BC 221 Frequency Meter tidy up

[Rod Salter]

Next came the testing of the unit itself, the headphones must be plugged in to 'Switch on' the unit.

Armed with a circuit I checked the filament circuit connections and applied 6 volts from a truck battery.

The leads soon started to give off “The Smoke”

After some poking around I found pins 2 and 7 of one of the valves were bridged together, snipping that allowed the valves to warm up. Good so far.

Next was the B+, I used a dropping resistor connected to a mantle set as the power source.

Some testing with a shortwave radio and a Digital frequency counter, I found - 'nothing'

There is re-soldering on the output valve's socket, the cathode resistor had come adrift.

5.jpg

This resistor and some others are not originals.

Resolder and more testing, Bingo, a signal although it seemed weak.

I left the valves on without the B+ while I had dinner when I got back I sensed the output valve was barely warm compared to the other metal valve.

It was a 12 SJ 7 for a 12 volt system, swapping it for 6 SJ 7 brought a hot valve and a good signal.

I manufactured an aerial connection from a .22 brass shell and a spare screw.
Putting it back in the case and powering up, it works well.

Some of you may have noticed the calibration book was not for this unit, believe it or not, the calibration sequences are spot on!

A power supply - in the next post.