Cobra 29 RX Problem making an old man go nuts!

[Crossbow]

Hey guys I need some help!

I really hate asking for help but this radio has just about made me check into the nut house!

I have a Cobra 29 LTD NW that has no RX what so ever. No RX/ S meter reading as well.

Radio was receiving ok I thought but the transmit was off frequency.
I found 10.240 Chrystal at fault and replaced it. Radio is now back on frequency and transmitting fine and still had receive up to this point.

I hooked the radio up to my HP Signal Generator to do an Alignment, Here is where things went bad.

As stated in the service manual from CB Tricks it says for receive alignment to place:
AF VR: Max.CW
RF Gain: Max.CW
Set SG MHz to 1KHz, 30% modulation.

More less normal settings but I have never done one with the volume wide open. Mid level seems to work fine for me but on this radio the reading on my ComTekk SINAD meter level would not come up enough to go green and stayed around 6db, So I turned the volume wide open on the radio and BAM!! Something Blew, No Receive, No S Meter Reading, Dead as a door nail.

All you can hear is a faint whining noise, Squelch is working as you can turn it up to knock the noise out but the RF Gain has no effect on it. If you turn the ANL on it will knock the noise out.

Ok so here we go searching for the problem. I pull the Transistor voltage chart again from CB Tricks which is not correct if no one has ever noticed it before. Stated voltages on some items are not correct!
I pulled a working 29 and laid it beside the dead one to pull my voltages from to cross-reference with.

Not a single TR was out of whack so I started pulling cans around the audio chip checking them on my tester. All checked to be working but I replaced them with new ones as this is an old radio and needed recapped anyway. Nothing changed still no RX.

Next step was check PA, Yep works good on mic audio out but no RX coming out. Now this tells me the audio chip is good and done the finger test over the pins and heard the roar. I changed it with a new one anyway. Still no RX.

Next step start pulling and checking diodes D1,2,3,4,5,6,7,8,9,10,24. The only one that tested bad was D6. I replaced it with a new one and thought my problem was fixed. Nope still no RX.

Ok so now I'm thinking maybe one of the diodes were bad even though they are brand new so I changed every dang one of them out with the working radio to make sure. Well working radio is still working and dead radio still has no RX.

Next step lets change out more parts from the working radio. Keep in mind I done these one at a time to see if my problem was cured. I changed out TR7,8,9,10,11,12,13, Fet 1, Fet 2, IC2, FL1, FL2 and IC1. Also changed out some of the disk caps C34, C36, C48, Resistor R31 to boot. Oh and pulling TR13 out to see if RX returns done nothing. Working radio still working and dead radio still has no RX.

Even thought maybe it has something to do with the volume/squelch pot so I changed it out along with the RF gain pot to boot. Nothing changed.

Also when trying to aline with scope and sinad meter none of the receive cans adjust change at all.

Oh yeah 1st mixer is showing on channel one 16.270, 2nd mixer is showing 10.240.

As you see I done everything the correct way to start with by checking voltages and everything checked good so there was no other choice but to do a shot gun approach on this thing to make sure.

I have checked every trace visually and with ohm/volt meter, Everything looks and checks out good so what am I missing here? Please save an old man from jumping over the edge!
Thanks in advance for any help to solve this mystery.

 

[Robb]

So; how much RF voltage did you feed from the sig gen into the radio when it popped the radio?

You have a sig gen; so feed a signal in and find out where it stops.

 

[Crossbow]

So; how much RF voltage did you feed from the sig gen into the radio when it popped the radio?

You have a sig gen; so feed a signal in and find out where it stops.

The normal 4uv. Yes I have tried to trace the tone up to 100uv and can not find it anywhere. Working radio I find it all over the place no problems. Test the same spots on dead radio and nothing. Sorry I should have added that in the main post but it was getting pretty long.

 

[Crossbow]

Radio Shop "Buddy" has a video with the sound I get. Check time 41 secs on this video and when he turns the radio on then up there is the only noise I get.

 

[Robb]

First time I messed with doing a receive alignment, I turned one of the coils and I lost all receive. Thought I broke the thing. Went back and played with a couple of the coils, and it came back. What a relief that was.

 

[LeapFrog]

This is a bit of a long shot, but have you tried another microphone yet?
The Cobra 25/29 radios need a microphone connected for RX audio to come in.
I had a wire come loose inside a microphone 4 pin plug, long story short I once ripped my radio in half for no good reason.

 

[Crossbow]

First time I messed with doing a receive alignment, I turned one of the coils and I lost all receive. Thought I broke the thing. Went back and played with a couple of the coils, and it came back. What a relief that was.

As said in post none of the receive cans adjust anything. Nothing had been adjusted before this happen. Just hooked it up seen signal was not high enough turn radio volume to max and BAM. This is mind blowing is the only reason I posted. I been working on this thing off and on for days now. Nothing makes sense about it what so ever. The noise is there with nothing but power hooked up. No coax just plain radio turn on and volume up and you can hear this noise.

 

[Crossbow]

This is a bit of a long shot, but have you tried another microphone yet?
The Cobra 25/29 radios need a microphone connected for RX audio to come in.
I had a wire come loose inside a microphone 4 pin plug, long story short I once ripped my radio in half for no good reason.

LOL Yeah frog it has. I made a switch to plug in to keep cord crap out of the way but yes it has had a couple bench mikes hooked up testing the output audio.

 

[loosecannon]

Crossbow,

yes there is a mistake on the cbtricks 29LTD page. when it lists the 29LTD service manual it's actually for the 25LTD.

click on the ones that separate the manual into two sections and you'll have the right one.

right now my money is on C8 being bad.
LC

 

[Crossbow]

Crossbow,

yes there is a mistake on the cbtricks 29LTD page. when it lists the 29LTD service manual it's actually for the 25LTD.

click on the ones that separate the manual into two sections and you'll have the right one.

right now my money is on C8 being bad.
LC

Thanks LC,
Yeah there is a few not correct on CB Tricks but I worked my way around them. Making me a chart from using a brand new un-touched 29 with all the factory voltages. One way to skin a cat right?

