Topband: Collins R390 and 75A4 on 160m

[Tom Rauch]

> I have a 1955-vintage Collins R390 here and am curious about whether
> anyone on the reflector has done any comparisons on 160m with the 75A4
> - or what anyone who has a R390 thinks of its performance on 160m.

I have both early and late R390's (the late is a 390A). That does not 
make me an expert, but at least I have looked carefully at the two  
units.
 
Neither is all that good for BDR or IM3DR, and both are pretty poor 
on narrow selectivity. The filters are so poor, it would be nearly 
impossible to do a really narrow test. 

The R390A is the better of the two, since in the narrowest positions 
it uses a 2kHz mechanical filter. As I recall, the 390A has 16,8, 4 
and 2 kHz mechanical filters reasonably close to the low IF 
mixer...but unfortunately has two RF amps and other many stages that 
cause problems ahead of the filters.

Unfortunately the R390A has only a single pole crystal way back in 
the IF chain to do the less than 2kHz stuff, so the filter skirts are 
horrid. The R390, with only L/C filtering is even worse. Both have 
more filter blow by than my IC706! 
 
My R390 sits on the AM radio band, so I can listen to AM BC while 
working. If it wasn't so heavy, I'd throw it in the attic of my barn 
with all the other things I'll never use. My opinion, after trying to 
do a IM and blocking test, is it is junk compared to almost any 
transceiver you can buy today.

I don't have a 75A4, but I wasted a lot of time converting a 75S3 to 
160. It is better than the R390A, but not by much. The 32S3 
transmitter is very clean. 

73, Tom W8JI
W8JI@contesting.com