August 1939 Radio-Craft
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
This Radio Service Data Sheet
is from the series published by Radio-Craft magazine in the 1920s and 1930s.
The "2-Unit" part of the title indicated, per the schematic, that the power
supply and the speaker output drivers were in a separate chassis that would have
been mounted in the trunk or under a seat. I could not find an example of an
actual RCA Model M-70 auto radio, but the thumbnail image to the left from
information on the RadioMuseum.org website at least gives an idea as to its
I am posting them for the benefit of people who restore those vintage models to
working order. It is still possible to buy
Sam's Photofacts packages
that include schematics and alignment data, but these info sheets fill the gap for
ones either not available, or for someone needing to get a feeling for what he/she
is up against before making a full commitment to the task.
RCA Victor Model M-70 2-Unit Auto-Radio (Chassis No. RC-394) Radio Service Data
This is No. 259 in a series of Radio Service Data Sheets within Radio-Craft instituted
with the October, 1929 issue. During this time the manner of presenting these Data
Sheets has undergone considerable modification in accordance with comments by readers.
Can you suggest any changes which will make these Data Sheets of greater value to
the majority of the Servicemen who read Radio-Craft?
For all alignment operations, connect the low side of the
test oscillator to the receiver chassis, and keep the output signal as low as possible
to avoid A. V .C. action.
Cathode-Ray Alignment is the preferable method. Connections for the oscilloscope
are as follows: Vertical "H1" to terminal "C" on 2nd I-F transformer; vertical "0"
Connect the output meter across the speaker voice-coil and turn
the receiver volume control to maximum (fully clockwise) and the tone control to
middle of range.
Rotate the gang condenser to its full-mesh (maximum-capacity)
position and then adjust dial scale so that the pointer is aligned to the last calibration
mark at the low-frequency end of the scale.
It is very important that these instructions be followed when installing the
The antenna circuit is designed to work with an antenna having a total capacity
including the shielded lead-in not to exceed 150 mmf. If an antenna having a larger
capacity is to be used, it will be necessary to add a condenser in series with the
lead from the antenna filter L1 to the antenna coil terminal ("A"). Where a "Doublet
Under the Running Board" type of antenna is to be used having a capacity of approximately
200 mmf., the condenser added should be approximately 500 mmf. The insulated running
board type having an approx. capacity of 550 mmf. will require a condenser of approximately
150 mmf. Cars using an insulated steel top of approximately 3,500 mmf. will require
a series condenser of 150 mmf.
Posted February 24, 2022
(updated from original post on 12/19/2014)