A question regarding initial checkout of a dead radio

[guitar_199]

Greetings to all. I have been a member here for a long time...but have not made much noise. Just a bit of introduction if you don't mind so that you know what you are dealing with!!!!

Back in 1975 - 1977 I went to DeVry Institute of Electronics in Dallas and completed the Associates Level course. Yes, I am an idiot for not finishing the Bachelor program but I got hired by a NASA contractor and it was just too tempting. Over the next 9 years I worked in hardware maintenance on the Space Shuttle flight simulators at NASA JSC. At that point I found a home in operating system software and took a detour to the "dark side" of software development.

Now 63, I find that I am longing to get back to my "electronic roots' more as a hobby than anything. So I do not present myself as a "knows all" about electronics. I am trying to get back into it by having fun going through some of my old CB radios... some of which don't work well by any means.... and others that don't work AT ALL!!! So I am anxious to "listen and learn" and just have some fun. Sorry if it was boring... but I wanted you to know what you were working with!

Now, to the Radio. It is an old Cobra 21 - 40 channel. Just for giggles, I wanted to do a recap... as this radio came form my father in law's collection when he passed away back in 1995. It has not been used since WAY before he died and there were only about 25 ish electrolytic caps in there..... so I figured it was a reasonable thing to do.

Well........ when I finished.... the power light comes on, the meter light lights up.... but that is about all. No audio at ALL... not even noise. (I had a Wilson 1000 hooked up as a temporary antenna/load). When I key... it looks like nothing happens. No meter movement... no real response of any kind.

Wanting to use this as a learning opportunity, it seems that I saw somewhere that, on a dead radio, one of the early things to check are the reference oscillator/VFO to make sure that they are running... AND that they are controlled by the PLL.

I do have a 100 MHz Tektronix scope but do not yet have anything in the way of a freq counter so I am flying a little but blind as of this time. When I scoped the reference oscillator I saw what I believe was a 1024 MHz oscillator running. Based on the timebase setting and observing the scope image.... I know that it was definitely in that neighborhood... but obviously I can not judge the frequency THAT closely.

When I went to scope the VFO I THINK I saw it....because I definitely picked up a high frequency signal (again, maxing the timebase and counting the boxes) it was in the neighborhood of the right frequency....BUT the level was significantly lower than the reference oscillator. With that, I was a little puzzled and stopped for the night. That was some time back in the Spring and I have not hauled it out again yet....but would like to get back to it.

With the story to this point....

1) am I headed a good direction?
2) can I tell enough with just the scope to make this judgement? Or do I really REALLY need to get a frequency counter before moving on?
3) any suggestions on checking out a PLL like this? What my head thinks is that..... IF the VFO is indeed running... it needs to respond to the channel switch. meaning..... there is a control voltage from the PLL to the VFO and, as I change channels, that control voltage should change. Also there is a locked/unlocked line coming out of the PLL that quite probably enables/disables at LEAST transmit circuitry if NOT receive. But I am thinking that I should be poking around in that area to see if the control voltage is working/ and the locked signal is correct, and go from there!

(Note: THIS is part of my issue, I understand the idea/theory... but have absolutely NO experience in the area.)

So, anyway, good to be here and hope that one/several of you might have some ideas or comments which I would appreciate!

Bob

 

[guitar_199]

Just as a "follow on"..... I have been back through every single capacitor changed (again there are not that many!) and:

1) they are all in with the correct polarity, and
2) there are no solder bridges/shorts around the pads.

Just thought I would add this....

 

[Robb]

Are you seeing any signal movement on the S/RF meter?
Can you get mic audio out of the PA?
Do you have the mic plugged in? (it is necessary)
Is the mic wired correctly and the wires in the mic plug properly soldered?

 

[guitar_199]

Each in order:

Are you seeing any signal movement on the S/RF meter?
1) No. Not even a little bit!

Can you get mic audio out of the PA?
2) I was way to caught up in the moment to think of trying the PA function! I do still need to do that!

Do you have the mic plugged in? (it is necessary)
3) yes I did.

