Cobra 142GTL Intermittent AM Transmit

[101 Creole Kid]

Hey Guys,

Thanks for your continued support. Purchased a Cobra 142GTL from eBay (1st Red Flag!) Good radio, but has had some work done in the past (2nd Red Flag). Talked with a bud locally on AM and he mentioned to me I was "Choppy" & "Intermittent". Changed the mic and still had the same issue. Switched radios and put the 142 aside. Well, got it on the desk and did the Clarifier Mod. While tuning the radio, I noticed what he was talking about. It'll key up and hold steady on the dead key and then is leaks off, falls off completely, comes back to 2W (preset dead key), and repeats that cycle until I let off the mic. I checked for loose solder joints around the driver and final with no avail. It doesn't react this way on SSB and I've made many contacts on SSB in the past. Only on AM. I haven't checked any voltages (planning) but have been searching for answers here with no luck to my issue. Radio has NOT been recapped. It seems there are some caps that have been replaced previously. There's also been some work done in the power supply area also the the power cord is only about 18in long. I'm baffled as to why.

 

[nomadradio]

Get a can of "duster" from the office-supply store. Key the mike until the transmit fails, then turn the can of duster upside-down. Now squirt the final transistor with the can inverted. This will spew liquid onto the final transistor and chill it severely.

If the wattmeter jumps back up immediately, the final has a thermal intermittent failure inside.

If you press the can's valve only partway you can direct the stream or better, just dribble onto the final without splashing too much onto surrounding parts.

Used to have a transistor we held onto after a repair. Hook the component tracer across its base and emitter pins and you would see the normal dog-leg pattern. Hold the soldering iron against it on the rear and the trace would flatline. Pop it with some chiller and it would come right back.

Every time.

73