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Cobra chrome no receive pure static

JNJ

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So there was another thread exactly like this on the same radio model. I’m going to use those hints and suggestions to try to fix this but the one thing I need help with is the bottom of the board looks very rough

I’m going to post a few photos of the board and see if someone out there can let me know if this is stock or not. I’ve seen multiple radios of this model have really ugly solder connections and surface mount parts attached to the bottom
 

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JNJ

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IC4 is the audio chip, thats why I’m wondering if it’s a mod causing the bad static
 

nomadradio

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First question: Does the volume control affect the static at all? If turning it up and down has no effect, the problem is located downstream from that control.

Does it transmit? And if so, do you hear the same noise in a receiver on that channel?

73
 

JNJ

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Jun 12, 2022
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First question: Does the volume control affect the static at all? If turning it up and down has no effect, the problem is located downstream from that control.

Does it transmit? And if so, do you hear the same noise in a receiver on that channel?

73
The volume control does attenuate the noise. TX is clean

Judging from the other post there are several capacitors around VR 1 one that could be a culprit

But the board definitely looks like someone’s been re-soldering on it however I’ve seen these surface mount parts on other chromes

And I can’t tell if it’s a modification or just someone’s attempt at fixing the issue.

I couldn’t find them on a schematic although I could be wrong. A board layout would help too, but I just couldn’t find it.

This radio was made in 2018 and it came from a trucker, I’ve heard that a could have a blown out front end

He had three radios in barely used condition that all stopped working I assume he bought them and had them modified immediately because they’re all modified

There is a blocking capacitor (C48) between the audio transformer and the audio amp that I’ve also heard can go bad

But I figured maybe it had more to do with someone playing around with it considering it’s a newer Radio

I do have a signal tracer if that helps you


Also I forgot to mention that The radio does not receive whatsoever there’s no mixing from the incoming signal

Here’s a schematic

COBRA CHROME CB TRICKS
 
Last edited:

JNJ

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Jun 12, 2022
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First question: Does the volume control affect the static at all? If turning it up and down has no effect, the problem is located downstream from that control.

Does it transmit? And if so, do you hear the same noise in a receiver on that channel?

73
The volume control does attenuate the noise.
 

Handy Andy

Do Your Research First, Then Decide...
Apr 23, 2018
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When I have a radio "fresh out of the box" and no receive, the "hiss" is the IF - a good quantity of it is a good sign that BOTH IF's the PLL one (16~17MHz) and the 10.240MHz (Xtal) IF are working.

But why dead?

Usually due to a bad solder joint - you may be seeing someones' effort at trying to get the radio to work again - but in rare instances - the OEM part is bad, not prime like it should have been and they didn't catch it or worse - they just pushed it out the door.

Some things to help you sort this out is to look for and locate VR1 - you said it had some caps around it - but are you sure it's VR1?

VR1 is the 455kHz IF trim - which does produce that hiss in major quantities if nothing else works.
1655036190100.png
Don't see any "Bad soldering" here but loud hiss usually means something high gain, but not in the band.

So, you have to go further back towards the Final Driver area and that section that pulls the RF Signal off the main output strip - that area is tricky sometimes.

Even the coils that tune for 27MHz band - some fixes I've had done for DOAs' have had coils gone "open" but nothing serious, if the coil wind from the trace to foil does not show a short but the schematic of it does - takes a good unsoldering tool or braid to remove that cans tabs then it loosens and pops up the cover/case and the coil can be inspected to see if the strands are not making contact like they should.

On to other things to check.

The thing comes with that wonderful "offset" - so double check the T/R function - make sure that PLL pin changes state JPEG_Cobra_29_Using2816PLL.jpg

Thats Pin 9...

Also related to RF gain, there is a section - two actually, that affect receive and if not wired correctly - will damage TR6 - the RF Gain control - Gain control.

Just locate this little "square" - that square pad helps TR6.

