Heathkit 309 C Owners Manual

Page 1

ASSEMBLY AND OPERATION OF THE

HEATHKIT R. F. PROBE
MODEL 309-c

The #309-C crystal probe extends the AC range of any 11 megohm input vacuum tube voltmeter
to over 100 megacycles. A crystal diode employed as a rectifier is capable of linear-response
over a very wide range of frequencies. Response of the #SOQ-C probe is substantially flat from
1000 cycles to over 100 megacycles. D. C. isolation is provided by the .005 ufd condenser. A
ceramic capacitor of this type has low impedance to RF and is ideal for use in this application.
The 4.7 megohm resistor acts as a combination isolation filter and multiplier. Voltage at the
cable end of the resistor is negative DC and the VTVM selector should be set accordingly.

A well shielded probe with the detecting elements contained in the housingis necessary for sat-
isfactory readings at high frequencies. The Heathkit #309-C RF Probe meets these requirements.
Input leads are extremely short in order to eliminate extraneous signal pickup which can give
false readings. The metal probe housing is grounded to prevent hand capacity effects from
causing deflection of the meter needle Etched circuit wiring cuts down circuit capacity, ime
proving high frequency response.

The input capacitor is rated at 500 volts DC and the probe should not be applied to circuits where
DC voltages higher than this are found AC or RF voltages in excess of 30 volts should not be
measured, as damage to the crystal may result. As this probe is designed primarily for RF,
signals below 1000 cycles will read low and the rectifier in the VTVM should be employed by
turning the selector switch to AC. The capacitive effect of the regular AC leads will be negli-
gible at frequencies below 1000 cycles.

Accuracy of the probe is maintained within approximately :10% and this accuracy is adequate
for RF work. The readings obtained are EMS and should be multiplied by 1.414 to convert to
peak readings.