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Cobra 29 ltd classic not transmit


Jan 21, 2018
I'm sure this has been brought up before but I can't seem to find an answer, I am trying to learn how to work on CBs, I have a stock cobra 29 ltd, I was going to try to align it but in the process I lost output on TX, the tx light changes to red when the mic is keyed but that's it, I have a Dosy meter, a signal generator, a frequency counter and oscilloscope which I am trying to learn..any help would be great..

most likely, you tweaked one of the TX mixer cans to the point where it wouldn't pass enough RF for the next stage, thus killing the output.

first thing is to do the PLL alignment with your scope, peaking those cans like the manual says.

then use your scope on the output of each TX coil. start with L23, and peak the coil in TX mode for max indication on your scope.

then move on to L21, then L20, then L17, and then L14.

the reason to do it that way is because you may have one coil so out of whack that no matter the tweaking you do on another coil upstream, you can't see an indication on the meter.

refer to this schematic:

i will give you the points to hook the scope probe to since you are new to this, but setting the scope properly to see the output is up to you. there are many places online to show you this.

for L23, connect the probe to pin 4 of IC3.

for L21, connect the probe to pin 7 of IC3.

for L20, connect the probe to the junction of R61, C85, and TR16.

for L17, connect the probe to the junction of R58, C155, and L17.

for L14, connect the probe to the junction of L14, L13, and C59.

i have a feeling that it's either L20 or L17 that is the culprit, but its just that, a hunch.

if you get to a coil that doesnt seem to respond at all when you turn it, stop there and post back here.

yes, there is a chance that you smoked the driver or final, but we can check that next.

there are much more knowledgeable people here on this forum than me, so if you get other advice that tells you to go in a different direction, their advice should be heeded also.
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i'm sure you can see how that answer doesn't leave me too much to go off of.

all i can tell you is that the more detail you put into your responses, the better help you are going to get.

this is because you may not know that the little side note you make about something might be the smoking gun to another person reading it.

take us through your alignment process, from start to where you lose the ability to tune something.
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SteveM, you need to start at the very 1st of the alignment then go through each step as Loosecannon told you to do. do not worry about output until you do each step then you will have to post what point you can not align. sounds like you are trying to do something you do not understand in the 1st place. if the radio was working before you started you have just turned one can out of spec and have lost every thing.
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Sonoma, your right about not really understanding, I'm just learning and trying to get figure it all out slowly, so far the most difficult part is understanding how to set up the scope.

It may also help to know if any previous work was done to that radio - but we'll be here to offer ...

When it comes to the 29 you need to be aware that there were several reworks of sections - including how the RX and TX align together.

To help, locate an old radio - and use it as a means to produce RF to get a signal that the radio you're working on, can RECEIVE on that radio you're working in - as a means to see if the Receiver side can get aligned to a point that the TX side can start to fire and at least get you some results you can listen for on that older radio.

Just be sure to make up several dummy loads for this because as you work thru, you'll need the RF output of one to let the other "sense" it and tune accordingly.

When stuff like this happens usually the misalignment comes from where the PLL has a SHARED output that goes to both the TX and RX so it can discern and tune - via image thru heterodyne - what channel you are tuned to.

That "shared" coil is usually L23/L19 - that gives the RX an Intermediate Frequency (IF) that the radio can parse down the 27MHz (or thereabouts) signal and develop a lower frequency (in MHz) secondary signal that the radio can process more efficiently.

It (L19) also gives that long narrow vertically mounted chip (7310) IC2 one side of the RF and L23 provides the other missing half for TX - that when combined with L19/L18 output - form the 27MHz TX RF carrier that pipes to the TX strip and it's output is L20.

L19 is usually the "wax" covered coil can that works with the PLLs L22 peaking coil for RX side of things - L18 is their peaking coil for RX out - L23 is to peak out the radios "delta tune RX" side - 10.240MHz...

Once you get an "RX heartbeat", and can see S-meter signal on the radio you're trying to retune - it's a good indicator that you have the radio's PLL pretty close to alignment that you can then focus on the OUTPUT strip (the TX stuff - INTO A DUMMY LOAD) and it'll "snap" into place when the strip has enough RF passing down the chain to get the strip to turn on the transistors and start to amplify within the stages.

