I can see where there may be some issues and I hope no one takes this wrong but...
Signal metering can be done with the AC meter setting of a DVM if you need help.
But, I also wanted to point out something that many of you are not catching...and perhaps a little "Musical" lesson may help you re-think the problem and develop a method of tuning (SIC) that you can re-train your hearing to listen for.
Any of you got an old Piano or Electronic Keyboard?
Just set it up and turn it on and hit the key of "C" - that is a tone, with it's full bodied sound - you can put this into a radio to send out - then you may have a better understanding of the colorization the EQ emphasis and Phase Equalization does to your audio as it goes thru the radio.
Ok, the reason why I brought the above up is due to the complexity you make in a simple act of tuning a radio.
Easy with the electrical - as Scotty once said "The more they add to the plumbing - the more they can stop up the pipes" - it's true - don't make it harder than it is.
Just understand the differences in "noise" you hear - a pure sine wave sound has a spectral tone that is predominate - it may help you to hit that key of "C" every now and then so you can tune to the note using a tone generator into a simple radio or even play the note on a tape you recorded it on earlier - just so you can "resonate" the speakers' sound to the note being played.
A triangle sawtooth signal has a sharper tone harsh to the ears - so does a satruated boxcar - almost bass rattily sounding - so between the two extremes - lies your ability to discern the tonal effects and develop your own method of tuning.
What then?
Well in a Cobra 25 you have VR2 a 200 to 500 ohm trimpot over by the 2nd IF amp stage - and it controls the amount of IF signal the section processes. Play with this control and understand it's effects of gain and its' use for the RF signal that is an IF image - also has an audio component to it too. So you can use this trimpot as a training aid to help you discern what is a pure signal versus and overloaded one - as well as the obvious - weak hissy noisy signal.
As you tune a radio - you need to be aware of your signal generators actual level input signal strength.
If you put too much in, AGC will kick in and push down all the noise and you also lose your ability to truly peak the signal the right way as well as center on the image you need that the radio is passing from the RF amp, past the 1st IF amp into this 2nd IF amp stage.
The sections I'm talking about are L5, L6 and L7 - with L6 being the main focus of nulling and finding the "dip" in noise that indicates you found the image you needed to obtain - the coil is aligned to capture the carrier in the image...
This is where the 10.69 ~ 10.7MHz IF is "mixed" with a 10.240 signal (your main PLL reference Xtal if that is needed to help you understand this) you don't tune for maximum noise, you actually tune to center in on a carrier (in your first steps you send out a simple dead key to peak out the RX tuning coil cans) as you peak them out - the signal may need to be attenuated - so do it!
If you need to make it easy, just use a weak signal and or a small wire in the antenna jack of the radio needing to be aligned so it doesn't keep overloading the receiver and AGC has to kick in all the time.
Then continue the peaking procedure as you work - just remember that the biggest effort for listening and "nulling" out noise - is to pay attention to the conversion of the 2nd IF and the 455kHz section - this is the most critical and if not done right - you will hear hissy carrier noise incoming with the audio signal - and you wonder why the NB and ANL can't quite work this out...
Pay attention to the S meter too - it can provide the "dip" indication - how? Well, review the detector circuit - the embedded noise in the radios own IF signal - is SUMMING in with that dead carrier - you want to have the best signal to noise ratio right? Well, IF can be considerably noisy - so we are doing this step to help reduce the noise floor for that purpose - the S-meter picks up that noise as part of the detection and it's turned into a voltage - a noise signal BIASES your S meter - so as you tune in to that dead carrier - remember to look for and listen to - a rush of noise, S-meter goes up - as you get closer to the image (L6 area is where this effect is most pronounced) the noise will start to fade - the S-meter will or can drop - then as you keep turning to tune - the noise will reappear and the S-meter will jump and then fade again as you tune away...ok if you saw that, then remember VR2 needs to be turned down a little more - then this helps that section "Narrow" down the filter and you can get the carrier centered and S - meter will also follow - means that the AGC has not kicked in yet, good - that is what you want - the weakest signal you can hear - and see - and max out for that.
Now, you can re-trim VR 2 to allow the S-meter to show proper actions - you can also verify you have centered on the carrier properly (why do I say carrier?)
You're focusing on the "dead spot".
The noise is the internal birdie the radios making against your efforts - so as you tune to the dead carrier - that is what it is a noise free signal - you obtain the image the carrier shows up in as the Audio from the Image of the Signal you want. That dead spot and dip - there is your audio signal image off of L6...
This is for Educational Purposes Only - Not Responsible for Articles Left In Your Clothing That You Are Too Lazy To Remove....
