29 WX NW ST No TX and No RX; Need Help Please

[BuckSkin]

Hi.
My first post here.

I have a Cobra 29 WX NW ST (in fact I have two of them) that I bought brand-new about three years ago.
It powers up and lights up (except the nightwatch; alas it quit on both radios soon after they were new --- it was cool while it lasted)
The weather stations come in loud and clear.
The PA speaker works good.
The onboard speaker works.
The external speaker works.
The mike works fine on a different radio.

When I key the mike, the green light turns red, but no meter movement and no transmit.
Squelch all the way counter-clockwise and RF Gain all the way clock-wise yields no noise whatsoever at the speaker.
A helper in the same driveway, in a truck with an excellent radio, hears nothing when I try to transmit.
When she tries to talk to me, I hear nothing.

I put the other identical working radio in this truck, connected it to all the wires and cables that had been connected to the radio that is not working, and it works fine; so, the problem is definitely in the radio and not the hook-up.

I am no radio technician; however, I have used a soldering iron and a soldering gun quite a bit; I am well versed in normal vehicle electrics and am quite handy at just about anything; that being said, I have never worked on the insides of a CB radio.

The only testing tools I have are a DVM, a test-light, a 12-volt power source, and common sense.

What is wrong with my radio and what do I do to fix it ?

Do repair and modification instructions I find for Cobra 29 LTD Classic apply to my 29 WX NW ST ?

Any help is appreciated and thank you for reading.

 

[Robb]

The mike works fine on a different radio.

Explain please.

 

[BuckSkin]

Explain please.

To eliminate the possibility that the mike was causing the problem, I tried it on another Cobra and it worked fine; I also tried a known good mike on the problem radio to no avail.

 

[nomadradio]

Gotta give credit where credit's due.

You did try the mike on another radio, so we know that the problem is with the radio, and not with the mike.

But you have a radio that won't receive another radio nearby, and won't transmit.

I'm a bit surprised that the green LED turns red when you key the mike on the broken radio. As a rule, when a Cobra 29 shuts down like this you won't see the LED change when the mike is keyed.

But that assumes the radio has just one single problem that would shut down both.

Could be that one problem has shut down the receiver, and a completely separate fault has shut down the transmitter. Most-common failure that shuts down both at once is for the PLL section of the radio that sets the channel frequency to lose its "lock" on the legal channel frequency. FCC rules dictate that an automatic shutdown circuit must sense this condition and disable the transmitter to prevent you from transmitting outside the legal 40 channel frequencies. A PLL failure frequently also shuts down the receiver, as well.

But your LED turns red when you key the mike. Far as I know, the PLL must be properly locked to the selected channel for that to happen.

Is there any chance that the root of the problem is a "golden screwdriver"? Someone who keyed the mike, focused his tunnel vision onto the wattmeter and proceeded to turn every adjustment in the radio?

Unless you have some history of how the radio went from working to not working, this is a possibility. A description of how the radio went from working to not working can eliminate a lot of wasted troubleshooting effort.

It's a little bit like telling a mechanic "my car won't start". If you also mention that the high-school motorhead down the street scrambled your spark-plug wires all out of order, it would save him a few wild geese that he won't need to chase figuring out the solution to your problem.

Just cranking one of the many adjustments inside too far can shut things down.

And if more than one of them is adjusted wrong, your tool set is probably not sufficient to bring it back to life. The tiny gray tuning slugs inside the small metal cans with a hole in the top are small and brittle. Metal tool tips are not appropriate to turn those for a couple of reasons.

73

 

[sonoma]

with the problems you have the 1st thing I would would see if there is voltage to the PLL and it is in lock. at least if you have a 10.240 reading at the crystal, but since you only have a DVM what you can do is limited.
you might just touch up some of the solder joints in the PLL and VCO areas and see if just maybe it could be a bad solder joint causing your problem.
with out proper equipment your trial to repair the radio is very limited.

 

[nomadradio]

A car with an older motor, using a distributor and vacuum advance can be disabled by simply twisting the distributor far enough out of time.

But getting it back to the proper adjustment will require a timing light.

Might be able to get it to run without that tool, but you'll definitely wish for it.

Never have learned to find or fix PLL problems with a meter alone. Figured out around 1975 that it takes a 'scope if that section of the radio has problems.

But since your LED on the face of the radio turns red when you key the mike, I still think that the radio's PLL is working okay.

Just knowing the history of the problem can be as valuable as a whole bench full of lab gear like you see on "Mike's Radio Repair" YT vids.

And without any history of how the failure took place, you may need every one of those fancy gadgets to make it work again.

I haven't used the word "sabotage" yet, but if the radio has been tweaked off the air, you'll find that fixing sabotage tends to take the fanciest tools to remedy.

73

 

[BuckSkin]

with the problems you have the 1st thing I would would see if there is voltage to the PLL and it is in lock.

I haven't used the word "sabotage" yet, but if the radio has been tweaked off the air, you'll find that fixing sabotage tends to take the fanciest tools to remedy.

73

History of the radio is I bought in brand-new in a sealed box about three years ago.
Since I have owned it the cover has never been off the radio.
It was not bought as "peaked and tuned"; it was bought on Amazon; I got two of them at the same time.
As for why and when it quit working, I just realized that I was meeting a lot of trucks and not hearing any of the usual chatter and when I would see someone I wanted to say something to, I thought they either had their radio turned off or were just ignoring me.
After a bit of this, I realized that there was no static or noise; so, I had a helper in a different truck in the same driveway see if they could hear me say anything and also see if I could hear them say anything.

There was no catastrophic event that one minute it was working and then BOOM it quit; I just realized something was not right.

