cobra 138

[papabear63]

i have a cobra-138 when i key the might down and i talk into my mike the power meter neddle gos to the left not the right and the modulation is not were it should be when i talk into the mike so what the problem and could i fix the problem myself

[Sandbagger]

Nov 11, 2008 1:31:02 GMT -5 papabear63 said:

i have a cobra-138 when i key the might down and i talk into my mike the power meter neddle gos to the left not the right and the modulation is not were it should be when i talk into the mike so what the problem and could i fix the problem myself

When you say the meter "goes to the left", what do you mean exactly? Are you saying it drops to zero when you key after having a strong signal on receive, thereby making it look like it "goes to the left"? Or does the meter jump up on dead key but back swing when you talk?

If there is no transmit power, you could have a bad final, or any number of age-related problems (the original Cobra 138 is over 30 years old now), including bad relay contacts. If you have some power but low modulation, there could be an issue with the audio modulator or just a bad misalignment. Does it work ok on SSB?

[2600]

Hmmm.

32-channel Cobra 138.

This one isn't the "A" version with the 4-pin mike plug, is it?

The original (pre-"A") version had a quarter-inch "phone" plug, like old-style stereo headphones, or a stereo guitar cord.

I sold those 33 years ago. Had a problem with sideband. If you turned the SSB limiter too high, the contacts of the relay would get hot. The plastic inside the relay would soften, and the contact points inside the relay would sag against one another. Eventually, the hot and ground sides would touch inside the relay, and blow the fuse.

And if you put a 'guitar-player' fuse (cigarette-pack foil) in the fuse holder, smoke would be the next symptom.

I'd guess that this is why you hear very few of this model still on the air. Sooner or later that relay probably went bad in every one of them ever made.

The relay is made from "unobtanium", and has not been available for at least 25 years. And no, it's not the same as an american-made relay that size. The pin assignments are all different. Sure, they will line up with the holes, but an american-made substitute relay will NOT work. Not unless you cut and jumper the foil pads beneath it.

You really need to put a meter in line with this radio's antenna so you can see what's happening when you key the mike. This radio used an old-technology final and driver transistor. If they blow out, it takes some know-how to substitute one that's new enough to get your hands on.

And if half the crystals have drifted off frequency so far that the trimmer caps won't put them back onto the channel frequency, that's par for the course. Age and use both will cause old crystals to drift and have to be replaced.

Before blaming any crystals for this kind of trouble, be sure to clean channel selector. The moving contacts are silver-plated and tend to get dark and gunky every ten or twenty years.

73