Start with L20 and work thru the range - listening on that older dummy loaded radio for a receive (same channel ok? Just use channel 20 it's in the middle of the tuning range so and see if the hiss volume starts to rise and peak out - then you've found that output level for L20 - doesn't always mean that the others further down the strip will be aligned enough to make that strip fire...so then you work back from L20 to L21 and L17 - L21 is a loopback to IC3 (another 7310 for TX mixer) and is a twin coil - so for best results here's what Cobra suggests...

Start on Channel 20...
Tune L22 using Oscope for max reading on R85...
(Bounce between Channel 1 and 40 to equalize/maximize this reading)
Use DC Voltmeter on R88 tune to Channel 40 - see if you get 3.2 Volts (aprox)
(If PLL doesn't lock or start to fire, R88 will show 0 volts - nothing to check)
Adjust L19 to bring R88 to 3.2 volts

Remember that RX check?

Adjust L18 to peak RX out for receiver on channel 20

So as you "align" listen to the radio you're working on - it's receive, thru the speaker to listen for a hiss or static as you backtrack and tweak the coils back to re-align and the PLL starts to fire and get IF signals to the RX side to make the receiver at least sense something.

Then L20 is the peaking out for TX - L17 and L21 are part of this section too...

L23 is the PLL's supply of the 2nd half - so it needs to be peaked too...
(but this one is the one that in TX mode, you start to listen for on that receiver radio)

I'd leave L20 at about mid-point for now and see it tweaking L17, L21 together (all three GENTLY bottomed out and turned 1/4 turn each check and see if the TX strip starts to fire with L17 as the first one to be backed out each time and follow to the right L21's left side then L21's right side) L23 at this time can be TWEAKED SLIGHLTY to help with developing output on L20 (Channel 1 and 40 are checked here in this step for bandwidth and are tuned to equalize the power level ok?)

Once it starts to TX on the RF meter you're using in-line with a dummy load on that radio ... look for and re-tweak L20 to start maximizing it's output at this time - that way you know you've got partial alignment and L20 is one of the better ones to "zero-in on" when it comes to TX RF strip power...THEN LEAVE IT ALONE except for final tweaking and equalization of the power along the channel spread (1 thru 40)

I hope the steps above help you...it comes from the repair / service manual for the LTD model...

So in light of the above, get the thing to RX and peak out RX side as best as you can then focus on TX - so if it don't TX more than likely it won't RX very well either...

:+> Andy <+:
Loosecannon, I'm going off the alignment procedure I downloaded from cbtricks and I started with the PLL alignment and that seemed straight forward then I followed the steps you provided but as I adjust each can I'm not seeing much change on the scope and the reading fluctuates quite a bit, I hope that helps, I am still learning..
Sonoma, your right about not really understanding, I'm just learning and trying to get figure it all out slowly, so far the most difficult part is understanding how to set up the scope.

Well, let's start with a list of the equipment you have and the actual brand and model numbers. This will help us all maybe help you. And welcome to the forum! And although it was over 55 years ago I to at one time had no idea how to adjust anything now the problem I have is remembering.


A simple guideline to help you know what and where you're going...

Cobras' 29's?

PLL develops IF of 16~17MHz (using - a general, tunable feedback loop centered around 17MHz and tweaked by adjusting frequency of oscillation using a Varactor via control signal - which is sent to the PLL and "compared" - any errors? Control Signal is modified and Varactor responds) PLL develops timing reference using 10.240 Xtal that's sent in via L23

L22, L19 and L18 - RX 16~17MHz (1st IF stage stuff to develop an IMAGE of 27MHz signal from RF amp - to produce a 10.7MHz IMAGE (27MHz - 17MHz = 10Mhz generalized)

1st IF (tuning) Handled by IC2

L23, is 10.240MHz (Delta Tune section) applied to that 10.240MHz IMAGE - to develop ANOTHER IMAGE (10.7MHz - 10.240MHz = 460kHz (455MHz IMAGE)) L24 just fine tunes this...

1st RF (From PLL) and 10.240 injection to make TX? - Handled by IC3

L17 takes in L20s' output from IC 3. To obtain 27MHz, two IF signals are combined from L18 and L23 - any mixing by-products are removed thru L21 - TX RF is PEAKED AND SPURIOUS STUFF IS REMOVED by L21's "twin coil" bandpass peaking coil feature - L21 is a BANDPASS coil (takes in a range of frequencies and sharpens their response curve and removes artifacts ABOVE and BELOW this determined range of frequencies) then AMPLIFIED by IC3 and peaked using L20.

