Aligning Cobra 29, stuck on the TX part

[TruckerKevin]

This old beater cobra 29 Ltd classic is just a radio I use for experiments that I’ve had around forever. It’s not about fixing it so much as it is learning what I’m doing. I have spend the last 2 mornings practicing how to do alignments, and I figured out a couple tough problems without asking for any help. I was rather proud of myself that I got it receiving hella good now, set the bias voltage that was way off and then got it back on frequency. Unfortunately now I’m at a stand still.

I got to the transmitter alignment part; according to the service manual, it said connect the scope to r61 and set the cores of L17 and L20 at the bottom and adjust L32 and L21 for a max. I assumed this meant to bottom out L17 and L20. I did this (took them all the way down) and noticed when I keyed up I had zero modulation. No surprise there. L23 and L21 never reacted on the scope at all for some odd reason like the manual said, so I moved on and brought L17 and L20 back up BUT played hell trying to get my modulation back. This radio was keying around 2 watts and swinging to around 7 before I got my palms on it, now it barely moves the meter on key up and barely swings at all. I see it moving as little on the scope when I key up but it does not move my external meter hardly at all. And this all happened after I moved those 2 pots according to the service manual.

I have been adjusting these pots relentlessly since. At first the meter was halfway up on dead key and actually swinging backwards. Then doing nothing at all. Moving back and forth between L17 and L20 I finally got my dead key to 1 watt or so and about 2 watts if I’m lucky on swing. At least it is now better than doing nothing at all or swinging backwards like it was.

What did I do to cause this? And equally as important, what do I need to do to get modulation back? Anyone have any ideas on what I did wrong? And its okay to call me dumb, I’m trying to learn and can take it like a man

 

[Handy Andy]

It's the "Bottom out" L17 and L20 part that gets me???

Did you happen to take pics of the Radio BEFORE you did this alignment?

Because, a photo or two of those slugs can help you get that radio working right again, quicker than by trying to revert this thing back... All you'd have to do is go back and review the photo to tweak those cans, pots and what-nots - back to the original spots

So unless you "counted the number of turns" you needed to turn those slugs to seat them (bottom out) there's the long process of tweaking those slugs back into place.

So in light of the above...which "service manual" did you use?

You may need to restart...

I've done up a Cobra 29 alignment "field guide" to help those that don't have scopes and frequency analyzers - you can find that here...

https://www.worldwidedx.com/threads/cobra-29-ltd-classic-no-rx-tx-help.253718/

And there are others too...

https://www.worldwidedx.com/threads/need-help-with-side-mic-cobra-29-ltd-classic.251271/

https://www.worldwidedx.com/threads/cobra-29-ltd-classic-not-transmit.228821/

There is more - I could go on, but if you read thru those threads, this may help you find a method YOU can develop from all of this - to get that radio back on track and working.

 

[TruckerKevin]

Hi Andy and thanks for the reply

I have a spare 29 that was in my parts box. I counted the turns in on L17 and L20, then reproduced them to my radio in hopes to get it back close, but to no avail.

I am going to review those links you gave me right now

 

[Handy Andy]

On a separate note,

You said you "retuned" L21...Are you referring to this coil?

L21 is the DUAL slug coil on the topmost side of this pic...

Be patient with tuning of this...
Both both are tuned together in small increments to each other to "max out" one side of the channels then you tweak both to max out the other side of the channels - then average out between in the middle at channel 20
​

 

[TruckerKevin]

That makes sense, I probably did not do the dual coil right. I think you found my screw up. Yes you are correct, that is the coil it said to tune when the others are bottomed.

Here is the particular manual

http://www.cbtricks.com/radios/cobr...st/graphics/29ltd-st_29wx-st_sm_pg12_pg34.pdf

 

[Shadetree Mechanic]

I think there is a misprint in the Cobra 29 book. I don't know exactly what it was though, I only remember hearing about it.

 

[TruckerKevin]

I think I found that thread that you are referring to about that. They give test points but it turns out to be wrong. I had a heck of a time finding a correct service manual for this to do the alignment. It’s only the most common CB in the world.

 

[Dmans]

It’s only the most common CB in the world.

All the more reason for people to publish their "own version" of the alignment procedure.

73
David

 

[Handy Andy]

That makes sense, I probably did not do the dual coil right. I think you found my screw up. Yes you are correct, that is the coil it said to tune when the others are bottomed.

Here is the particular manual

http://www.cbtricks.com/radios/cobr...st/graphics/29ltd-st_29wx-st_sm_pg12_pg34.pdf

I'm well aware of that one as well as how it works a particular coil, L21, being the "key" to making the TX mixer "mesh" the Loop frequency that is clocked to the PLL, against the 16MHz that the "Varactor-side" puts into it.

Between the two - you get a set of frequencies both above and below the main fundamentals.

You're trying to get 27MHz, but also remember that your PLL shifts it's output gear to remove the 455kHz shift it uses for the 2nd (or last) IF stage to get a RX to work.

