[Hallicrafters] SR-400 1st mixer mod

[Walt Cates]

I have an SR-400 on the bench that everything works fine, but sensitivity on 80, 20, 15 and 10 meters was in the mud. The 40 meter band worked fine. So I thought I would try Mr. Orwin's (SK Hallicrafters engineer) mod to the first mixer. I used the version where he put the 4.7K from the plate of V2A (pin 1) to the reg 150V line. And added a 150pf from V2 pin 1 to V2 pin 6. AMAZING difference. The 80, 20 and 15 meter bands all increased 15db and 10 meters increased 9db. The 40 meter band seem unaffected, which seems a little Unfortunately Unfortunatly I did not test for birdies before the modification. 

With my test equipment I can only get reliable measurements on birdies down to 35db below a 0.5uv carrier signal. So this is what I found after the mod.

21.413mhz audible not measurable.
21.483mhz -22db (I can live with this one)

14.623mhz -32db
14.136mhx -18db (need to work on this one)

3.587mhz audible not measurable.
3.760mhz audible not measurable.

No other birdies audible but it looks like i have some bird hunting to do. 

Any way, I found the mod to be an improvement and thought I would pass on my experience with it.

Best Regards, Walt Cates, WD0GOF, http://www.myhamshack.com/WD0GOF/<http://www.myhamshack.com/WD0GOF/>