[ARC5] Frequency shift with percussion

[Bill Cromwell]

Hi Charles,

The main selectivity should be as close to the antenna as possible. But. 
I use a NEScaf filter at the audio output. That functions much as a 
'Select-O-Ject" would with it's variable center frequency and variable 
width passband. It's a digital circuit but is NOT a computer sound card 
gadget. No computer. No sound card. No software. It is even useful on my 
40 meter receiver with the wide IF. The NEScaf can choke down to 
something slightly less than 100 cycles, open up to allow AM or anywhere 
in between. You are still responsible for avoiding overload in the rest 
of the radio so you'll have to resort to being a radio operator <grin>. 
I suspect you are up for it.

I have considered inserting a small converter to move the IF signals to 
a frequency where xtals and entire filters are more available and then 
back to the IF chain after filtering. If some solid state bits living 
inside your Navy receiver is a repulsive idea just scratch that off the 
list from consideration. So far the audio filter has been adequate and I 
am avoiding the 'effort' to build that up converter/filter/down 
converter. You will have choose frequencies carefully.

I also piped the audio output into my computer sound card and processed 
it with SDR software. I got mixed results and I suspect it was from 
overload in the sound card. Forty meters was fading out so I put that 
experiment aside. Wide passbands are ideal for the SDR software approach 
so I'll revisit that. I've had good results with some other radios using 
the SDR approach. Note this is a sever abuse of SDR software and the 
purists will be whining and moaning about it.

73,

Bill  KU8H


On 03/01/2015 09:10 PM, Charles wrote:
> I found another screwup from the previous owner of my Navy 3-6 Mc. receiver... he’d disconnected an important resistor from the detector diode plate to ground, and I hadn’t spotted it when I reconnected the audio to the grid of the 12A6. That explains the very weak audio and strange transient response! Also, the lug he’d messed with on the .006 uf mica broke off when I moved it, so I had to dig into the chassis and replace that. Now I’ve got lots of audio – in fact it’s better than my 7-9.1 Mc unit with the same speaker.
>
> Next I proceeded to install a “gimmick” and cathode pot on the 1st IF but it was too touchy. Put it on the 2nd IF instead, but then noticed my dial was way off frequency. WTH??
> Eventually I discovered that rapping on the oscillator coil can with a screwdriver handle would move the frequency anywhere from 2 to 10 KHz up. And if I turned the receiver upside down or on its side, and banged on the coil, it would jump back down in steps. The only thing that could cause this behavior is a loose slug in the osc coil. I took the osc coil can off and looked closely at it – poked the slug with a stick, it didn’t move. Put it back together, behavior still there. Finally applied the Sherlock Holmes maxim, “Eliminate the impossible and what remains, no matter how improbable, must be the truth”. Took it apart again, heated the wax with a heat gun and tried it. Much less microphonic (microtunic?)...So I melted some candle wax onto and around the slug and that fixed it.
>
> Now if I could find one or two crystals at the IF (1.415 MHz), a simple crystal filter like the one I installed in my 7-9.1 Mc receiver would improve selectivity quite a bit. Conditions are pretty rough on 80/75m with a broad receiver but I could copy most of an ongoing AM QSO on 3873. The 40m unit would not be usable without the filter since its IF is twice as wide as this one!
>
> -Charles
> WB3JOK/0
>