[ARC5] Frequency shift with percussion

[Charles]

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