No receive and other issues on Cobra 29 NW ST

[JDE]

Doing my first repair on a cheap radio from ebay, sort of jumping ahead a bit while I wait on some new equipment, Lou's books, etc so please bear with me as I am definitely a rookie at this and trying to learn.

Radio arrived with blown open D23, swollen/leaking C123, clipped D11, bad/loose solder joints on + and - leads, missing power socket.

Removed D23 and C123 with the intention of replacing.

Re-attached D11 with a dab of solder, will end up replacing.

Radio powers on, internal speaker dead, external speaker cracks and pops sometimes but no noise with squelch opened, meter dead, NW panel not illuminating, and there is talkback on Dynamike knob. Radio keys, talkback works, no watt meter/scope/monitoring radio yet to test any further. I am TXing in to a dummy load.

There are a few globs of clear hot glue in several places on the front of the board and a newer looking blue capacitor up by the final, the back has several jumpers and resistors which may not be factory, and there are two red wires from the board spliced to two blue wires going in to the side of the meter and not entirely sure what they're for(unless those are for meter illumination). Unless something was very poorly installed or the person had terrible soldering skills, hard for me to say what's factory and what isn't since I'm so new to this.

Questions;

What is C123 for?

Did this radio come with talkback? I'm pretty sure it didn't.

How much of a pain is it to troubleshoot/repair the NW panel illumination?

From what you can see of the boards in the attached photos, any identifiable repairs or mods? That blue capacitor up by the final appears to be new, and I imagine other work has been done since D11 was clipped. Rear shot is after I removed D23 & C123 and resoldered the power leads.

 

[Handy Andy]

C123? Power Filter Capacitor - working with the POWER Filter choke - keeps noisy ignition and power supply noises out of the box it's in (Your radio)

Actually, I think it did - its "resistor" is located under the board by the PA and EXT Speaker leads.

​

Found some other stuff too...I wish I had a Cobra 29 still around I'd let you see that board to help guide you in repairing this one.

I found other spots that need attention, but this will help you get started.

​

The above is your board with some highlited areas that will help you in trying to get the radio to RX again.

To do that, you need to get audio working - so right now, lets' fix JP36, and see about that Talkback resistor getting removed.

Then switch the radio over to PA - and install a External Speaker in the PA jack and see if the radio will produce any audio (adjust volume and Dynamike) for volume control in PA - then we're halfway home.

Then we will have to sort out the NW Nightwatch dimmer circuit and see if the EL panel even works - it looks like some work has been done back there and in looking over the foil side of the board - you have 1 varactor - and the other varactor that would be used in a Cobra that has Delta-Tune - instead uses a FIXED value cap.

To help further - go to this post...

Cobra 29 no rx or tx Help please!

So I have an older cobra 29 that has no transmit or receive. The rx/tx light changes when I key the mic. The PA works so the audio chip seems to be good. All I hear is a click in the speaker when key up. The meter does not move on rx or tx. I replaced all the caps in the entire radio after...

www.worldwidedx.com

That has some of the more recent posts and tuning tips and tricks to help you.

 

[JDE]

Awesome info as always, Andy.

-I'll remove the capacitor and resistor at JP36 and replace with a jumper.

-On the talkback, you mentioned Pin 4 being disconnected and not sure what that means.

-On your orange/green/blue boxes, I'm not quite there yet and assume I need more than a DMM and a soldering iron for those.

 

[JDE]

TB resistor is removed, JP36 cap and resistor replaced with a jumper.

PA works to external speaker, turning Dynamike up turns up the volume of the PA but can't go very high since this is at my bench so it's feeding back and squealing.. Volume knob didn't seem to do anything, only Dynamike.

Still no receive, no static, just a faint hum and some crackles here and there. Closing squelch and messing with NB switch doesn't seem to make a difference.

Edit: Forgot to mention, I do have a green RX light under normal operation and it does switch to red on TX. Light dimming on TX is very faint, and probably due to the 20+ year old RadioShack 3a power supply I'm using. Meter is still stuck on the far left side and doesn't budge at all, but I have a few coming in from ebay next week and will get that swapped out once these other issues are resolved.

 

[whiteastro]

Did this radio come with talkback? I'm pretty sure it didn't.

No that radio doesn't come with talk back. I own one because it is the model I bought when I first got back into CB in the late 90s I think.

 

[whiteastro]

It's still a good receive & talking plain Jane AM radio if that's all someone's looking for.

 

[JDE]

That's basically all I'm wanting out of it, get it repaired back to OEM specs and toss it in one of the vehicles, or give to a buddy.

