Cobra 142 gtl am power issue

First of all, welcome to the WWDX Forums! There's a great group of people here to chat with, get advice from, and maybe even learn a thing or two, so sit back and enjoy the forums.

I'm going to ask a bit of a silly question, but what kind of meter are you using to measure your output? Does it show PEP? If you're using something that only measures RMS or average power, it can show backswing, especially if you turn up the deadkey wattage.

Let me know what kind of meter you have, that might have something to do with your backswing issue.

~Cheers~

Sounds like a meter issue to me too. The KL-60 will realistically only do 25-35 watts out with 1-4 watts of drive on AM. Try a better, peak reading meter.

ExitThirteen said:

First of all, welcome to the WWDX Forums! There's a great group of people here to chat with, get advice from, and maybe even learn a thing or two, so sit back and enjoy the forums.

I'm going to ask a bit of a silly question, but what kind of meter are you using to measure your output? Does it show PEP? If you're using something that only measures RMS or average power, it can show backswing, especially if you turn up the deadkey wattage.

Let me know what kind of meter you have, that might have something to do with your backswing issue.

~Cheers~

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Good morning, RAYBO here and need some advice,I have a cobra 142 gtl base & do I need to turn dead key wattage down to pair with an amp & to do that it is vr6 correct?

279er said:

Hello everyone. I am new to here so let me start by introducing myself My name is Luke and i am into the hobby for aout 4 months now. Im having some issues with a cobra 142 that I picked up. Ok here is the skinny of if am power has issues. At first I thought it was a driver final issue with radio so i replaced the driver with a 2166 and the final was also checked and good it is the 1969

Here is my issues. With vr6 set at 1w carrier she will swing up to 1-1/2 watt max If I set it at 2 watts i get half watt swing like previous. Now if i set vr6 past 3 watts the power starts to backswing. The max i can set carrier to is 6 watts and with full modulation she swings backwards down to 4 watts.

Modulation is great and ssb works great she swings 15 watts no problems Also i am runnng it on the dc side not thru ac so i dont think its a power supply issue. I was told to check the vco pots to see if that could be a issue but after tinkering with L13 and L14 it made no diff now i am stumped

Any help or suggestions on wha i should be looking for or do would be greatly appriciated. I am no tech but I would like too se if its somthing i can correct before i spend the money to take her in. I just want the radio to run a 1 to 2 watt deadkey and swing up to atleast the legal 4 watt limit lol

thanks in advance. 279er

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On a 142, don't bother going above 4w carrier, it simply ain't got the guts. As for the downward modulation issue, two things come to mind. The less likely is a mismatch between the final and antenna due to someone tinkering with the output coils as reflections can surely cause this, but more likely than that is an improper RF alignment. I will explain.

With a 4w unmodulated carrier, the collector is at about half the supply, call it 6v. With the proper RF drive level, the 6v at the collector is turned into an RF signal varying between near 0v and 6v, because in class C, the RF pulls the collector down on + going RF signals. It looks like this:

Applying modulation, the waveform looks like this:

Let’s say someone screwdrivered the alignment and the RF drive level is weak, now the unmodulated carrier looks something like this:

And if we apply modulation, look what happens:

In the case of a properly tuned radio, The RF waveform can get bigger and smaller with the modulating waveform, but in the screwdrivered case, the RF waveform can get smaller, but due to the insufficient RF drive, it cannot get larger. The net result is a decrease in RF power with modulation, aka, downward modulation.

Evidence of this problem, in extreme cases, might be a higher collector voltage than expected to get the 4w dead key, so give that a try, turn the dead key to 4w then see if the collector voltage is higher than it should be. If so, you need a TX alignment.

EDIT: quoted 3rd post instead of first by accident, corrected that
EDIT2: a similar problem can happen when adding a bunch of channels because American radios have narrow IF transformers and getting outside the passband can produce similar results.

I totally didn't realize I was replying to a decade old post. Sorry @Raybo. Yes, VR6 controls the deadkey.

@brandon7861 - no you did fine, it makes the thread more relevant again.

I liked the way you draw the wave as to show how RF and audio - being impressed one to another - can show "gaps" in the power envelope. We often times relate it as negative swing - which is what it is. A loss caused by RF swamped with Audio and the mixed results are - an Audio wave that has RF characteristics, versus an RF dominate wave - having Audio characteristics imposed on it.

A better way to say this would be a Audio wave with not enough RF - so the other radios listening to and everyone else's equipment tuned to that RF Spectrum the radio is (supposedly) on, see this as a negatively swinging radio - more often than not - poorly sounding to everyone's ears.

It may be a Zombie thread to some, but the effort you put into it, makes this "Frankenstein" of a thread - work - still to this day.

Almost every 142 I have ever repaired, aligned, tuned would backswing with the deadkey around 5w. After alignment, I normally set the deadkey to 3w and it will swing forward up to 5.5w in average, and around 12w to 15w peak.

I would say old dried up caps may be the issue. Or it could be that weak flimsy passthrough regulator they put in these radios. Typically it's either a 1419 or 1135, I always replace those with 6487G's.

Make sure the driver and final bias is adjusted correctly also.

Don't put any swing mods in a 142, they don't need it, 142's are great modulating radios when aligned correctly. And definitely don't remove or disable the limiters. The limiters will prevent your negative peaks from going over 100%.

Just my opinions....
Just thought I'd throw this in this post.

brandon7861 said:

I totally didn't realize I was replying to a decade old post. Sorry @Raybo. Yes, VR6 controls the deadkey.

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Lol thanks for your help I'll try it