Hello all,
Back with another project, unfortunately my last stomper projects ended up getting picked up before I could figure them out. I'm hoping this time around I have a bit more of a casual fault and not such a head scratcher.
I have 2 Cobra 200 DX radios, in almost mint condition. The issue is that the previous owner ended up over volting them one after the other. Story goes on both occasions what killed them was his power supply failing and sending 18+ into the DC input. Among the things that got fried were the output transistors. He said he sent them to be repaired but not much was done besides changing the 2SC2290 transistors and then they gave up.
I've picked what seems to be the better one out of the two and have started reviving it. The radio receives and is on frequency. Could probably use a little more reception sensitivity but besides having zero output power it seems to be alive and well. The first issue that I found with it is a bad driver mosfet at the output stage. When measuring the gate voltage I was only getting about 0.5V and after taking the mosfet out and testing it I found that it was leaky between gate and source. I reset the gate voltage to 3.3V and installed the driver mosfet out of the other radio which seems to be good. I can now see a really decent signal coming out of C305 coming from the mosfet stage. It's about 12Vp-p and shows up at the correct frequency and modulated on AM. From here however I cannot find any RF signal at all being applied to the bases of the 2SC2290s. Collector and Base voltages are present and correct just nothing modulating the stage.
Looking at the schematic I can't exactly understand how the signal coming out of the mosfet driver stage ends up entering the power amplifier. Whatever the case it seems like the signal being put out by this driver stage ends up getting lost somewhere.
Back with another project, unfortunately my last stomper projects ended up getting picked up before I could figure them out. I'm hoping this time around I have a bit more of a casual fault and not such a head scratcher.
I have 2 Cobra 200 DX radios, in almost mint condition. The issue is that the previous owner ended up over volting them one after the other. Story goes on both occasions what killed them was his power supply failing and sending 18+ into the DC input. Among the things that got fried were the output transistors. He said he sent them to be repaired but not much was done besides changing the 2SC2290 transistors and then they gave up.
I've picked what seems to be the better one out of the two and have started reviving it. The radio receives and is on frequency. Could probably use a little more reception sensitivity but besides having zero output power it seems to be alive and well. The first issue that I found with it is a bad driver mosfet at the output stage. When measuring the gate voltage I was only getting about 0.5V and after taking the mosfet out and testing it I found that it was leaky between gate and source. I reset the gate voltage to 3.3V and installed the driver mosfet out of the other radio which seems to be good. I can now see a really decent signal coming out of C305 coming from the mosfet stage. It's about 12Vp-p and shows up at the correct frequency and modulated on AM. From here however I cannot find any RF signal at all being applied to the bases of the 2SC2290s. Collector and Base voltages are present and correct just nothing modulating the stage.
Looking at the schematic I can't exactly understand how the signal coming out of the mosfet driver stage ends up entering the power amplifier. Whatever the case it seems like the signal being put out by this driver stage ends up getting lost somewhere.
Last edited: