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AC Line Voltage Controller for Atwater Kent Set Model 40
By dsen333v
Fall/Winter 2011-2012 (Revision A, March 2012)
Rev. A. March 2012- