The old saw goes "Measure once cut twice. Measure twice, cut once."
Lost track of how many times I advised someone to check the base voltage feeding to a new replacement final before soldering that lead to the pc board. Shoulda taken my own advice.
Customer states no transmit from his Cobra 2000.
Heard the echo of "There's your problem" when I saw how the driver transistor was almost soldered.
No challenge, just hook it up like it should be. I did test the driver transistor while the leads were disconnected. Said it was okay.
Shoulda checked that base voltage first.
Didn't even have to key the mike. Smoked L42 within seconds of applying power.
Turns out the final was partly bad, leaking current out into the final's bias circuit, leaking into the driver transistor's base circuit. Turned on the driver full bore without keying the mike.
But not for long.
More-detailed inspection revealed that the driver's bias diode was an open circuit. This will cause a similar runaway when you key the mike. Didn't get the chance.
The collateral damage isn't as bad as it looks. New collector choke for the driver, new bias diode for the driver, two new bias trimpots and both the electrolytics in the two bias circuits.
And a driver transistor. And a final transistor.
The final might have worked at one time in the past, but it's a counterfeit. Has the Mitsubishi 3-diamonds logo, but it's 10mm wide, not the 9mm width that signals the genuine Mitsubishi product.
Just because it's counterfeit doesn't mean it can't work. This one probably used to, but not no more.
The 1N4148 works okay to replace the MV1Y that was bad. Should probably put a dab of fingernail enamel on it to make it stick to the driver transistor.
So, put in all this new stuff, set bias for the driver and final. Now I get a half Watt.
Nice.
Didn't take long to identify this little crock. C146 is in parallel with the RF output circuit. This one was the troublemaker. Also replaced C147. It's in parallel, just adjacent to this one.
Now it tunes up to full output. For a 1983 Cobra 2000 this is around 15 Watts peak. But it ain't broke no more.
Woulda been quicker and easier if I had just followed my own advice and checked base voltage while the driver transistor's leads were unsoldered.
Never too late to learn, even if it takes more than once.
73
Lost track of how many times I advised someone to check the base voltage feeding to a new replacement final before soldering that lead to the pc board. Shoulda taken my own advice.
Customer states no transmit from his Cobra 2000.
Heard the echo of "There's your problem" when I saw how the driver transistor was almost soldered.
No challenge, just hook it up like it should be. I did test the driver transistor while the leads were disconnected. Said it was okay.
Shoulda checked that base voltage first.
Didn't even have to key the mike. Smoked L42 within seconds of applying power.
Turns out the final was partly bad, leaking current out into the final's bias circuit, leaking into the driver transistor's base circuit. Turned on the driver full bore without keying the mike.
But not for long.
More-detailed inspection revealed that the driver's bias diode was an open circuit. This will cause a similar runaway when you key the mike. Didn't get the chance.
The collateral damage isn't as bad as it looks. New collector choke for the driver, new bias diode for the driver, two new bias trimpots and both the electrolytics in the two bias circuits.
And a driver transistor. And a final transistor.
The final might have worked at one time in the past, but it's a counterfeit. Has the Mitsubishi 3-diamonds logo, but it's 10mm wide, not the 9mm width that signals the genuine Mitsubishi product.
Just because it's counterfeit doesn't mean it can't work. This one probably used to, but not no more.
The 1N4148 works okay to replace the MV1Y that was bad. Should probably put a dab of fingernail enamel on it to make it stick to the driver transistor.
So, put in all this new stuff, set bias for the driver and final. Now I get a half Watt.
Nice.
Didn't take long to identify this little crock. C146 is in parallel with the RF output circuit. This one was the troublemaker. Also replaced C147. It's in parallel, just adjacent to this one.
Now it tunes up to full output. For a 1983 Cobra 2000 this is around 15 Watts peak. But it ain't broke no more.
Woulda been quicker and easier if I had just followed my own advice and checked base voltage while the driver transistor's leads were unsoldered.
Never too late to learn, even if it takes more than once.
73