I will try C8 and see what happens. It will be Monday before I will have a change to work on it again.

 

[loosecannon]

Crossbow,
maybe i'm missing something.
I just watched Buddy's video, and you say that you get the tone from your signal generator coming through the speaker like he does?

I thought you said that you had a dead receiver.

If you are hearing that tone from the sig gen coming through your speaker, regardless of how loud or how weak, then you are getting RF through the radio, just at too low a level, and the problem is not going to be C8.

If that is your issue, then my first guess would be to start checking the mixer sections.
This radio uses different mixing freqs when you are transmitting then when you are receiving, and im thinking that one of your mixer signals is at too low a level.

start by trying to do the alignment of the PLL and VCO and see how far you get before you encounter a problem.

If either the 16.270 or the 10.240 signals are at too low a level, then you will get the very weak receive symptom.

let me know what you find on monday and i will look into it further.
LC

 

[Crossbow]

Crossbow,
maybe i'm missing something.
I just watched Buddy's video, and you say that you get the tone from your signal generator coming through the speaker like he does?

I thought you said that you had a dead receiver.

If you are hearing that tone from the sig gen coming through your speaker, regardless of how loud or how weak, then you are getting RF through the radio, just at too low a level, and the problem is not going to be C8.

If that is your issue, then my first guess would be to start checking the mixer sections.
This radio uses different mixing freqs when you are transmitting then when you are receiving, and im thinking that one of your mixer signals is at too low a level.

start by trying to do the alignment of the PLL and VCO and see how far you get before you encounter a problem.

If either the 16.270 or the 10.240 signals are at too low a level, then you will get the very weak receive symptom.

let me know what you find on monday and i will look into it further.
LC

No LC,

There is nothing hooked to the radio other then power and mic. You hear the noise with the radio just laying there with nothing hooked to it. Wish I had taken a video of it because I know you guys think I am crazy..LOL

 

[LeapFrog]

There is nothing hooked to the radio other then power and mic. You hear the noise with the radio just laying there with nothing hooked to it. Wish I had taken a video of it because I know you guys think I am crazy..LOL

That's a "squelch threshold adjustment" or something different I think.
I'll try to find a video demonstrating what I think you are talking about.

Just because you can hear (anything) without coax/antenna connected does not mean the radio has an issue some sets buzz/ static hum faintly with nothing connected, have you tried connecting an external speaker at the Ext. jack or a different internal speaker?

How do incoming stations sound when the radio is connected in a normal setup with a pre-tested antenna/coax system, or can you not get any audio in a test condition like that?

 

[Crossbow]

That's a "squelch threshold adjustment" or something different I think.
I'll try to find a video demonstrating what I think you are talking about.

Just because you can hear (anything) without coax/antenna connected does not mean the radio has an issue some sets buzz/ static hum faintly with nothing connected, have you tried connecting an external speaker at the Ext. jack or a different internal speaker?

How do incoming stations sound when the radio is connected in a normal setup with a pre-tested antenna/coax system, or can you not get any audio in a test condition like that?

Frog,
Your missing what I am saying all together. As said before there is no RX to this radio! With or with out Ant hooked up, With or with out my SG hooked up. The RX is pure dead!
No S RX on meter and no RX coming in from the good radio. If you read my post where I spoke of putting two radio's side by side hooked to the SG and using my tracer from good radio to bad radio there is no sound coming from any of the test points nor from any part of the receive side of this radio. Only thing it has is a faint whine noise and yes I always use an ext speaker on the bench as I take the lids off and put them out of the way.

 

[2FB327]

I don't know if this will help you. Something I found.

The Receiver

This looks so complicated. And over whelm the new person trying to read the schematic. So lets block this together to make it a bit easier.

Let’s get to know what is happening in the AM receiver of this Cobra 29.
Go here and look at the block diagram:http://www.cbtricks.com/radios/cobra/29ltd_29ltd_st_29wx_st/graphics/29ltd-st_29wx-st_sm_pg1_pg11.pdf

RF signals are picked up by your antenna, converted into energy and sent to the so-239 on the back of you radio. We will call these 26.965 to 27.405 mhz signal. This signal is brought to TR 7. This is your RF amp. This amplifies the signal to make it more usable. The signal is then fed into FET 1. This is the first mixer. Lets say you radio is tune to channel 1. The frequency is 26.965 Mhz.. At the same time the PLL (IC 1) with the respect of the channel selected, IC 2 the VCO mixer sends the signal of 16.270 Mhz to FET 1 also. The signal produced at IC 2 will be from 16.270 to 16.710 (1 – 40).

The signal of 16.270 is then subtracted from the 26.965 Mhz signal yet producing our 10.695 Mhz signal.
26.965
-16.270
=10.695
From there the signal passes through the 10.965 Mhz filter (FL1) and delivered to FET 2. This is our second mixer. At this point we are getting a 10.240 Mhz signal from TR20 (10.240 OSC) This is mixed with the 10.695 Mhz signal
10.695
-10.240
= .455 Khz
The signal is then passed through the 455 Khz filter (FL2) and then sent to the second IF amplifiers, TR8, TR 9, TR 10. After the amplification the signal is passed through D4. This is our detector. Its purpose is to demodulate the signal for speech signal recovery. The recovered speech signal is sent through the volume control, to IC 4 (audio chip) and finally to the speaker. Where we plainly hear “Breaker 19, anyone got yer ears on comone?”