How does the noise sound on the antenna - change when the NB/ANL is turned off, or just to ANL.
This may mean the TR6 is damaged.
Asking this to help understand - for the way you lose the linearity of the RF gain control - it's all scrunched up near the top of it's rotation - meaning the "noise floor" and the RF Gains' power used to adjust RF gain - is very close to each other - as if one sets the mid point for the other.
Quick question - did you check the PIN diodes for shorted or blown open condition...
TR6 is more on than off when the RF gain knob is turned down,
sending power thru itself to make it appear more as a open line as far as the PIN diode section is concerned - even though it is nearly acting more like a dead short- Power present (RF) at the antenna input section to the PIN diodes see the Diode as a reverse Biasing no power flow thru the section - it appears as "nothing" until the power present in the received signal forces Pin diodes so they only conduct in spiking or extreme input level condition, it's also pulling the power from D5 / Last IF can - if that power balance is right, you see carrier power when someone is keyed up and you have a signal to hear - when it's "Dead" on the channel - there is still an IF carrier - it's a balancing condition so you're seeing IF carrier power all the time - tells me TR6 or the Pin diode section can be bad.
- The above description is to see RF gain knob all the way down - but some people they see RF Gain goes UP - they think it fully receives, and they'd be correct, but the operation appears backwards for as you turn the control - you're changing the amount of power that goes thru TR6 - in parallel to the Pin diodes off of D3.
- AGC action affects the BASE of TR6 - which in turn affects how much the RF Gain knob has control of R9 and D3
- Did you ever locate R101? D20? R113? Locate L3 and it's mess, you'll see something...
- I don't want to get into an argument with anyone over it - but...it is interesting...
- It's part of the FEED to both RX/TX switch but also for voltage to the RF amp, thru R12/R13 and R15 - R18 and R26.
Why this comes up? Well, if you have an older Cobra 29 without ST, WX or non-LX models, you would see a normal compliment of parts located all around the TR21 RX/TX switch
On these newer models - you might want to re-check that.
The above is from a KEPC 1141 board.
The issue lies in how you work the RF gain, one way it's an attenuator - another way, it is an amplifier for attenuation...
But both use what is derived off of D5 C29 and R128 - AGC goes up and down all the time varying with noise level and the incoming signals - all of which will reflect on the S-meter.
[There is so much more to add, this will have to wait . for I'll still have to stick with the adage of IF leakage but the route to give you this answer was a strange one!]
Here's why...
There are several errors in schematics
that parts don't match up to the radios' 100%
Traces too...
So to wait for the next round...
If that is not the case, then again, IF signal leakage is going on somewhere and the radio's picking this up.