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cobra 142 problem
- Thread starter fred42
- Start date
Okay, so just so I have it straight.
Trouble on AM receive. Trouble, or dead?
And trouble on AM transmit. If AM transmit is completely dead, that points directly to TR34. Most older radios used type 2SC1491. It's easy to test in the circuit. You should have 13.5 Volts on the center leg, transmit or receive so long as the mode selector is in the AM position.
IF YOU DON'T THIS SUGGESTS THE MODE SELECTOR SWITCH HAS GONE BAD.
Probably didn't need to shout, but if that happens there is only one cure. A new mode selector switch.
If you DO have 13.5 Volts DC on the center leg, key the mike and see what DC voltage you see on the leg nearest the front of the radio. If it's zero or close to it, you should now check the rear-most facing leg. If you have 5 or 6 Volts DC or more, TR34 is definitely dead.
Any time we replace TR34, we also replace TR35 as a precaution. Might check okay, but it saves us repeat problems if it turns out to have been damaged when TR34 failed.
But this has NOT ONE THING to do with the receiver.
A 142 that receives on SSB but not on AM is a bit of a puzzle. The receive signal takes the same path through the receiver for AM and SSB both. This is not a common fault for that model.
Have you tried cleaning the controls, especially the mode selector? If the contacts are badly oxidized that can cause things to cut in and out, or quit altogether.
Really sounds like a radio with more than one problem, or with a mode selector that has more than one bad section in it, possibly.
But the only thing that AM transmit and AM receive have directly in common with each other is the mode selector switch.
73
Trouble on AM receive. Trouble, or dead?
And trouble on AM transmit. If AM transmit is completely dead, that points directly to TR34. Most older radios used type 2SC1491. It's easy to test in the circuit. You should have 13.5 Volts on the center leg, transmit or receive so long as the mode selector is in the AM position.
IF YOU DON'T THIS SUGGESTS THE MODE SELECTOR SWITCH HAS GONE BAD.
Probably didn't need to shout, but if that happens there is only one cure. A new mode selector switch.
If you DO have 13.5 Volts DC on the center leg, key the mike and see what DC voltage you see on the leg nearest the front of the radio. If it's zero or close to it, you should now check the rear-most facing leg. If you have 5 or 6 Volts DC or more, TR34 is definitely dead.
Any time we replace TR34, we also replace TR35 as a precaution. Might check okay, but it saves us repeat problems if it turns out to have been damaged when TR34 failed.
But this has NOT ONE THING to do with the receiver.
A 142 that receives on SSB but not on AM is a bit of a puzzle. The receive signal takes the same path through the receiver for AM and SSB both. This is not a common fault for that model.
Have you tried cleaning the controls, especially the mode selector? If the contacts are badly oxidized that can cause things to cut in and out, or quit altogether.
Really sounds like a radio with more than one problem, or with a mode selector that has more than one bad section in it, possibly.
But the only thing that AM transmit and AM receive have directly in common with each other is the mode selector switch.
73
I seem to remember a Superstar 3900/ept3600 chassis radio I had worked on, had a similar problem.
Turned out to be a diode.
Turned out to be a diode.
Last edited:
One big difference between the Superstar and this one. That radio has a double-conversion receiver on AM and FM modes. It's single conversion in SSB. This means that the radio has a whole separate section for AM. Things can go bad in that part of the radio without affecting SSB receive.
But his radio has a simpler receiver setup. AM and sideband both travel through the exact same circuits. A diode fault like that one should cause trouble for both, not just for AM.
Probably.
73
But his radio has a simpler receiver setup. AM and sideband both travel through the exact same circuits. A diode fault like that one should cause trouble for both, not just for AM.
Probably.
73
