[Milsurplus] Transplanting R-390 IF modules to R-390A

[pete williams]

Convention has it  that directly  substituting the R-390 IF module  to a R-390A to 
simulate a R-725/URR wasn't possible  without making  changes to the  heater wiring. 
Reason being  the original  R-390 module was  wired for  a  25V  heater and the 
R-390A used 6.3 V. It was  further  assumed that the transformer of the  R-390A had 
insufficient capacity if its 26V  secondary was used.--- the  6.3 V line  was  ample.
Not impressed  with the work needed to rewire in a  cramped environment, a look at 
the heater layout  for  both  receivers in the  IF  area, showed  the  following.
The R-390 needed 25V on pin #20 from the plug P-117 . On the  R-390A, pin #20 had 6.3 
V.
also,
The  R.390 needed 6.3V on pin#8  from the plug P117.. On the  R-390A, pin #8 had 25V.
Question..1.. why not  transpose wires on pin  #20 and  #8 and  satisfy the  voltage 
requirements  without the hard  work?
Question 2.. would the  xfmr in the  R-390A handle  the  extra  demand?
Doing it   and  measuring the  results gave the following...
....Total current from the 25V tag on the  xfmr  was 1.2 A with  12BW4 tubes  as 
rectifiers, and   3TF7 in use.--- just on the maximum continuous  rating  for the 
xfmr. Obviously would be less if SS  diodes  used in  lieu of  tubes.
Current drain on switching to CAL rose to 1.45 A but well within the 4.6 A  allowed 
on a 70% duty  cycle (see xfmr  specs)
RESULT..
No problems  experienced  with Xfmr  heating or  drop in voltage...   and the  radio 
performs.
Changing the pin connections  as  outlined in P -117 not  too difficult.
These  results do NOT include the  use of the  switched ovens but it's  your  choice.
NOTE.. further reduction of the 25V  current  drain  could be  achieved by removing 
the   ballast tube , running a  connecting wire  from pin#8 to pin #19 and  chassis 
grounding  pin 7 of the  ballast tube   socket. This  the  allows 6.3 V to  be 
applied  to the  BFO  and  VFO  tubes .You'll need to be  running  6BA6.
The  army specs would, no  doubt be  exceeded but then  as  casual users  we don't 
run 24 7 365!-- do we.?
Maybe it's  a  reinvention of the  wheel, but if  so  I haven't had it rotated  here.
Thanks  to Gary  Gitzen   and  Tom Marcotte who  have  both been across these 
developments
Any questions?
Cheers
Pete VK3 IZ





Pete D..Williams
METUNG, 3904
AUSTRALIA
jupete at bigpond.net.au