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

[Dan Merz]

Hi Scott,  an R-725 is more or less  what I  now have,  though built by
contractors, and had the 390 non-a type i.f. in a 390a radio,  and a couple of

other mods,  one to improve PTO stability and an added 25 volt filament supply

to power the R-390 i.f. chassis.  The purpose was to eliminate the non-liner
phase shift caused by mechanical filters,   which caused errors for direction
finding equipment,  and I guess to have a current production radio at the time

the decision was made that was nearly the same as the 390a otherwise,  in the
early 50's.  Approximately 300 were made according to Tom's article.   I don't

know more details of exactly how it was used.  Yes I did it for the fun  of
the project and to explore and perhaps enjoy the audio characteristics of a
390 non-a i.f. ,  which is reported to be more pleasant to the ear.  Why not
just use a 390?  I don't have one and decided I would rather not obtain one
for now because of space and effort to get it going and the 390a I have is a
clean set - one 390 type radio seems to be enough so far.  They are somewhat
inconvenient to casually move around from a behind-the-scenes location and use

when the fancy strikes you.   Who knows what the future will bring. I've been
known to change my mind about such things,   I have several other types of
communication radios competing for shelf space where I can operate them so my
decision was biased by that consideration. I am pleased by the consideration
that the chassis can be easily swapped in about 15 minutes, and either will
work interchangably in the 390a radio. I can't think of a reason to have this
conversion if you already have a satisfying 390 non-a unless you want to see
if it will perform a little better,  or are just very curious and like the
unusual.   Marcotte wrote in an earlier ER article about the R-725 he had at
the time,  April 1996:  "How does it work?  Quite well, just like an R-390,
but with the smoothness of the fewer gears of the R-390a gear train.  It is
actually more sensitive than my all-Collins R-390 and R-390a,  and has none of

the ringing caused by the mechanical filters in the R-390a."

 So that's my story and I may not stick to it.  If I missed some aspect that
interests you,  ask away,    best regards,  Dan.

Scott Seickel wrote:

> Hi Dan,
> I do not receive Electric Radio and have no idea what a R-725 is.
> Would you be so kind as to clue me in?
> Did  you do it for the fun of the project or is there some performance
> advantage that I am missing?  I would imagine that it would make the
> radio better for SWL'ing but why not just use a 390?
>
> Thanks,
> Scott
>
> ----- Original Message -----
> From: "Dan Merz" <djmerz@3-cities.com>
> To: "R-390 List" <r-390@mailman.qth.net>
> Sent: Monday, January 20, 2003 9:59 AM
> Subject: [R-390] 390 non-a i.f. in R390a
>
> > 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
> >
> >
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