Hallicrafters HA-1, Operating And Service Instructions

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3-4-1-2.KEYING CONTACTS (Pin 2, 3 and 4)
Pin 2 (normally open to ground) is normally used for transmitter keying
purposes.
Pin 3 and 4 (independent, normally closed to ground) are provided for
auxiliary control of station facilities.
IMPORTANT
The mercury-wetted relay contact rating is 5 amperes maximum, or 500
volts maximum, the product not to exceed 250 voltamperes with contact
protection. For example, the maximum allowable voltage with a 5-
ampere load is 50 volts ( 5 A × 50 V = 250 VA); the maximum allowable
current with a 500-volt supply is 0.5 ampere (500 V × 0.5 A = 250 VA).
Refer to paragraph 3-5 for method of determining required contact
protection network.
3-4-1-3.Speaker (Pin 6) AND AUDIO OUTPUT (Pin 7)
A standard circuit transfer type PHONES jack (J1) on the rear apron
automatically disconnects the internal speaker when the phone plug is inserted.
Output impedance is 3.2 ohms, although it is suitable for use with any low or
medium impedance headsets.
Figure 4. Typical Station Installation.