1655045926905.png

It's best to troubleshoot - to check remove the two RF Gain wires - remembering where they go - and remove TR6 - see if RX comes back in spades - if it does - then you'll have to figure out what took out TR6 - just replace it, but carefully check those wires - it "pulls" RX signal out of the RX input by throttling the PIN diodes.

1655046462644.png

So to have TX work, and the noise to be attenuated, leads me to think the two things - one the T/R pin is not working or the RF gain section is goofed up.
 
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JNJ

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Jun 12, 2022
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When I have a radio "fresh out of the box" and no receive, the "hiss" is the IF - a good quantity of it is a good sign that BOTH IF's the PLL one (16~17MHz) and the 10.240MHz (Xtal) IF are working.

But why dead?

Usually due to a bad solder joint - you may be seeing someones' effort at trying to get the radio to work again - but in rare instances - the OEM part is bad, not prime like it should have been and they didn't catch it or worse - they just pushed it out the door.

Some things to help you sort this out is to look for and locate VR1 - you said it had some caps around it - but are you sure it's VR1?

VR1 is the 455kHz IF trim - which does produce that hiss in major quantities if nothing else works.
Don't see any "Bad soldering" here but loud hiss usually means something high gain, but not in the band.

So, you have to go further back towards the Final Driver area and that section that pulls the RF Signal off the main output strip - that area is tricky sometimes.

Even the coils that tune for 27MHz band - some fixes I've had done for DOAs' have had coils gone "open" but nothing serious, if the coil wind from the trace to foil does not show a short but the schematic of it does - takes a good unsoldering tool or braid to remove that cans tabs then it loosens and pops up the cover/case and the coil can be inspected to see if the strands are not making contact like they should.

On to other things to check.

The thing comes with that wonderful "offset" - so double check the T/R function - make sure that PLL pin changes state View attachment 59322

Thats Pin 9...

Also related to RF gain, there is a section - two actually, that affect receive and if not wired correctly - will damage TR6 - the RF Gain control - Gain control.

Just locate this little "square" - that square pad helps TR6.

View attachment 59323

It's best to troubleshoot - to check remove the two RF Gain wires - remembering where they go - and remove TR6 - see if RX comes back in spades - if it does - then you'll have to figure out what took out TR6 - just replace it, but carefully check those wires - it "pulls" RX signal out of the RX input by throttling the PIN diodes.

View attachment 59324

So to have TX work, and the noise to be attenuated, leads me to think the two things - one the T/R pin is not working or the RF gain section is goofed up.
Wow I really appreciate all the work that you put into this and I will definitely take the time to sit and absorb it all.

Thank you very, very much.
 

JNJ

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Jun 12, 2022
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Way the static continues even though the microphone is not plugged in and ic4 is getting hot

OK so here’s what I figured out

RF gain (collector to tr6) does not attenuate the signal

However turning off the talkback circuit completely stops the noise (with or without a microphone)

So the radio is completely silent without any receive when the talk back function is off.

Until… you plug in the mic and boom, noise comes back

Talkback doesn’t work either, It’s putting out Power just fine and sounds clean on air but the talkback is completely disguised by the noise

The signal meter still jumps with the static both in PA and when the talk back circuit is off

But the noise is louder on pa than cb (talkback disengaged)

Dynamike does not reduce anything
 
Last edited:

Handy Andy

Do Your Research First, Then Decide...
Apr 23, 2018
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Then this area is part of your "mess"
1655076606494.png

Pin 4 connects to this area

- there is a "row" of pins alongside J2​
  • - one wire should have a silkscreen of "SP -" (SP minus symbol)
- the one to the right is PA (J1)​
  • - it's ring goes straight to ground there is also an "ear" that must remain open or unsoldered.
- in the very bottom of the pic, in RED LETTERS is SP +​
and the outline of a dropping resistor - this SP positive.​
It goes back to CB PA switch up front.​
The SP - (minus)_ wire is OPEN when the mic is left out - the only time it grounds is when the mic cord is plugged in (to a cobra 4-pin wired mike)​
Then the circuit is complete - the speaker works - but if the RING of J2 is shorted to ground (not the ear) all the time - you'll get results like what you describe.​
So remove the talkback mods - restore the unit to OEM - then continue to properly re-do the work to make the MODS work like they should. One step at a time and verify the radio works each time - don't hurry...​
 

