Cobra 29LTD Chrome need Schematic.

[Handy Andy]

down on the bottom of the list here is some info for 2 of the newer radios Andy. see if one of them match up with his radio.

http://downloads.cobra.com/CB/Models/29-SERIES/

Sigh...no luck, this newer chassis is not from the Chicago labs, this is direct from China as in they are taking over that chassis which makes no sense - Dynascan still owns the rights to it. So apparently China (origin) is making changes to this chassis without express consent because the Main office don't know what they are doing or even know of this Encoder and 24-pin PLL chip revision either.

It's painful to admit, but this may be the last of the discrete Dinosaurs that we've enjoyed working on.

 

[Handy Andy]

Andy here a youtube post of the sound If you see the meter its up from the dead key from another radio with modulation just a dead carrier.

Here is the second with just receive if you see the meter jump with the static on the meter

thanks Bob

Some of that appears to be bleedover "crosstalk" for the quieting occurs inbetween keyups - and the jump is from the key and unkey - that's what that sounds like..the one on the bottom #3319.

The one on top though #3320 - should not have any noise and yet it's there - ok this "static" is from the Audio Chain - but to localize it requires to adjust volume all the way down - still hear it at same magnitude? then the problem is between the Volume control and Audio Amp chip or it's feedback components.

Best to keep a Dummy load on the rear antenna jack too - for is the noise disappears from shorting resistor - then you have a bad solder joint in the rear panel - The SWR function - works ok? This little part can easily show if you have mounting and solder issues on the inputs.

Did you reheat - reflux/solder some of the area by the audio chip? The noise certainly there - and I'm surprised those green mylars were not the cause of this...

Now this is a long shot, but are all the POTS Volume RF Gain and those controls - are they soldered good? I know of only two radios with "crackle" noise that had their returns to a spot of the board that was not soldered well and they were a PITA to locate - because the resistor checks were ok, just the noise was from their RF Bypass to a board mounting screw right near it.

Check RF gain control - are you seeing any voltages at all? Check Pot terminal to Tank Can case (Foil ground)

I do see "flinching" on peaks but man if it's in the RF chain Double Check VR1 - it uses a cap to shunt IF signal. It uses carbon composition and although used, it can also be removed and observe the operation of the IF to make sure it's amplifying.

Now, as you dead key, I notice the meter does not "bury in the Cherry" so you lost a lot of gain in this area - like a bad connection or coil wind not making the best of connection.

Just some thoughts...

 

[dss56]

Andy I did try the heat with the mylar caps I scanned the complete board with a microscope. All seem to be good. I also flexed the board from top and from bottom.
I also used my audio tracer to and from the audio chip and both sides are static.

With the keyed up video I tried to adjust the volume and lowering and upping the audio doing this would not eliminate the static it bleeds right through.

thanks Bob

 

[Handy Andy]

Unless I have it in my hands - I've done all I can...

 

[dss56]

I appreciate all you help and on some of my other post your an asset to this site in helping people with your knowledge.
I'm going to spend 1 more shot tomorrow with the radio and then i will be give back to customer.

I'm sure not all radios can be repaired

thanks Bob

 

[Handy Andy]

I'm sure not all radios can be repaired

thanks Bob

Well, there is a difference between "Repair" and Rebuild.

Rebuild is a lot more involved - and one of the issues I've been seeing, are the leads of various components I've replaced or resoldered, were not tinned - and instead were rusted. I find that a lot even in the later model radios.

I do know from shop experience, that the leads whether they were tinned or not, had to be roughed up and burnished to clean off oxides. It's simple prepping for soldering adhesion and wetting. From the "flux" residues I see as of late - it is from their lack of performing such a step that can make or break connections.

We're left to clean up their mess due to it.

 

[dss56]

I agree on that when I buy parts I hate when res and caps are taped and some people sell res and caps that the legs are all oxidized and suck to solder so as you mentioned I do check first.
Example I just bought some vari caps yellow ones and they are a bitch to solder as they are all oxidized and not tinned properly. I worked 37 years for the government doing tech work then supervisor, all parts were bought were mil spec parts and had to go to NASA soldering school every 2 years to be certified.
People selling parts dont think about the end use when they have tape and residue on the leads.

 

[kaos513]

Try deoxit all your contacts.

 

[dss56]

Here is an update to the static.

See picture attached and the vr1 area green mylar caps. I did put some heat in that area with my shrink heat gun and the static stopped and now will not come back for the 4 days I had it static now no static and rf gain works no as it should.

Any suggestion/Ideas on what could it be? I flexed the board both ways and still nice smooth receive no jumping static. Let radio run for 45 minutes and still ok no static.

thanks Bob

 

[Handy Andy]

Those Green Chiclets - I'd replace them with Ceramic Disc or similar value right away. (Don't try to re-use Polystyrenes' or Mylar)

The Mylar's and their "innards" are susceptible to the environment. So if there was a lot of heat or moisture or both - that accelerates their demise. They are sealed with that cheap epoxy that works well if you keep the radio in a suitable environment - like the Home - but little help for Mobile setups with vibration - breaks down the setting of the epoxy resin coating.


The only reason they used these was - they are cheap and good for audio response (has more of a higher frequency roll off than a Ceramic) - but they also can be problematic.

Tap on them again - or just let them sit - after a while they will collect moisture again and you have that same problem.

 

[Chevboy0167]

Hi fellas. Check VR1 with a meter and make sure it changes in a smooth linear fashion. I had a 29LX that had no rx with static only and that crap plastic VR was bad. I didn't have a replacement on hand so I measured a known good VR in another LX and just used regular resistors in the range I needed. Worked perfectly.

Also maybe that 455 filter is on the fritz?

 

[Handy Andy]

Hi fellas. Check VR1 with a meter and make sure it changes in a smooth linear fashion. I had a 29LX that had no rx with static only and that crap plastic VR was bad. I didn't have a replacement on hand so I measured a known good VR in another LX and just used regular resistors in the range I needed. Worked perfectly.

Also maybe that 455 filter is on the fritz?

Good Idea, I had only presumed the OP had checked that - I mentioned something similar in a previous post - note the VR1 rotation "location" nearly full on - which adds a level of hiss that can make the static problem even worse in discerning the sources of noise you're hearing.

Only time, patience and due diligence will tell...

 

[sunbulls]

I agree on that when I buy parts I hate when res and caps are taped and some people sell res and caps that the legs are all oxidized and suck to solder so as you mentioned I do check first.
Example I just bought some vari caps yellow ones and they are a bitch to solder as they are all oxidized and not tinned properly. I worked 37 years for the government doing tech work then supervisor, all parts were bought were mil spec parts and had to go to NASA soldering school every 2 years to be certified.
People selling parts dont think about the end use when they have tape and residue on the leads.

I usually spend an hour scraping leads during a complete recap job. I have a dedicated small pocket knife just for that purpose. I don’t think I ever saw that much lead corrosion back in my Heathkit building days. Must be the waters of Lake Michigan off Benton Harbor are less corrosive than Pacific Ocean salt.