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Cobra 29 LTD classic output power

Ocean cruiser

Active Member
May 20, 2019
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Hello, I’ve noticed that my cobra 29 puts out a little more watts on channel 40 then channel 1. Also channel 40 draws more amps then channel 1. What do I need to do to balance this out? Also on the service manual, the pll alignment section says to check the voltage output but it doesn’t state where and how to take the measurements. Any help would be appreciated. Thank you
 

Use this if you want...
upload_2021-4-7_16-19-44.png
Belongs to a Cobra 29 - but one thing I do want to caution you on.

Your meter reading, you might want to double check for the existence of a tuning trap set up for 52MHz (2nd harmonic) - if it's been tweaked the wrong way - the rise in power may signal your radio may be producing a spurry - a 2nd signal - as a harmonic of the first (27MHz x 2) or 52-54MHz (TV Lo-Band).

Cobra29TXStripBipolar.png

You may need just to retweak and balance out the L11 thru L21 mess back there.

The Harmonic trap - if it's there; is L11 and C56, by the antenna connector - that's the tuning slug - it's a "notch" filter so it should not affect 27MHz - if you find it does - check the cap it series tunes with. May be it needs a value range up or down to re-center the notch frequency that filter is made for, and help you balance the power.
 
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Hello, I’ve noticed that my cobra 29 puts out a little more watts on channel 40 then channel 1.

If you're seeing this difference connected to an antenna, the wattmeter may be the root of the different readings.

If you were using a dummy load, it's SWR would be exactly the same on channel 1 and 40. Should show the same power that way, more or less.

But the SWR on an antenna changes when you change frequency. Will typically be different on channel 1 and 40. Small changes in SWR will affect the accuracy of a wattmeter reading.

Could be the radio has no problem at all, if it were to show equal power transmitting into a dummy load.

73
 
Use this if you want...
View attachment 43957
Belongs to a Cobra 29 - but one thing I do want to caution you on.

Your meter reading, you might want to double check for the existence of a tuning trap set up for 52MHz (2nd harmonic) - if it's been tweaked the wrong way - the rise in power may signal your radio may be producing a spurry - a 2nd signal - as a harmonic of the first (27MHz x 2) or 52-54MHz (TV Lo-Band).


You may need just to retweak and balance out the L11 thru L21 mess back there.

The Harmonic trap - if it's there; is L11 and C56, by the antenna connector - that's the tuning slug - it's a "notch" filter so it should not affect 27MHz - if you find it does - check the cap it series tunes with. May be it needs a value range up or down to re-center the notch frequency that filter is made for, and help you balance the power.
Thanks Andy. Really appreciate the info.
 
If you're seeing this difference connected to an antenna, the wattmeter may be the root of the different readings.

If you were using a dummy load, it's SWR would be exactly the same on channel 1 and 40. Should show the same power that way, more or less.

But the SWR on an antenna changes when you change frequency. Will typically be different on channel 1 and 40. Small changes in SWR will affect the accuracy of a wattmeter reading.

Could be the radio has no problem at all, if it were to show equal power transmitting into a dummy load.

73
It is into a dummy load. Thanks for the reply.
 
Use this if you want...
View attachment 43957
Belongs to a Cobra 29 - but one thing I do want to caution you on.

Your meter reading, you might want to double check for the existence of a tuning trap set up for 52MHz (2nd harmonic) - if it's been tweaked the wrong way - the rise in power may signal your radio may be producing a spurry - a 2nd signal - as a harmonic of the first (27MHz x 2) or 52-54MHz (TV Lo-Band).


You may need just to retweak and balance out the L11 thru L21 mess back there.

The Harmonic trap - if it's there; is L11 and C56, by the antenna connector - that's the tuning slug - it's a "notch" filter so it should not affect 27MHz - if you find it does - check the cap it series tunes with. May be it needs a value range up or down to re-center the notch frequency that filter is made for, and help you balance the power.
Hey Andy, thanks again for the info. I have another question. So on rx I noticed that on ch 1 the amp draw is .19 and on ch 40 it’s .30. What’s causing this, is it normal and how do I fix it if it’s not? Lmk please. Thanks
 
This varies, - you're saying RX, so again, look for various culprits, Meter LED segments onto the Varactor voltage and the effort the PLL is making in keeping time in in the local oscillator.

Not much to worry about unless you didn't say something more about speaker, noise or other activity.

You bring up a good point though, some Local and VCO sections of Cobras, fail due to a resonance "ringing" effect caused by poor component quality - it can sink a lot of current just trying to hold the channels' frequency. You'd see it as a rise, even smell - from the heat the current draw makes on the VCO section.

In this case, I think the current sinking the LEDS (Channel display) are causing is more of the issue than anything else.

If it's an older chassis - this heating will be noticeable - may indicate aged components even mean a Recap
 
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This varies, - you're saying RX, so again, look for various culprits, Meter LED segments onto the Varactor voltage and the effort the PLL is making in keeping time in in the local oscillator.

Not much to worry about unless you didn't say something more about speaker, noise or other activity.

You bring up a good point though, some Local and VCO sections of Cobras, fail due to a resonance "ringing" effect caused by poor component quality - it can sink a lot of current just trying to hold the channels' frequency. You'd see it as a rise, even smell - from the heat the current draw makes on the VCO section.

In this case, I think the current sinking the LEDS (Channel display) are causing is more of the issue than anything else.

If it's an older chassis - this heating will be noticeable - may indicate aged components even mean a Recap
Got it. Thank you very much.
 
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