JNJ

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Jun 12, 2022
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Hey so I was reading through your suggestions and I checked the voltage on pin nine I have about a 4.8 V as well as on R86

IC4 kept getting hot enough to melt the solder around it which is why the solder joints looked cold

Took it out and there was a short between the output pin and ground that measured about 12 ohms

I traced the noise all the way up to D12
I noticed the noise was gone when Set the switch to SWR but came back on CAL and was twice as strong on S/RF

I decided to do what you suggested and remove TR six and both receive and transmit lines

This did not seem to change anything
So I kept chasing the static until I ended up on FET 2

I replaced it and the static is gone and I can see the meter swing when I key another radio


So now I guess I just need to work on fixing the voltage issue and ordering another IC

Oh and I need to figure out why turning the dim switch on stops Radio from keying


Looks like someone’s definitely been messing in this Radio

And I’m just starting to learn about radio repair, so it’s very difficult to picture this stuff in my mind and try to think through it

Oh and I forgot to mention that the talkback function is actually stock, I guess it was just something they added to bump the price up a bit



Hey, wait a minute

There looks to be a lot of solder work near the pll where the meter lights are.

I wonder if maybe somethings soldered incorrectly and draining voltage from the pll?

Because after I replaced fet 2 I checked the voltage and it was around 8 V and then slowly diminished
 
Last edited:

Handy Andy

Do Your Research First, Then Decide...
Apr 23, 2018
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Look or search for Cobra 29 mods or Cobra 29 Repair for keywords to find several pages of hundreds of posts about Cobra 29 tweaks fixes, plus some have a lot of documentation.

Also - there are parallels to the Cobra series, like 29 is to Uniden PC 76 or 78 as Cobra 25 is to the Uniden PC - 66 / 68 series.

Lots of good hints...

As far as the PLL goes, that voltage on Pin 9 is the trigger - so if you have some way to hear that TX side of the radio and monitor the frequency of the PLL in RX the method I speak of is the PLL will offset the RX side from the TX side - so the 2nd IF conversion that 455kHz one where VR1 is at - works and stays aligned to the channels (1 - 40) of the CB band. So, the 16.430 MHz with shift to 16.885 in TX changes the way the IF works or parses out the signal for the channel.

1655118637932.png

IF that shift doesn't work, the radio appears deaf, because the RX bandpass itself for 27MHz - isn't wide enough to capture any signal 455kHz "off" to the radios' channel selected frequency.

So Pin 9 is pretty important to make the system tune correctly to hear RX channels from the PLL IF shift to work.
 
Last edited:

JNJ

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Jun 12, 2022
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Look or search for Cobra 29 mods or Cobra 29 Repair for keywords to find several pages of hundreds of posts about Cobra 29 tweaks fixes, plus some have a lot of documentation.

Also - there are parallels to the Cobra series, like 29 is to Uniden PC 76 or 78 as Cobra 25 is to the Uniden PC - 66 / 68 series.

Lots of good hints...

As far as the PLL goes, that voltage on Pin 9 is the trigger - so if you have some way to hear that TX side of the radio and monitor the frequency of the PLL in RX the method I speak of is the PLL will offset the RX side from the TX side - so the 2nd IF conversion that 455kHz one where VR1 is at - works and stays aligned to the channels (1 - 40) of the CB band. So, the 16.430 MHz with shift to 16.885 in TX changes the way the IF works or parses out the signal for the channel.