Is the mic wired correctly and the wires in the mic plug properly soldered?
4) To the best of my belief, yes. I freshly rewired a stock mic prior to doing this and wired it from the schematic.

Are you seeing any signal movement on the S/RF meter?
Can you get mic audio out of the PA?
Do you have the mic plugged in? (it is necessary)
Is the mic wired correctly and the wires in the mic plug properly soldered?

 

[sunbulls]

If you have another radio that works, use it as a quick test to see if it receives anything when you transmit on your bad radio. Also, transmit on your good radio into a dummy load or antenna to see if your bad radio has any response. Those two quick tests will at least tell you if the PLL and early mixer stages are working. Spray some contact cleaner into your PA switch. A dirty PA switch is common cause of your dead symptom too.

 

[Handy Andy]

Welcome to WWDX forum.

Well, I've been down that road too - still am - looking for an Exit - but while were here...let's trade some notes...

Ok, when a radio first powers up, it has to charge up all those caps you just replaced, on top of that it also has to apply power to places like the Audio amp. That "flat pack chip" with all those pins on it. There are a lot of caps in that area alone, and some rather big ones too. They are that big because they have to squeeze a large volume of capacitance into a small package.

So back to the start up - you should hear a "thunk" from the speaker - that is the Audio amp itself powering up and stabilizing itself so the amplifier in it can work.

Every CB radio produces a "thunk" on one form or another - so in light of the Cobra 21 - if everything is wired up right you should hear a thunk when you turn it on.

IF you don't - then you may have a wiring issue onto something more exotic.

Cobra 21 is a simple AM only radio - and it uses the MIC cord itself - and the handset - to provide switch ground for the speaker. So if you don't hear a thunk - when powering it on, then you may need to look at the Mic and make sure it is for the radio you have. Cobra 21 uses PA so it has switches, the board and the speaker itself - along with the Mic -and all route thru the switches on the front panel to get to the speaker.

But that is only part of the story.

Double check your work - a simple re-cap may have caused a broken speaker wire onto something yet to be discovered...

CBs' use Mics and so they have Jacks, or Sockets - and "mate" with a plug connected to the cord that connects to the Mic handset. If the Cord is wired for a particular radio - in this case a 4-pin wire Cobra Uniden style - if the Jack done the same way?

There is also the issue of the TYPE of Cobra 21 it is. It came in several "flavors" one is a, 21 Xtal type, 21 LTD, 21GTL and 21 Plus - there is a difference. Mics for the 21 Plus were ELECTRETS - meaning they used power from the radio to make the Element in the HANDSET work. But the LTD or GTL - used DYNAMIC - meaning they GENERATED power at the Handset which sent this audio signal power into the radio to be amplified.

So if your Handset is for Electret - and the 21 is a GTL - then it won't work - you'll need to find a Mic with w properly wired and supplied handset to make the radio work properly.

If the radio uses Xtals - the mechanical selector assembly may have corroded - you'd have 23 channels but if the Xtals or the Rotary Switch was bad - there goes the neighborhood.

Another aspect is the "Squelch" control. If that is not working correctly - then no matter what, you wouldn't be able to make the radio do anything except change channels - you wouldn't even hear a thunk. Because the Audio amp is clamped - muted - off - by the squelch control or the PLL telling to stay off because its not in lock and can't make the receive work.

So, mull this over and see if you can fiddle and discover anything that might help us in helping you get that thing on the air.

An external speaker can help - if the internal speaker "froze" due to age and the coil in it is seized to the core - well , an external speaker can at least let us hear a thunk and maybe a "Hiss" afterwards...

 

[guitar_199]

Well I am sure already that it is not transmitting. Well I SAY that.... when I key, the meter doesn't move at all, the TX lamp on the front doesn't light up. That said.... what you suggested certainly wouldn't cost anything to try!

If you have another radio that works, use it as a quick test to see if it receives anything when you transmit on your bad radio. Also, transmit on your good radio into a dummy load or antenna to see if your bad radio has any response. Those two quick tests will at least tell you if the PLL and early mixer stages are working. Spray some contact cleaner into your PA switch. A dirty PA switch is common cause of your dead symptom too.

