Help with Cobra 142

[Thestonerguy]

Just picked up a 142GTL, receives great, but have problem with transmit. When you first turn it on and tx, the meter swings with the modulation for about 2-3 seconds then apparently, it dies. The meter drops to zero, and if I unkey it and do it again, the same thing happens (for about 2 or 3 more tries, then nothing until I turn the rig off and wait about 10 minutes). When talking to someone a couple miles away, they get the same thing (I come in loud, then fade to nothing), however, I can still hear myself on a radio in the same room (so I know I still have tx audio). My first thoughts are either the final or driver, but hopefully I can get some second, third, fourth, opinions on what to check -

 

[nomadradio]

First, I would check the bias current (mic gain at zero, SSB transmit) on both driver and final. Sometimes a screwdriver expert will crank them dangerously high.

THEN, I would want to see what kind of power it delivers on SSB transmit. If the "fade out" problem remains, this points to the driver, final and parts wrapped around each.

If the problem is confined to AM transmit ONLY, this points to the AM modulator/carrier regulator transistor TR34, the one bolted to the heat sink between the audio power chip and the rear panel. If the DC voltage coming out of the emitter lead (the one towards the front) falls off in step with the wattmeter falling off, that's a clue. And if TR34 proves to be bad, I tend to replace the 2SC945 transistor that drives it, TR35, even if it checks okay. Cheap insurance.

Unless it's something else, like the power supply falling off? If the meter lights didn't fade while all this was going on, that's not it, anyway.

73

 

[Thestonerguy]

First, I would check the bias current (mic gain at zero, SSB transmit) on both driver and final. Sometimes a screwdriver expert will crank them dangerously high.

THEN, I would want to see what kind of power it delivers on SSB transmit. If the "fade out" problem remains, this points to the driver, final and parts wrapped around each.

If the problem is confined to AM transmit ONLY, this points to the AM modulator/carrier regulator transistor TR34, the one bolted to the heat sink between the audio power chip and the rear panel. If the DC voltage coming out of the emitter lead (the one towards the front) falls off in step with the wattmeter falling off, that's a clue. And if TR34 proves to be bad, I tend to replace the 2SC945 transistor that drives it, TR35, even if it checks okay. Cheap insurance.

Unless it's something else, like the power supply falling off? If the meter lights didn't fade while all this was going on, that's not it, anyway.

73

Tnx Nomad, I'll follow your checklist - Woody