Vintage Ham Scopes

Prior to 1935 hams relied on station
monitors or used their receivers to check
the quality of their phone signals.1 Unfortunately,
not every station sounded good
and a better method was needed. ARRL
encouraged hams to clean up their signals
and from the mid 1930s The ARRL Handbooks
provided schematics, so hams could
build a basic oscilloscope for phone station
use. At about the same time the National
Radio Company offered the CRO, a 3 inch
model of similar design. At $29.50 plus
the cost of the 3 inch 906 cathode ray tube
(CRT) tube and one #80 tube, this was
considerably more than the weekly wages
of the average ham.

But before the CRM could be developed,
it was RCA that designed and built
the new 913 tube that made it all possible
and then they reduced the cost to $5 each
(see Figures 1 and 2). National quickly
designed a new scope around that tube.
The CRM (see Figures 3 and 4) would sell
for $11.10 plus tubes. It was much more
affordable than their larger 3 inch CRO.
Thus, with the CRM, National made it
possible for most amateurs to improve their
AM phone signal.