 

[Tallman]

Change your microphone to one that is known to work, and did you plug the speaker in?

 

[guitar_199]

I'm going to sneak into the quoted message and type responses in BOLD!

Welcome to WWDX forum.

Well, I've been down that road too - still am - looking for an Exit - but while were here...let's trade some notes...

Ok, when a radio first powers up, it has to charge up all those caps you just replaced, on top of that it also has to apply power to places like the Audio amp. That "flat pack chip" with all those pins on it. There are a lot of caps in that area alone, and some rather big ones too. They are that big because they have to squeeze a large volume of capacitance into a small package.

So back to the start up - you should hear a "thunk" from the speaker - that is the Audio amp itself powering up and stabilizing itself so the amplifier in it can work.

Good point, and I can not tell you at this moment whether I heard it or not. Perhaps I can get this thing back out on Friday (after the kitchen is clean, of course!!!! ) and check that out.

Every CB radio produces a "thunk" on one form or another - so in light of the Cobra 21 - if everything is wired up right you should hear a thunk when you turn it on.

Agreed!

IF you don't - then you may have a wiring issue onto something more exotic.

Understood and agreed!

Cobra 21 is a simple AM only radio - and it uses the MIC cord itself - and the handset - to provide switch ground for the speaker. So if you don't hear a thunk - when powering it on, then you may need to look at the Mic and make sure it is for the radio you have. Cobra 21 uses PA so it has switches, the board and the speaker itself - along with the Mic -and all route thru the switches on the front panel to get to the speaker.

I will recheck that to the best of my ability. When I wired this mic I went right off of the schematic AND made sure whether the image was referenced from the back or from the front.... i.e. I did it by pin numbers to avoid that!

That said, I will recheck it!

But that is only part of the story.

Double check your work - a simple re-cap may have caused a broken speaker wire onto something yet to be discovered...

CBs' use Mics and so they have Jacks, or Sockets - and "mate" with a plug connected to the cord that connects to the Mic handset. If the Cord is wired for a particular radio - in this case a 4-pin wire Cobra Uniden style - if the Jack done the same way?

There is also the issue of the TYPE of Cobra 21 it is. It came in several "flavors" one is a, 21 Xtal type, 21 LTD, 21GTL and 21 Plus - there is a difference. Mics for the 21 Plus were ELECTRETS - meaning they used power from the radio to make the Element in the HANDSET work. But the LTD or GTL - used DYNAMIC - meaning they GENERATED power at the Handset which sent this audio signal power into the radio to be amplified.

So if your Handset is for Electret - and the 21 is a GTL - then it won't work - you'll need to find a Mic with w properly wired and supplied handset to make the radio work properly.

I had not thought of that....but it makes sense. It's on the list!!!

If the radio uses Xtals - the mechanical selector assembly may have corroded - you'd have 23 channels but if the Xtals or the Rotary Switch was bad - there goes the neighborhood.

Another aspect is the "Squelch" control. If that is not working correctly - then no matter what, you wouldn't be able to make the radio do anything except change channels - you wouldn't even hear a thunk. Because the Audio amp is clamped - muted - off - by the squelch control or the PLL telling to stay off because its not in lock and can't make the receive work.

Another good thing to check!

So, mull this over and see if you can fiddle and discover anything that might help us in helping you get that thing on the air.

An external speaker can help - if the internal speaker "froze" due to age and the coil in it is seized to the core - well , an external speaker can at least let us hear a thunk and maybe a "Hiss" afterwards...

Incredible!!! Have you seen that happen??
I can check the speaker as well.... that is a good thought.

 

[Handy Andy]

In fewer words - I have several older radios where age, moisture and corrosion has set in to the speaker cone - where it had "froze" in place after sitting for so long. Replacing the speaker is the only option - the thing sat in it's spyder cage and rotted away in the storage conditions.

If you can carefully remove the speaker from the panel - you can check by pressing gently around the webbing to see if the cone is free - you can also visually inspect the speaker and the "braid wire" that routes to the cones core coil that floats over the magnet in its' own cage. And poked holes or weak webbing - replace it - you can try to repair it but do you have the time and tools? Easier to replace...

