[AMRadio] Collins R390/URR versus R390A

[D. Chester]

Most of the changes from the 390 to the 390A, according to Signal Corps 
documents, were made as cost-saving measures, not for the sake of improving 
performance.

The mechanical filters result in tighter selectivity, but at the expense of 
nonlinear phase shifts inherent to mechanical filters.  That is what causes 
the harsher audio quality in the A version, as well as in receivers like the 
75A-4 that use mechanical filters.

I have both  receivers, and find the 390 better suited for most amateur AM 
reception.  The mechanical filter selectivity of the 390-A can be wonderful 
under congested conditions, but both  receivers should have included an 
additional intermediate selectivity position at 6 kHz.  With normal phone 
band congestion on 160-80-40, the 4 kHz filter is often too narrow for AM 
reception while the 8 kHz filter is too wide.  With the 390, although the 
nominal 4 kHz selectivity seems to work a little better for amateur AM, the 
8 kHz position is still often too wide.

I have added additional filters to my 75A-4 using an outboard adaptor, and 
have selectivity positions available @ 300~, 3.1, 4, 6, 8, 9.7 and 16 kHz. 
When the bands are lightly congested, I prefer the 8 kHz selectivity, but 
most evenings when operating in the 3870-90 kHz Ghetto, I use the 6.  When 
there is heavy QRM I often switch to the 4 or even the 3.1.

Don k4kyv