Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
Have you ever heard of a Mershon condenser? Me neither, but this Crosley Model AC-7 radio used one
(or more). Ohio State University alums are familiar with the name Mershon, if not the man.
Ralph
Mershon was, among other things, an early manufacturer of electrolytic capacitors. This 1930
service sheet for a Crosley radio mentions the propensity of Mershon capacitors to experience
breakdown of the electrolyte when the radio is not used for many months. Rejuvenation is necessary
via a prescribed procedure to ensure effective operation in removing the 60 Hz hum in the audio.
Crosley AC-7 and AC-7C
This receiver employs one (first) stage of tuned "push-pull" R.F., a second stage
of R.F. amplification (in which circuit oscillation is prevented by a "losser "
resistor R12 of 750 ohms and the reversed tickler winding T of L2), a regenerative
detector, and the usual two stages of transformer-coupled A.F. amplification. The
tubes used are as follows: V1, V2, V3, V4 and V5, X-'99s; V6, '12; V7, BH-type
gaseous rectifier. (The specified tubes must be used.) The filaments of the
amplifier and detector tubes are connected in parallel, and the filament current is
obtained from the rectifier V7 and high-voltage winding S1 of the power transformer
PT. The manner of obtaining grid bias for these tubes is indicated in the detail
circuit.
An insulating film on the plate of the Mershon condenser is built up at the factory; but this gradually breaks down if the receiver is not in use for some time. To build up a film on the plates the receiver is put into operation with all tubes in their respective sockets. At the start of the reconditioning process, resistor R16 should be
turned to extreme left, and resistor R1 set mid-way. Operate the set for ten minutes to half an hour, noting the current reading on the milliammeter MA; the value for correct operation of the set is between 55 and 60 milliamps. As the current increases, R16 should be adjusted to maintain this reading, A greater length of time than fifteen minutes is seldom required before the set begins to play well. If C10-C11-C12 is defective, the Mershon
unit should be replaced.
Posted September 21, 2016
Radio Service Data Sheets
These schematics, tuning instructions, and other data are reproduced from my
collection of vintage radio and electronics magazines. As back in the era, similar
schematic and service info was available for purchase from sources such as
SAMS Photofacts, but these printings
were a no-cost bonus for readers. There are 227 Radio Service Data Sheets as of
December 28, 2020.
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