I have a Cobra 29 on the bench I'm having a little trouble nailing down the problem on. I'm getting a good signal to the base of the driver but not to the final. The signal on the collector of the driver less than half of the signal on the base. For instance on 19 when keyed the signal on the base reads 27.185 which is what I would expect. RX works as it should. I believe the problem is in the PA section of the radio but at this point I'm stumped.
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Cobra 29 No TX Output
- Thread starter Derrick Walls
- Start date
sounds like the driver isn't amplifying.
check to see that you have 13.8 volts going to the collector, and after verifying that is there, unsolder the leads of the driver transistor from the board and test it using your multimeter.
here is a link to an article on how to do it if you need it: http://www.learnabout-electronics.org/transistor_faults_02.php
if you don't find the 13.8 volts on the collector of the driver, unsolder the collectors of the final and driver one at a time and check the PCB trace for that voltage.
If it is there when the transistors are out of circuit, but is not there when they are in circuit, then one of them likely has a short.
if that voltage is not there even when the collectors are lifted, then trace back to D8 which may be blown, or the modulation transformer itself.
LC
check to see that you have 13.8 volts going to the collector, and after verifying that is there, unsolder the leads of the driver transistor from the board and test it using your multimeter.
here is a link to an article on how to do it if you need it: http://www.learnabout-electronics.org/transistor_faults_02.php
if you don't find the 13.8 volts on the collector of the driver, unsolder the collectors of the final and driver one at a time and check the PCB trace for that voltage.
If it is there when the transistors are out of circuit, but is not there when they are in circuit, then one of them likely has a short.
if that voltage is not there even when the collectors are lifted, then trace back to D8 which may be blown, or the modulation transformer itself.
LC
I would start by checking bad solder joints some cobra 29s are know for that.
d8 diode checks good also getting 13.8 voltage to the collector of the final and driver. I put two new transistors in. still no output. Note: I have a oscope, multimeter and a bench freq counter. Not as familiar with the Cobra radios as i should be because I rarely work on them
ok found part of the problem. had a bad driver. just low output now. Radio has a mosfet installed on the final. getting little less than half watt deadkey with a swing up to 2 watts.
Sorry I was a bit tired last night. It does have a mosfet final but has all the parts install and removed for it. Reason I put a mosfet final in originally was because the customer wanted a rfx85 amp added to the radio and was using it to drive the amp. Radio is the 29LX been working great for him up until a couple of weeks ago. I'm beginning to think something is shorted between the final and the antenna connector. I know in most cases the final or driver would be bad. I ruled out the rfx85 amp as I disconnected it and hooked the antenna connector back up to just the radio. I'll find it eventually.
Correct me if I'm wrong shouldn't the final have some gate voltage to turn on the mosfet so it would amplify the signal. At this point it has none when keyed
yep that was it. the bias component was shorted to ground. got it fixed. The rfx85hd is shot. the transistor is gone in it. radio works fine without it. Is there a feasible replacement for a on board amp?
This statement is a bit confusing, as one end of your "bias component" goes to PC board ground.
Unless you just put a piece of wire in place of R56, i can't see how this could happen.
As for the RFX85, there are two amplifying transistors in it, and both are available so it shouldn't be too hard to repair the RFX85 unit.
LC
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