L21 has to mesh the 16MHz output from L19/Varactor with the 10.240 Xtal from L23 - together and remove any and all spurries - so the mixing products are kept to within the bandpass of L21 - it's why it has two sets of coils to allow the BPF it is, a range or room to allow the 27MHz - but keep out the 16MHz and 10MHz and all their offspring from getting amplified and put into the output that goes to the Pre-Driver.

Since L21 works only with 27MHz it's the unique "kid" on the block that requires a special trick to tuning and you work it's output to build power into L20 the actual 27MHz that goes to the Pre-driver.

So, really, L19 and L23 are "peaked" but you don't want those "spurries" damaging further down the TX stip, so they tell you to "detune" L17 and L20 - this allows you to "see" the 27MHz the strip is tuned for.

L17 just keeps power levels low - attenuated - so you can see and focus on the main power band the TX mixer is producing. You're complexity arrives from looking or using the older "Standard" equipment that now has been replaced with a spectrum analyzer which is the BEST tool for this cleaning up of details, but is not REQUIRED as the engineers are not making this as a broadband mess some think these things really are.

Once you have the L21 peaked to obtain the most power possible in the "skirt" or band pass frequency range - the L20 and L17 can be brought to full peak - then the rest of the Strip is designed not just for 27MHz, but to keep the other fundamentals from getting re-mixed back into it.

 

[Handy Andy]

Heres something I just did up today, hope it's something you can use as a guide.

This is from recollection of memory as well as my own notes, so this tends to follow a little more stricter guidelines, but when your Boss pays your your check that helps pay YOUR bills, it tends to be an accord many techs follow so they can continue employment - for if the Boss looks bad, so do you - and that means you might....well enuff' said.

 

[TruckerKevin]

I actually ******* got it finally. I discovered that L20 was seized up and my plastic alignment tool was not adjusting it like I thought. I sprayed some deoxit down in there then some electronic lube, let it sit overnight still could not get it. Switched to a metal screwdriver to see if I could get it unfrozen,,,, no go.

I desoldered a good pot off a junk radio in my pile and put it in its stead. Spent about another 20 minutes adjusting them, did it as I was supposed to. Detuned L17 and L20, tuned L21 together until they both peaked. Then tuned BOTH D17 and 20 (despite the manual saying only do 20) and got it just about dialed in.

Thanks Andy. This one about costed me my religion. I even gave up at one point and threw it in the junk box, only to dig it back out

 

[Handy Andy]

This one about costed me my religion. I even gave up at one point and threw it in the junk box, only to dig it back out

You're not alone!

Thanks for sticking with it!

Another radio pulled from the Landfill...


Had to Fix that Graphic from earlier - this is the REVISED one fixed L24 tuning section

​

 

[Squints]

I have a cobra 29 classic thats constantly transmitting on its own and it wasnt forced nor jarred in any way it just suddenly went to transmit without warning it has nothing to do with the mic orthe mic jack on the radio
The light indicater shows its still in rx mode but the needle is burried and lights are slightly dim and its showing dead key on watt meter.
I been through the whole radio looking for anything shorted or grounded out no twisted componant legs mic jack is disconnected as well.
Why ?

 

[Handy Andy]

By your word the TX light is on, even when the Mic handset is not plugged in?

You'll need to take a look at the TX Circuit.

Firstly - with the Radio off - Locate Pin 1 and Pin 3 of the MIC jack...
Do an ohmic - continuity check between BOTH pins - one pin one lead, the other pin the other lead of the tester - and what is the reading?

​

Usually in these cases it is something with assembly - like the socket is failing - jack side - not the Mic side. IT may have twisted and now a filter cap lead is striking the case forcing it to ground or a wire has broken loose, and it touches ground or case. You may not always see it.

You might need to post pics so we can guide you better...

 

[Handy Andy]

Then re-reading some more...

If you have lights dimming - like it's in transmit all the time, you won't see carrier or hear it on 27MHz but it can send IF as a "carrier" because of a short there's just nothing tuned by you to hear it.

It's the 10.240 side of things...

​

- could be from the case to wires or a components' BARE LEAD pressed against along the side panel
- they can pinch and short out or the Pre-driver in the TX side

- has a short jumper wire from where it's at by the heatsink the driver and Final mount to
- and heads towards where the Mic socket is at
- it "grounds" when you TX - so locate that pre-driver and make sure you don't have any components shorting out or pressed towards each other to cause a short.​

Another 29 that shows this "harness" and Loom wiring better - you can see there is a possibility that the case can pinch a wire - or two and cause a short...

See the Yellow wire by that rear heat sink panel?
That is the RX./ TX switch - it uses Mic jacks' Pin 3
See those cans that have "yellow" towards the left side in the above photo?
That is the TX side - the Driver is behind there, mounted to that screw on the rear heat sink panel.​