Thanks so much for taking of your time to help me.

There isn't a genuine radio fix-it shop within a hundred miles and if there are any radio hobbyists anywhere close, they are keeping it a secret; so, I hope to be able to fix it myself.

 

[loosecannon]

this may sound like a weird test to do, but do you have the ability to measure the current flowing into the radio?

If your DVM measures current on a scale that goes to 10 or 20 amps, then you can do this test.

put the leads in the correct holes of the meter for doing current measurements (can be different than the ones used for voltage and resistance measurements).

remember that current is read in series, and voltage is read in parallel.

this means that you have to disconnect the red (positive) wire from its voltage source (probably your battery or fuse block) and put the meter in series with it. (one lead to the source, the other to the red wire going into the radio)

now, with radio on, take note of how much current is being drawn. (will be less than one amp)

now key the mic, and take note of the current being drawn. (should be upwards of 2 amps in a normally working radio)

this won't necessarily lead us to the problem, but it will allow us to start painting a picture.

heck, you could have a shorted final and a bad component in the receiver.

LC

 

[BuckSkin]

this may sound like a weird test to do, but do you have the ability to measure the current flowing into the radio?

If your DVM measures current on a scale that goes to 10 or 20 amps, then you can do this test.

put the leads in the correct holes of the meter for doing current measurements (can be different than the ones used for voltage and resistance measurements).

remember that current is read in series, and voltage is read in parallel.

this means that you have to disconnect the red (positive) wire from its voltage source (probably your battery or fuse block) and put the meter in series with it. (one lead to the source, the other to the red wire going into the radio)

now, with radio on, take note of how much current is being drawn. (will be less than one amp)

now key the mic, and take note of the current being drawn. (should be upwards of 2 amps in a normally working radio)

this won't necessarily lead us to the problem, but it will allow us to start painting a picture.

heck, you could have a shorted final and a bad component in the receiver.

LC

It doesn't sound weird to me as I am totally in the dark.
I can do this on the work-bench with an old magnet-mount antenna connected and a float-charged 12-volt battery, instead of having to put it back in the truck, right ?

I actually have a clamp-on current reading meter besides my regular DVM; maybe it has the capability of measuring current if my DVM were not to have this ability;I will have to check that out.

 

[loosecannon]

that sounds like it will work.
not sure on the clamp on meter, but if it says it can read that range of current, then i don't see why it wouldnt work.

really im just trying to see if the current draw goes up when you key the mic.
LC

 

[BuckSkin]

that sounds like it will work.
not sure on the clamp on meter, but if it says it can read that range of current, then i don't see why it wouldnt work.

really im just trying to see if the current draw goes up when you key the mic.
LC

I will give the test a try, probably some time tomorrow, and come back with the results.

I really appreciate the help.

 

[Robb]

I didn't see anyone post the fact that this may be just as simple as a blown driver transistor . . . did I miss something?

 

[BuckSkin]

now, with radio on, take note of how much current is being drawn. (will be less than one amp)

now key the mic, and take note of the current being drawn. (should be upwards of 2 amps in a normally working radio) LC

With the current running through the meter "in series", --- radio ON = 0.27-amp; when I key the mike, it increases to only 0.36-amp.

I also noticed that, when I unkey the mike, the speaker makes a quick radio static noise, like for maybe a tenth of a split second.

What do I do next ?

 

[loosecannon]

Robb, it's the no receive symptom that is adding to the confusion.

Buckskin, my first guess is that T1 is open, as that is a part that sends audio to the speaker and also sends voltage to the driver and final.

This is assuming that your S-meter moves with incoming signals, but you just cant hear their audio.
if you can confirm with another radio, that your radio does not pick up the signals at all (no RF getting in to the radio) then the problem will be somewhere else.

check the voltage at pin 1 of the audio chip (pin closest to the front of the radio).
you should read the supply voltage here (13.8 volts approx.)

then read the voltages on either side of D8 (big black diode near T1).
post the voltage readings on both the banded and un-banded sides of this diode.
LC

 

[nomadradio]

If what he reports in the first post is correct:

The weather stations come in loud and clear.
The PA speaker works good.
The onboard speaker works.
The external speaker works.

This should scratch T1 from the list.

Says the green LED turns red when he keys the mike.

Unless they changed the design, that red LED should be disabled if the PLL is not locked properly to the selected channel.

In my experience, the above observations make T1 and the PLL wild geese. Not productive to chase.

Somebody needs to write up and photograph the "Digital Manual Signal Injector" method for receiver troubleshooting.

You take a digit (your index finger) and lay it alongside the metal shaft of a small screwdriver. This turns your body into an antenna, and the screwdriver tip into a signal-injection probe.

Touching the tip of your probe to the output of the 455 kHz ceramic filter should get you a LOT of random noise, and maybe a nearby AM radio station if the transmitter is within a couple of miles.

Next, touching it to the input pin of the ceramic filter should get you noise, but a noticeably less.

Next, the input and output pins of the 3-pin 10.695 MHz ceramic filter should work sorta the same way. Lotsa noise on the output pin, less on the input side.

Following the receiver's block diagram upstream from there, touching first the output, and then the input of each stage. is what you do if you get good noise on both ceramic-filter tests.

This is a pretty sketchy test, but it's quick and can point to trouble downstream from those filters if you hear nothing. Or to trouble upstream from them if you DO get a noise level this way.

No expensive signal generator toys required.

But pics with arrows to the proper foil traces would really round this out.

For better or worse, troubleshooting often involves identifying the stuff that's NOT broke, one by one until this leads you to the circuit that is broke.

73