[R-390] 390 non-a i.f. in R390a

[Dan Merz]

Hi,  I completed the conversion of a 390 non-a i.f. chassis to go in a
R390a radio.  This follows the article of Tom Marcotte in Electric Radio
Dec 2000 describing the conversion.  I found his article flawless in the
detail given for point-by-point wiring changes and thank Tom for writing
the article and providing  me a readable schematic for the 390 non-a
chassis and some other information on the R-725.  I deviated slightly in
a couple of cases to utilize some of the existing wiring but this was
just a preference on my part and amounted to the same connections.  Most
of the wiring mod's involve converting the series connections for 25 v
filament supply used in the 390 non-a to the 6.3 volt connections
required in the 390a,  plus dropping the B+ voltage a bit.  This is all
explained clearly in his article,  along with the background for the
R-725 radio which the final set emulates.   No modification of other
parts of the 390a radio are required other than taking the 390a i.f.
chassis out.   The conversion was done on a junker 390  non-a i.f.
chassis sold on eBay and it worked immediately upon putting it into the
EAC 390a set,  which brought a feeling of relief since I had no idea
what might be wrong with the junker chassis and I didn't have a 390
non-a radio to test it in.  One of the bigger tasks was to make a cable
adapter for BNC to mini-BNC,  or MB,  connectors for the two inputs from
the 390a r.f.  chassis. Tom used two of the 390a i.f. output adapters of
the type on the rear panel of a 390a. I had trouble finding these, and
I'd like to thank Roy Morgan for his assistance in trying to get the
relevant connectors from  The RF Connection - I opted not to use their
MB female to BNC female adapter because the two critical parts would
have cost $20 each plus shipping and I would have to make up two short
BNC to BNC cables as well. Instead I used some chassis mount MB
connectors from Fair and made a small aluminum box with the MB
connectors on one side with RG 58 entering on the opposite side,  with a
BNC connector on the outside end of the RG 58.  I can't say cheaper is
better but it was a bit of fun and turned out well.   If there is any
interest,  I'll post the details of how I did that but it's not too
complicated.  I'd recommend buying the RF Connection adapters unless you
enjoy cutting and bending metal like I do. I'm assuming the adapters
from RF C  look more or less like the 390a back panel i.f. adapter.

I was quite impressed with the appearance of the  390 i.f chassis,
though it's much more crowded than the 390a i.f. chassis and not the
easiest thing to reach into with a soldering iron, even a small one. My
first reaction on looking at what had to be done was "ughh."  As Tom
advised,  use any trick you can think of to do the soldering.  So when I
finished the soldering,  I felt I had done something challenging and had
not destroyed the beauty and function of the unit in the process.  I
hope the next guy looking at it will find equal enjoyment with the
unit.  This project took about 2 years to complete,  most of that time
looking for the i.f. chassis after I read Tom's article.  I got this
chassis  Dec 27 so it took about 3 weeks to round up the connector stuff
and make the conversion after that.

I'll post my impressions once I test the set a bit more;  I'm going to
add the two diodes ala Lankford and increase the bfo coupling cap.
Thanks to all for earlier comments regarding the connectors,  Dan