View attachment 59334

IF that shift doesn't work, the radio appears deaf, because the RX bandpass itself for 27MHz - isn't wide enough to capture any signal 455kHz "off" to the radios' channel selected frequency.

So Pin 9 is pretty important to make the system tune correctly to hear RX channels from the PLL IF shift to work.

I just want to tell you, you’re incredibly beneficial as a teacher and I’m very appreciative of all this help.

Don’t stop doing what you’re doing.

Anyways I have a lot of research to do and I will update the thread if anything new develops.
 

Handy Andy

Do Your Research First, Then Decide...
Apr 23, 2018
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To help - if you have access to a simple frequency counter capable of 5 to 6 digits so you can see the "Dimes" versus the "Nickles" (27.185 - Nickels or 16.270 - Dimes) the IF can be seen at TP3 right by FET1 -and TP4 shows your TX frequency.

Also the PLL generates a signal around 16 to 17MHz - the 1st IF downmix (a subtraction) which gives you 10.695 strip - but uses the 10.240 main Xtal to down mix that (Subtracts) to 455kHz and that's your 2nd IF - and the deconversion is a cap tap - off of that...

So why am I telling you this?

IT would help guide you better to know if the PLLs' IF it generates, is working for RX and TX and you're not left wondering where is it really listening at?
 

JNJ

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Jun 12, 2022
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To help - if you have access to a simple frequency counter capable of 5 to 6 digits so you can see the "Dimes" versus the "Nickles" (27.185 - Nickels or 16.270 - Dimes) the IF can be seen at TP3 right by FET1 -and TP4 shows your TX frequency.

Also the PLL generates a signal around 16 to 17MHz - the 1st IF downmix (a subtraction) which gives you 10.695 strip - but uses the 10.240 main Xtal to down mix that (Subtracts) to 455kHz and that's your 2nd IF - and the deconversion is a cap tap - off of that...

So why am I telling you this?

IT would help guide you better to know if the PLLs' IF it generates, is working for RX and TX and you're not left wondering where is it really listening at?
Thank you, I do have an old RadioShack frequency counter I use often but it has an antenna on it.

The antenna is connected to a BNC connector so I can probably use another BNC connector to make a high impedance probe with a ground for testing those frequencies.

I’m quasi familiar in a textbook way of super-heterodyning but it’s still not completely clicked in real world terms for me.

I have to be able to see it in my mind or it drives me crazy

I spent a long time visualizing reactance in order to get it down, I don’t like just doing the math I like knowing what’s going on as closely as possible.

Anyways IC4 had to be removed and I ordered another one. Whenever it gets here I will try to get this radio back to working order. Right now I guess I need to start on making a probe.

By the way I replaced FET 2 with a similar model from a cobra 29. The model numbers were almost identical except for the last few.

Do you think I can get away with this?
 

Handy Andy

Do Your Research First, Then Decide...
Apr 23, 2018
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Lots of times yes, the FET series gets similar "better performance" sand grains that can use that same number - but offers "enhanced performance characteristics'" that if it got renumbered - well - it's screw up life because they threw out the die they used to use - out with the bathwater.

In a way, they re-invent the wheel (here we go again - ed.) when it comes to semiconductors and so instead of making life a living hell for JEDEC and ANSI - EEIC and Warner Brothers - amongst many others.

To fix this - they just affix a suffix to help spell out the trim line like they do on automobiles - S, SE, SUX (Not a popular trim suffix) SEX (to Risque) LUX (tends to rub well) SRX PDQ LEV EGR and many others we haven't thought of yet.

So, 945 AQ (Audio Quality) or other non-such monikers - which can make you bonkers...it's the suffix that fixes the problem - much like the bumper sticker of a de-badged car - you don't care what they drive... you just remembered it for the sticker that said...

Caution: Contains Nuts

Still has four wheels...but the FET has 3 legs, insulted er - insulated Gate and everything thing from the Source goes down the Drain.
 

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