I understand why one would want to eliminate the mechanical filters in the IF
module. L/C filters are less harsh sounding then mechanical filters. Looking
at the comparison between the R-390 and R-390A (enclosed), it seems that the
only real weak link is the R-390 regulated power supply. The other differences
between the two radios are minor.
Why not eliminate the voltage regulator circuit in the R-390 in favor of the
simpler design of the R-390A L/C filter? It appears that the high voltage
winding of both power transformers have the same rating. This would eliminate
a lot of heat, increase reliability, and perhaps provide the room for the two
added filter inductors.
I can see that using a Zener diode to regulate the B+ in the various
oscillators would be desirable but that should be an easier task then rewiring
the tube heaters in the non-A IF amplifier. Comments?
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence. Murphy
On Monday, January 27, 2025 at 04:40:22 PM CST, Tom M. via R-390
<r-390@mailman.qth.net> wrote:
Attached are two documents when pasted together yield the schematic for the
R-725 IF deck. I used this to guide me when doing my conversion. Note there is
a B+ dropping resistor added in the R-725 IF deck
On Monday, January 27, 2025 at 01:08:59 PM CST, Byron Tatum
<bjtat...@att.net> wrote:
A quick question regarding the filament string re-wiring in R-390 IF deck to
allow use in an R-390A. I am assuming 12AU7A tubes should be used (as marked on
IF deck chassis) due to their slightly lower filament current draw. Are 5814A
tubes to be avoided in regards to this string re-wiring?Byron W5FH
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