A simple ohm meter ohmic check (1X or <1K ohm setting) can tell if the coil is ok - it's usually stamped on the back of the magnet housing 16 ohms or 8 ohms 1 to 2 Watts power rating is usually found in these things.

There is a Positive (+) and Negative (-) so they have a polarity so the speaker offset is positive - moves outward then inward to keep clearance for the coil housing doesn't SMASH into the magnets bottom base and then you have the seizing that follows - the drag will lower the cones ability to move and provide fidelity - eventually leading to a fail - done - as in - seized or open circuit (overheat)

 

[nomadradio]

A dead radio is missing one or more vital functions.

Changing all the electrolytics is a good policy when they are that old, but it introduces the opportunity for errors. Getting them turned the right way sounds easy, but you only need to reverse one of them to cause trouble.

A tiny bridge of solder across the gap between two solder pads will also be a risk, even for a pro.

My first go/no-go test for a dead receiver is the "digital manual signal generator". Lay one finger (digit) alongside the shaft of a small number zero phillips screwdriver, and touch the tip to the input and output solder pads of the 455 kHz ceramic IF filter. If you get a lot of raucous noise on one side, you know all the circuitry downstream from there is at least partly working. Touching the input pad of the filter should still get you some noise, usually not quite as much.

There are a number of 40-channel Cobra models with the digits "21". All of them have suffix letters after the digits. The original "21" model with no suffix digits was a 23-channel radio.

That being said, the internal layout of the "21XLR", the "21GTL" and the "21LTD" are not the same. And that's only three of them.

Getting any more specific about where to look after the 455 ceramic filter starts with being specific about which "21" you have.

73

 

[guitar_199]

Now THAT is a cool thought. Signal injection..... from a fingertip!!!!! If I get a chance to pull it out this weekend, I'll give it a shot.

I had trouble finding the right schematic but finally did when I got some help from The Defpom on his site. Mine is the 21 LTD but NOT the 21 LTD Classic. Those two are tow VERY different radios for certain!!!!


"My first go/no-go test for a dead receiver is the "digital manual signal generator". Lay one finger (digit) alongside the shaft of a small number zero phillips screwdriver, and touch the tip to the input and output solder pads of the 455 kHz ceramic IF filter. If you get a lot of raucous noise on one side, you know all the circuitry downstream from there is at least partly working. Touching the input pad of the filter should still get you some noise, usually not quite as much."

 

[Robb]

Did you test the PA for mic audio yet?

 

[guitar_199]

No sir. Much as I hate to say it, "mama" has had me busy prepping for family coming in for Thanksgiving. It will be after that is over before I can dig it all out and go for it again.

 

[Handy Andy]

Ok! Well, enjoy your Turkey Day!

Wanted to share this with you - was not sure but I do remember coming across several Cobra and Uniden Radios of older vintage that have 3 Pin power cords. Well, all of them do, but what I found was several 21's of the "flavors" I've talked about before - have more than one of the pins at the power plug in the back - providing power.

Some radios won't even work unless all the pins at the plug have power - so that means anyone wanting to use this on a base or mobile - would need to wire up the power wires properly to make it even power up and display any channels.

So if you can open the radio up and look for this...

2 Pin Power Cord (Red and BLACK)
​

You can see the BLACK wire towards the corner panel inside the radio with the BARE wire using a Ferrite Bead to POWER the radio.

2-Pin Rear Panel Power Jack Pinout


Now 3 Pin Power...


3 Pin Rear Panel Power Jack Pinout
​

Some of these radios have WX function which carried over from those days and would allow the radio to just sit there until a WX alert came in - so you'd have a WX alert and listen to WX as needed. But until you started the vehicles' engine or applied power to the other pin the radio would NOT allow you to use the radio - the CB side of it.

So when you are working on the radio - remember to apply power to all pins as you see from the INSIDE - so if you only have a 2 pin power cord, but the Radio requires 3 pin power hookup - then be sure to use the proper plug for that jack and wire accordingly.