 

[JDE]

Are these compatible replacements for D11 and C123?

https://www.digikey.com/en/products/detail/onsemi/1N4148/458603

https://www.digikey.com/en/products/detail/kemet/ESK108M025AH4EA/3082784

Since I'm about to put in a Digikey order...any other commonly replaced components I should stock up on? I'm using cheap Cobra 29s from fb and ebay to learn diagnosis/repair. Might end up having a 25 or 148 cross the bench at some point, just depends on what I can get my hands on.

 

[whiteastro]

If that Cobra 29 NW ST has the stock mic I was told by a radio guy they weren't too good. I replaced mine because it quit working. I think they have CA 73 on the back of them. Check that out for problems.

 

[nomadradio]

Those part numbers look legit.

You don't mention a candidate to replace the reverse-protection diode. We use a generic 6 Amp rectifier, soldered directly across the inside of the power socket. Got in this habit working for slip-seat drivers. They would board a tractor with the Plus and Minus rubbed off the doghouse 100,000 or so miles ago. Made the alligator-clip hookup into a game of Edison roulette. The 6 Amp diode would happily trip all the 2 or 3 Amp fuses you like with no damage to the radio. And a bigger fuse would not likely blow it into two pieces like the tiny 1-Amp factory diode will do.

And even if all you have is a 1-Amp rectifier like the original, it's best to have that diode in the radio. Safer, anyway.

The most common major casualty we see in a radio with that big cap blown out is for the audio power chip to croak. Count your blessings that part has survived.

73

 

[Handy Andy]

-On the talkback, you mentioned Pin 4 being disconnected and not sure what that means.

In the radio the Mic Jack and Plug, you already know the Pinout code...

Pin 1 Ground
Pin 2 Audio
Pin 3 TX
Pin 4 RX

To help you, remember when you unplug the Mic cord from the radio?

- the speaker goes dead. That is becasue the main way the Speaker even works is lost. A completed circuit is broken.​

​

How?​

​

Pin 4 is the radios' SPEAKER return - the audio line is wired in a way so that the audio from the Audio amp and transformer - are present all the time, the speaker uses one wire from that output of the transformer directly to the speaker thru the connectors on the rear panel - those EXT SP and PA jacks.​

• - this line is also shared by the Final and Driver - they get power from it too - again, all the time.
• When the Handset is unkeyed, RX Pin 4 SHORTS to Pin 1 completing the Speakers circuit.
• When the Handset in Keyed, in TX - Pin 4 OPENS - removing power from the speaker circuit - has nowhere to go - opens like a fuse and pin 4 just sits there.
  • During the Keyup, Pin 3 is Grounded - this TURNS ON a circuit designed to send power to the TX side of the radio. This makes the Pre-Driver TR16 turn on and produce power, enough power to turn on the Final and Driver.
  • The Audio present that would normally go to Pin 4 - now goes to the driver TR17 and Final TR18 - and out the antenna.
  • When you unkey, Pin 3 OPENS - the TX Circuit turn off. The Pre-Driver stops producing power and the TX Strip shuts down.
  • Pin 4 now SHORTS to Pin 1 and completes the Speakers circuit and Audio is heard from the RX side - as the TX side is turned off because of Pin 3 no longer can turn on TX and Pin 4 is now shorting the speaker (-) lead to ground - completing the circuit for the speaker.
    • Remember the ASPECT of Pin 1 - is main ground - power is looking to complete a circuit - so Pin 4 still has Audio On It (it's why that resistor for talkback works) even though Pin 4 is LIVE, Pin 1 is not connected to it in TX, it is connected to Pin 3 so it simply is turning another section of the radio on using Pin 3 to Pin 1 connection.

​

It's why mics use a DPDT switch and are wired the way they are...

In the above photo,
White Wire is common to BOTH MICROPHONE Element
and the SPEAKER-RX and TX switch.
The Mic wiring completes a circuit, and the plunger completes
the switching of the RX and TX circuits at the handset
- Pin 4 has to go all the way up here to finish the connection to the speaker.​

Attached are several large files to help orient you.

They are wiring diagrams for various Pinouts of 5-pin Cobra/Grant and Realistic 5-pin DIN types...also a primer for Turner Mics to help understand the wiring of the DPDT switch.

We're here if you have more questions...

 

[JDE]

If that Cobra 29 NW ST has the stock mic I was told by a radio guy they weren't too good. I replaced mine because it quit working. I think they have CA 73 on the back of them. Check that out for problems.

It came with a destroyed mic so that went in the trash and I'm using a new Cobra HG M73 mic on the bench.

