blew the final in the radio. I found another final and replaced it and the power input diode that was shot. The radio powers up and the final doesn't heat up like it was doing when he brought to me, however the radio only keys aout a 1/10 of a watt. I checked the pre final and it is fine. What could be causing the non existent transmit power? Bias resistors or is it like a electrolitic is bad??? Also, I cannot find a schematice for this radio to even identify what is what.
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Re: Mini Magnum Radio not transmitting after Final...
Not knowing what kind of equipment you have, its hard to say, but .10 watt? Sounds like the buffer or driver also got involved. Usually you get 1/4-1/2 watt to the driver on a stock radio. I would also check the bias resistors like you suggested. Also check for burnt foil around the final you replaced. Good just be a slightly damaged circuit. Let me know if this helps. Tom
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Mack's cophased harness just merges the two wires at the connector going to your radio. If the radio falls out of compartment and is caught by the coax it could damage this connection. Outer shield and inner lead could short here. I have replaced many a final due to bad coax for my company which primarily Mack. Well i say that but it is the beloved company driver that continues to talk on a bad antenna system. It actually take a while to kill a final even with out an antenna attached. Good Luck...........
If there is no movement on the meter. have the SWR's check on the antenna system.. Free to have it checked. at a good shop.. You may have whats called a blown final..( final transmitter transistor) from high swr's. although you may hear, doesn't mean the antenna is OK. the other problems are a bad Audio transistor on the back corner, its the Large TO-264package/TO-218/TO-220 Styles. also. there a small diode on the solder side of circuit board near the 10.240 crystal, if its loose will also not let it key up or TX. make sure all your coax cables are tight, and all the switches are in the correct positions... also doesn't hurt to have a second radio nearby to see if it will transmit to it. for testing. If you need more help let me know.
I noticed finals are mosfet they will take alot of punishment from a bad antenna system match "high SWRs If the meter is moving at all on keyup i would not suspect finals. Finals seldom degrade in power they just quit. The driver for the Final transistor another transistor can produce low transmit power with a failed Final.
You say it is in a Dumptruck lots of vibration and dust. It could be cold solder joints..cracks in solder and corrsion in cracks reducing voltage to components. Dust or dirt in trimmer pots which are varible resistors.
Open case on solder side (top of radio)and tighten phillips head screws dont over tighten these boards can be fragile. After opening that side you will understand the dust issue.
I suggest checking the easy stuff first.
1. check power leads 2. Swap mic. 3. check coax connections. 4. check coax for damage. 5. swap antenna they can have broken wires. 6. make sure the plastic insulator on top that antenna is inserted in is on top and coax side is contacting antenna mount for ground.
when mic is pressed transmitter is engaged. First is the Fuse the proper fuse? Could be an antenna-coax problem. Get a dummy load and hook up to your CB and try it again. If it doesn't blow the fuse, the problem is indeed in your coax antenna system. another way to test your antenna coax is to disconnect it from the radio, take a multimeter, set it to ohms 200 ohm scale, put one lead on the center pin of the coax and the other on the outside of the connector on the coax.if it shows anything at all, you have a short and that will not work. It needs to not read anything. To be sure the antenna system is ok other than swr you can also check the coax an antenna for continuity. one lead of meter on the coax center pin then other on the antenna. should get a reading, a low value. again on the outside of the coax at the connector and at the antenna side on the outside of the coax connector. should be a low reading. but you cannot have a reading from the center of the coax to the outside of the coax. disconnect the coax at the antenna and measure from center to outside at the coax if the previous reading showed a value.If you see a value on the meter, any value, your coax has a short. not good. most likely at the coax connector. redo the connector. It is possible that the coax got compromised somehow and got smashed to the point the outside coax meets the inside and touches. Now the other possibily your fuse blows, kis because the final output transistor is bad. So if you hook up a dummy load to your radio and the fuse still blows, then the final transistor is bad. This is probably due to a bad coax-antenna system. So before you replace any parts check the coax-antenna system first or you will have the same problem and burn out another transistor in the final. If none of this helps then just get another radio or have a god buddy look at it that knows about radios. Trade him another radio or something to fix it.
There is a good chance you could have blown your final transistor. It could also be something is no malfunctioning in your mic. Is it on frequency? Hook it up to a frequency counter and make sure channel 19 reads 27.185. If it is your final transistor it blew for a reason. Most likely it got hot from a high SWR. 1-1:5 is the goal and no higher than 2. If it he SWR is higher then 2 the check your coax cables and adjust your antenna. Good luck.
That is your final output transistor. You probably have a very high swr due to a poorly tuned antenna or bad coax.
They normally do not fry. They just stop working. If its actually frying, you had better check your antenna system. If your antenna is not tuned for 27mhz, your coax has a short, or you have no antenna at all, the energy that normally radiates from the antenna does not, and it all goes back into the radio, and fries that final transistor, amongst other components at time.
This is an older radio they do not make any more. I have never been in one so I have no clue what final it has. You may be able to get the part number and possibly the part, if they still make it. If they don't make it, they may be able to tell you suitable replacement final for it.
I would go on ebay, and find a similar midland radio, made around the same time that is being sold for parts cheap. Get the final and other possible replacement parts as well.
Firts you have to check for carrier.For this you need an swr meter.2) Check continuity between the pl259 tip and antenna.3) Check for modulation (voice) if all of the above are OK. 4) Check if your display changes to transmit when you press the mike. IF all fails,then try another mike.
I tried to answer this the first time you posted :). If the finals got burned that bad I found that the bias resistor sometimes will get a burned foil and not make good connection. Also 1/10 watt sounds like the driver isn't amplifying from the buffer. Might want to check for burns on the foil all around the final stages, including where the antenna connector mounts to the board. Don't believe the caps are blown but as a last resort.... Its really hard to troubleshoot without the radio on the bench . Good luck. Tom
Not having the radio in front of me,I would check the following things.
1: Loose wire on the antenna connector inside the radio. (cols solder joint)
2.Turn radio upside down with covers off, look for bad connections on the legs of the buffer, driver, or final. Also check the voltage regulator. It should be mounted on the side of the chassis on the same side the mike connector is. If you don;t see any loose connections I would suspect a bad final, or a bad connection in the antenna sysem.
When you say high band wiorks great, I would lean more on the antenna system. Some antennas do not Broadband well. A good going over on the system from coax to antenna is a good idea. Check for ground, corrision, swr. etc. The thing that gets me going is when things get warm it works better. Well when things get warm, they expand and possibly a bad connection can fix itself. The connex radio is a good radio, but a big truck can play hell with the circuit board.