Current-operated 'S' Meter for Crystal Radios - UK Vintage Radio Repair and Restoration Discussion Forum

vu2nan-nandu · 13th Aug 2015, 5:25 am

Crystal radios with high-impedance voltage-operated phones require, as the 'S' Meter, a high-resistance microammeter connected across the phones. In other words, a sensitive voltmeter that does not bypass headphone current significantly.

On the other hand, crystal radios with relatively low-impedance current-operated phones require, as the 'S' Meter, a low-resistance microammeter in series with the phones. That is, a sensitive ammeter that does not drop headphone current signnificantly.

For my crystal radios, using sensitive balanced-armature phones (having an impedance of the order of a few hundred ohms), a voltmeter was picked up from the junkbox. On removing its series and shunt resistors, the resistance of the 1mA FSD movement turned out to be only 60 Ω hence quite suitable for the purpose. The voltage markings on the meter face were removed and the graduations marked 0-10 instead, using a lead pencil.

The meter was already mounted on a square bakelite flange. Mounting holes on the flange served as terminal- mounting holes, obviating the need for an enclosure.

The result - an interesting weekend project and a handy crystal radio test aid.

13th Aug 2015, 5:25 am	#1
vu2nan-nandu Pentode Join Date: Jul 2014 Location: Bangalore, India Posts: 132	Current-operated 'S' Meter for Crystal Radios Crystal radios with high-impedance voltage-operated phones require, as the 'S' Meter, a high-resistance microammeter connected across the phones. In other words, a sensitive voltmeter that does not bypass headphone current significantly. On the other hand, crystal radios with relatively low-impedance current-operated phones require, as the 'S' Meter, a low-resistance microammeter in series with the phones. That is, a sensitive ammeter that does not drop headphone current signnificantly. For my crystal radios, using sensitive balanced-armature phones (having an impedance of the order of a few hundred ohms), a voltmeter was picked up from the junkbox. On removing its series and shunt resistors, the resistance of the 1mA FSD movement turned out to be only 60 Ω hence quite suitable for the purpose. The voltage markings on the meter face were removed and the graduations marked 0-10 instead, using a lead pencil. The meter was already mounted on a square bakelite flange. Mounting holes on the flange served as terminal- mounting holes, obviating the need for an enclosure. The result - an interesting weekend project and a handy crystal radio test aid. Attached Thumbnails