This equipment consists of a high grade two tube repeater amplifier circuit with either high or low impedance input and output. The amplifier has it maximum gain of 25 db. and the frequency response characteristic is linear within plus or minus 1 db between 25 and 10,000 Hertz.


The Loudspeaker Unit comprises a high quality permanent magnet dynamic loudspeaker mounted in a special baffle and finished to match the other units on the rack. Its impedance is matched to the high impedance output of the repeater amplifier by a specially designed transformer incorporated in the unit..


The Rectifier Unit provides plate supply and tube heater current for the complete equipment except for the modulation monitor which has
a separate heater transformer. It is capable of delivering 5 amperes at 6 volts for the tube heater supply and 100 ma at 275 volts for the B+. A
voltmeter and main line switch are provided on this unit, also taps on the power transformer to accommodate line voltages from 105-120 volts.


Each type of monitoring equipment consists of a single 19 inch rack assembly of various panel mounted units arranged in the following
order from the top downwards:-

Assembly A-39865

Loudspeaker 38950
Meter panel 39100
Repeater amplifier 38938
Switching panel 39869
Modulation meter 39864
Frequency meter 39863
Frequency sub standard 39757
Blank panels 34825-34825
Rectifier 39102

Assembly B-40078

Blank panels 34825-39296
Meter panel 39100
Blank panel 39296
Switching panel 39869
Modulation meter 39864
Frequency meter 39863
Frequency substandard 39757
Blank panels 34825-34825
Rectifier 39102

Each rack is fitted with a small shelf at table level for the convenience of the operator. Metering and control facilities are provided in easily accessible positions. All the units ate removable and each is completely shielded against electrical interference. The arrangement of removable units allows complete flexibility of the system in that additional equipment may be added if required. The panels are  1/4 inch aluminum of standard height and are finished in St. James gray or black duco as desired. The external connections necessary are the pick up lines for modulation meter and frequency meter and the 110 volt 60 cycle or 25 cycle A.C. line. The A-39865 monitor requires one additional line for checking the audio input to the transmitter.


The tubes used in this equipment consist of the following:

Modulation meter Two RVC-25Z5
Frequency meter One RVC-37 , One 1 RVC-89, One RVC-79
Frequency substandard One RVC-6A7
Repeater amplifier One RVC-89 1 R VC- 37
Rectifier unit Two RVC-80 .


A-39865 20.5 in 79.5 in 30 in 350 lbs
B-40078 20.5 in 79.5 in 30 in 300 lbs

B40078 image provided by Lewis Bodkin

Contributors and Credits:

1)  Lewis Bodkin  <05bodkin555(at)gmail,com>
2) B40078 Data Sheet

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Feb 7/08