 

[JDE]

Those part numbers look legit.

You don't mention a candidate to replace the reverse-protection diode. We use a generic 6 Amp rectifier, soldered directly across the inside of the power socket. Got in this habit working for slip-seat drivers. They would board a tractor with the Plus and Minus rubbed off the doghouse 100,000 or so miles ago. Made the alligator-clip hookup into a game of Edison roulette. The 6 Amp diode would happily trip all the 2 or 3 Amp fuses you like with no damage to the radio. And a bigger fuse would not likely blow it into two pieces like the tiny 1-Amp factory diode will do.

And even if all you have is a 1-Amp rectifier like the original, it's best to have that diode in the radio. Safer, anyway.

The most common major casualty we see in a radio with that big cap blown out is for the audio power chip to croak. Count your blessings that part has survived.

73

Do you have a part number for that so I can toss it in the digikey cart? I had just grabbed a 20pack of "1000v protection diodes" from one of those CB parts sellers on ebay for 7 bucks, not sure of their amp rating since he doesn't say and they aren't here yet.

When you put the diode across the power socket, the D23 spot on the board just stays empty/cut?

 

[JDE]

In the radio the Mic Jack and Plug, you already know the Pinout code...

Pin 1 Ground
Pin 2 Audio
Pin 3 TX
Pin 4 RX

To help you, remember when you unplug the Mic cord from the radio?

- the speaker goes dead. That is becasue the main way the Speaker even works is lost. A completed circuit is broken.​

​

How?​

​

Pin 4 is the radios' SPEAKER return - the audio line is wired in a way so that the audio from the Audio amp and transformer - are present all the time, the speaker uses one wire from that output of the transformer directly to the speaker thru the connectors on the rear panel - those EXT SP and PA jacks.​

• - this line is also shared by the Final and Driver - they get power from it too - again, all the time.
• When the Handset is unkeyed, RX Pin 4 SHORTS to Pin 1 completing the Speakers circuit.
• When the Handset in Keyed, in TX - Pin 4 OPENS - removing power from the speaker circuit - has nowhere to go - opens like a fuse and pin 4 just sits there.
  • During the Keyup, Pin 3 is Grounded - this TURNS ON a circuit designed to send power to the TX side of the radio. This makes the Pre-Driver TR16 turn on and produce power, enough power to turn on the Final and Driver.
  • The Audio present that would normally go to Pin 4 - now goes to the driver TR17 and Final TR18 - and out the antenna.
  • When you unkey, Pin 3 OPENS - the TX Circuit turn off. The Pre-Driver stops producing power and the TX Strip shuts down.
  • Pin 4 now SHORTS to Pin 1 and completes the Speakers circuit and Audio is heard from the RX side - as the TX side is turned off because of Pin 3 no longer can turn on TX and Pin 4 is now shorting the speaker (-) lead to ground - completing the circuit for the speaker.
    • Remember the ASPECT of Pin 1 - is main ground - power is looking to complete a circuit - so Pin 4 still has Audio On It (it's why that resistor for talkback works) even though Pin 4 is LIVE, Pin 1 is not connected to it in TX, it is connected to Pin 3 so it simply is turning another section of the radio on using Pin 3 to Pin 1 connection.

​

It's why mics use a DPDT switch and are wired the way they are...

View attachment 57294
In the above photo,
White Wire is common to BOTH MICROPHONE Element
and the SPEAKER-RX and TX switch.
The Mic wiring completes a circuit, and the plunger completes
the switching of the RX and TX circuits at the handset
- Pin 4 has to go all the way up here to finish the connection to the speaker.​

Attached are several large files to help orient you.

They are wiring diagrams for various Pinouts of 5-pin Cobra/Grant and Realistic 5-pin DIN types...also a primer for Turner Mics to help understand the wiring of the DPDT switch.

We're here if you have more questions...

Great info once again, this thread is turning in to a goldmine so definitely bookmarking it!

 

[JDE]

@nomadradio - Digikey has slim pickings on 50v/6a diodes, basically I have two in-stock options and they both require a min order of 1000.

https://www.digikey.com/en/products/detail/diotec-semiconductor/UF600A/13153812

Would that one work, but in say 100v instead of 50? These are going in to a 13.5v circuit so voltage rating of the diode shouldn't matter, just trying to find one that doesn't require me to order a thousand of them. Something like this;

https://www.digikey.com/en/products/detail/micro-commercial-co/6A1-TP/773647

https://www.digikey.com/en/products/detail/comchip-technology/6A04-G/1979682