[Collins] R390A query

[Dr.Gerald Johnson]

Fundamentally the limit is that the two RF stages and slugged tuned 
coils can't tune that far down. That's why they made a R-389. Or for the 
51S-1 they made an external tuner, the 55G-1.

I think (can't find my 390A manual right now) there is a test point for the 
first mixer grid. There is in the 390 book that I found. You can connect a 
LF tuner to that test point and then the radio should come alive on 
the VLF frequencies.

Another alternative is a mixer and a crystal oscilator at some 
convenient even frequency with a low pass filter with about 500 KHz 
cutoff as a receive converter. I've used that many times with that low 
pass filter for VLF. Without the low pass filter notching out AM 
broadcast band, broadcast band was all that I could find.

There are signals at VLF up through 500 KHz, though without weather 
the aircraft beacons get boring, there isn't much random CW on 500 
KHz anymore, and most of the other signals are specialized like LORAN-
C or WWVB or narrow shift encrypted FSK. The Omega beacons are 
gone. Not even WWVB is interesting to listen to, takes a special 
decoder to read the modulation, and when using to check a 
frequency standard, a superhet receiver is useless, it takes a good TRF 
(which I have) and then lots of work to filter out local and distant 
noise.

For a mixer, I use something from MCL that I've packaged up with 
coax connectors, probably like the SLB-1 running the LF signal to the IF 
port. I have an assortment of crystal oscillators in boxes with coax 
connectors that I've built over the last 40 years that still work for that. 
But these days a computer type crystal oscillator module comes for just 
a couple bucks, and some need 3.3 volts, some need 5 volts. I don't 
remember the source of the low pass filter that I built, I could have 
designed it from scratch or copied something from a handbook.

Those are the options I know of.

73, Jerry, K0CQ, Technical Advisor to the CRA
 

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Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.