[R-390] R-390 IF Module (was: Wanted)

[Drew Papanek]

On the topic of swapping the non-A IF module into the R-390A, Don wrote:

>They're out there.  I had no trouble finding one 5 or 6 years ago, when the
>harsh, fatiguing sound of the mechanical filters in my 390A finally drove
>me to do something (swap in the 390 IF strip).  It wasn't even expensive.
>But what an improvement!  It's now the second-best sounding radio I have,
>next to the Drake R8B (which, coincidentally, also has LC filters in the 
>IF).

Some years ago I worked on a 75A-3 for a friend and I noted that it had only 
the SSB filter and that the other filter switch position had no filter 
plugged into its corresponding socket.  On a whim, I inserted a cap (for 
coupling) and a couple of resistors (one for plate load, the other for grid 
bias) into the unused filter socket.  I then noted that reception in the 
now-enabled second filter switch position had quite good audio, if a bit 
broad.

I've pondered a modification for the R-390A along somewhat similar lines.  
The 16 KC filter would be bypassed and the q-spoiler resistors in the IF 
cans would be increased in value or perhaps eliminated entirely so that the 
IF cans alone would determine the radio's bandwidth in the 16KC position.  
The IF would be realigned to provide a bandwidth for good audio quality and 
would not affect operation with the other filters since its bandwidth would 
be wider than those of the other filters.

The IF module so modified would have higher gain than original, particularly 
in the 16 KC position (no filter insertion loss) and that would need to be 
compensated for.  Perhaps the modification could incorporate a switching 
arrangement to alter the cathode bias resistor on just the first stage or 
maybe also on subsequent stages.

Another although somewhat inferior gain compensation arrangement would just 
replace the 16KC filter with an attenuator to compensate for the now-missing 
insertion loss, and the IF gain pot would be reset.  (The Q-spoiler 
mod/realignment would also be done.)

Just my thoughts on conserving the diwndling supplies of non-A IF modules 
while helping to insure that the comparatively plentiful -A IF modules get 
irreparably hacked.

Drew