Cobra 29 RX Problem making an old man go nuts!

[Crossbow]

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?”

Thanks 2FB,
Buddy has this on his site and I read it then went through it step by step.
Every part checked good and the freq's are on.
Even changed each part to make sure.
I think LC is on the right path telling me C8. There is something blocking the signal from entering and C8 makes sense as its the first thing standing in the way on that side of the circuit. I failed to see that end of the circuit as I went down the middle receive path and not the outer side. I will know tomorrow and report back!

Thanks everyone for the help. This is not one of those run of the mill easy repairs.
Just needed someone smarter then me to see what I missed.

 

[loosecannon]

ok crossbow, im back to thinking about C8 after reading your post.

I can see the possibility of some oscillation causing that slight tone.

very curious to hear what you find.
LC

 

[Crossbow]

Crossbow,
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

C8 was not the cure. PLL and VCO alignment checks out good.
R85 set to max on scope
R88 set to 3.2 volts
R17 set to max on scope
TP3 "R17" 16.270 is good
PLL and Fet 2 showing 10.240

 

[Crossbow]

Update! I found the problem! L6 seems to have a problem. After ohm and volt testing all the receive cans I found L6 dead. Replaced that can and she is now back up and running.

Thanks again everyone for the help.

 

[Crossbow]

For this radio we have found D6, D316 and L6 all being bad.
Hope this will help others with the same kind of problem.