[R-390] A vs non A

[Charles P. Steinmetz]

Robert wrote:

>I may be picking up up what I believe to be a R390. When I was talking to a
>friend about this he kinda scoffed saying they were less desirable and
>harder to work on than the 390A.

As others have said, everything about the 390 is a bit higher-grade 
than the 390A.  (The 390A was designed as a cost-reduced version of the 390.)

Note that the 390 has LC IF filters instead of the mechanical filters 
in the 390A, and for that reason sounds much, much better due to the 
more linear phase characteristics of the LC filters.  For this reason 
(better phase characteristics/less group delay), some special 390As 
were made with the LC filters for specific applications (these are 
designated R-725/URR).

A possible down side of the 390 is that the LC filters are 
stagger-tuned in a way that is not easy to duplicate at home -- you 
really need to sweep align them, and to know what you are doing, 
which is best done with the IF out of the radio and installed in a 
test jig.  Chances are they have not been adjusted, so if you just 
leave them alone they should be fine.  If someone has been there 
before, however, all bets are off.  Note that the IF alignment 
procedures given in the TM-11-5820-357-35 (1966) and TM-11-856 (1955) 
manuals do NOT stagger-tune the IFs -- if you read carefully, they 
both state "Perform the procedure outlined . . . below only when the 
transformer cores have been displaced greatly from their normal 
positions within the cores."  In other words, "This procedure will 
get you back on the air, but will not return the radio to its proper 
IF alignment."

Other than retuning the IF if it has been messed with (which, 
hopefully, it hasn't), and the heat generated by the voltage 
regulators, which can be controlled with fans, there is absolutely no 
reason to avoid a 390.

Best regards,

Charles