[Boatanchors] Collins receiver distortion

[Gary Schafer]

I have always noticed audio distortion in the Collins S line receivers.
I have acquired an HP 3586C "wave analyzer" also known as a selective 
voltmeter. Made some measurements on the audio of my Collins 75S3. With 
a carrier tuned in (crystal calibrator) to produce 1000 hz audio tone I 
measured the 2nd and 3rd harmonic levels in the audio.

With the agc on and full rf gain the second harmonic was down 30 db. The 
3rd harmonic was down about 40 db.

With the rf gain backed down to just below the point where the agc is 
active the distortion is much better. The 2nd harmonic is down 40 db and 
the third is also down 40 db!

It is noticeable when listening to any of the S line receivers that as 
soon as the rf gain is raised so that the agc is working the audio 
distortion goes up. This can be heard on SSB signals also.
I have in the past played around a bit with the bypassing in the audio 
stages to try and clean it up but have been unsuccessful.
The problem has something to do with the agc. Maybe it clips the rf at 
the detector and some of the artifacts get through to the audio?

Now that I have identified it as second harmonic energy it may help in 
resolving. Haven't had time to dig in further.
Anyone else had any success in improving the Collins audio?

As a comparison I also measured my Kenwood ts430 at 1000 hz. 2nd 
harmonic was down 57 db and 3rd was down 53 db! Much cleaner than the 
collins.

Just to see if the audio circuit was rolling things off in the ts430 I 
also used a 500 hz tone and got 54 db down for the second harmonic and 
47 db down for the third.

It did not make any difference in distortion on the ts430 whether or not 
the agc was functioning.

73
Gary  K4FMX