455kHz? An RX offset - for RX only. It's Processed and amplified then sent to Detector and the rest is history...

The radio uses two signals one about 16~17MHz and the other about 10.240MHz
combined to form ~27MHz for TX - same process is used in reverse save for an extra IF stage step of 455kHz for further emphasis and processing for AM detection...


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Stevem, will you show us the alignment procedure you are using?

There is one that is online that says its for the 29 but is actually for the 25 and i want to see if this is the one you are using.

If you are having trouble with the scope, look up "rf probe" and build one. Very simple circuit.
You can attach it to your voltmeter and use it instead of the scope in order to peak the cans i listed.

Just like the others are saying its important to start with the pll alignment and move one step at a time.

What is the very first thing you are trying to do?

Just focus on L22 peaking for Channel 20...then you're really done with the O-scope for now...

Check DC volts for Varactor tuning - look, clip, hook, or probe tip at R88 for ~3.2V on Channel 40 - adjust L19 for the reading...you don't need to TX either...you can use the RF weak signal coming from that dummy loaded radio TX'ing it's heart out for you if you need to verify that Channel 40 or any channel your workhorse mule dummy load radio is on...when the PLL locks, L19 just needs to be "tweaked" so it can lock into the range of tuning the varactor is working with on the PLL. Once the PLL locks onto the channel frequency even if it's a little off - you'll get some noise out of the speaker, just make sure Squelch control is off, NB and ANL are off and the Channel 9 or 19 switch is not on.

Note - some radios you can't get a stable reading at the ends of their tuning range bands - the PLL sense and lock signal is so unstable at times - like cold mornings - so when you and your radios are at room temperature - you are in a stable environment and all the PLL is gonna' try and do is lock the Varactor thru L19 and L22 into a stable loop ... L23 is from that Xtal so if that's screwed up - you're really in a world of hurt because L23 passes signal, just not a lot of it at it's extreme ends (top of bobbin - bottom of bobbin) so you don't lose the PLL unless the L23 has failed - did you break the slug? If so there's a bigger problem...but if you didn't - let's move on...IF you're not sure - just set L23's slug at mid-point of turns in the coil form ...

Once those two L22 and L19 are done LOCK AND LOAD on these - because L23 and L18 come into alignment when you tune for RX - they align themselves as you try to get a receive back into the radio...You do not need to go back to L19 or L22 for any reason - they give the RX side the 1st IF signal needed so the PLL can function - L23 just "peaks out" the 2nd IF - L18? Peaks out the 1st IF for your RX.

Then it's a simple process of RX peaking and broadbanding (channel 1 to Channel 40) via L2, L3, L4, L5, L6, L7 L8 and L9. What about - L1? That's NB - for best results leave NB off but turn ANL on once Signal and S-meter indication is noticeable and workable then use the Work-mule radio for signal on channel 1 (tune those RX cans) and Channel 40 (tuning those RX cans) ... see how this comes together - if you've done this right...

A trick I use ...
Once the Radio has settled in and is TX-ing and RX-in like it should - you may notice that the channels signal in the receiver level is not quite equal - you can retweak by adjusting L2 and L3 - by setting L18 to the bottom of it's core and then use your dummy loaded radio on channel 1 and re-tune L2 and L3 - note which coil really peaks out on this channel versus channel 40 - do the same to these two on channel 40, again looking for peaking favorite - you'll find a good range of "offset tuning" between these two channels of peaking set the one coil that favors or peaks more toward channel 1 and set the other that peaks better towards channel 40 - there's your "spread" and means to equalize signal strength between all channels for best results.

In many ways, that older radio on the dummy load is your signal generator and all you need is a simple test jig for antenna on the SO-239 jack - even a spare test probe with a banana plug works - just stick it in there and listen for noise and signal from that dummy loaded radio and the channel it's TX'ing on...

TX is just tweaks to L20, L21 (twin slug) and L23 and L17.

L24 adjusts Delta Tune...

You Do Not need to make this harder than it is...

You'd be surprised as to how quickly you can learn to tune a radio by ear and layout pattern, so hang in there - we are working right alongside you...

:+> Andy <+:

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