Type A Power Supply

An a-c operated power supply for instruments which use the Signal Corps BA48

Battery Block (Burgess Type 6TA60)

Figure 1. View of the Type 1261-A Power Supply.

• BATTERY OPERATION of a measuring instrument is necessary if the instrument is to be truly portable and easily used in any location. Many instruments designed primarily for portable use can be and are used in production test set-ups where they are in continuous use. A-c operation for such instruments would eliminate the need for frequent battery replacement, but would limit portability.

The Type 759-B Sound-Level Meter is a good example of a portable battery-operated instrument which is often used in production testing. Early in 1942* the Type 759-P50 power supply was brought out as an a-c operated power source which was interchangeable both electrically and mechanically with the battery block in the sound-level meter. Thus, the sound-level meter became readily adaptable for either production work or for field use. This power supply could also be used in the older Type 759-A Sound-Level Meter.

Other instruments intended for portable use, and hence battery operated,

*See "An A-C Operated Power Supply for the Sound-Level Meter,'* by H. H. Scott, General Radio Experimenter, January 1942, Vol. XVI, No. 8.

have been developed and manufactured since the Type 759-P50 Power Supply became available. A number of these instruments use the same battery block as the Type 759-B Sound-Level Meter, but differences in plate or filament supply loads and other small details prevented the use of the Type 759-P50 Power Supply in these later instruments. For all these instruments, a new power supply, the Type 1261-A, has been developed which incorporates the changes required to make a more complete replacement for the BA-48 battery block.

This new supply, by means of suitable selector plugs, can be used in the following General Radio instruments: Type 759-A and Type 759-B Sound-Level Meters, Type 720-A Heterodyne-Fre-quency Meter, and Type 1231-A Audio Amplifier and Null Detector. Octal selector plugs inserted into a socket on the top of the power supply are used to select either a 1.5- or 3.0-volt filament supply and, in cases where the current drain of the instrument to be supplied is less than required for normal operation of the power supply, the plug is used to add load resistors which insure normal operation.

The Type 1261-A Power Supply is not limited to use in General Radio instruments. An unwired selector plug, which can be wired by the customer, is available so that the power supply can be used in any battery operated instrument which has plate and filament requirements within its scope. Intended as a general purpose replacement for the BA48 battery block, the power supply is a light and compact unit that fits in the battery compartment, and has a four-

terminal output socket which fits the plug on the battery cable of instruments using the BA48 battery.

The filament supply, which consists of a selenium rectifier with an L-C filter, has two small flashlight cells connected across the output. These cells are connected in series for 3.0-volt output, and in parallel for 1.5-volt output by means of the selector plug. The cells act as a regulator for the filament supply and also provide an extremely low a-c output impedance. Hum and low-frequency changes in output caused by a-c power line fluctuations are reduced to the point where satisfactory operation of the high-gain amplifiers used in the sound-level meters and in the Type 1231-A Audio Amplifier and Null Detector is obtained.

A small push button switch, located on top of the power supply, makes it possible to disconnect the cells at any time and to set the output voltage equal to the cell voltage. Under operating conditions, normal line voltage variations cause the cells either to charge or to discharge slightly, and when the instrument is turned off a small relay opens the circuit so that the cells will not be damaged. These flashlight batteries cost only ten cents apiece and, as used, their life is extremely long. A conventional vacuum-tube rectifier is used to obtain the d-c output voltage for the plate supply. A four section R-C filter attenuates the hum or a-c ripple voltage to an exceptionally low value. The plate supply is not regulated.

The Type 1261-A Power Supply replaces the older Type 759-P50 for use in the Type 759-A and Type 759-B Sound-Level Meters, but is not directly interchangeable with it. A modification incorporated in the new power supply to protect the small flashlight cells, used in the filament circuit, requires a simple wiring change in the battery cable of older instruments. This change does not affect battery operation of the instrument and has been made standard wiring for current and future lots of instruments. Complete instructions are furnished with the powrer supply so that the customer can make the required change in older instruments, and once the change is made the Type 1261-A Power Supply is directly interchangeable

Figure 2. Schematic circuit diagram of the power supply.

Figure 2. Schematic circuit diagram of the power supply.

with the BA48 battery block. Similar quency Meter. For instruments shipped changes must be made in existing models in the future, no changes are necessary, of the Type 720-A Heterodyne Fre- —E. E. Gross

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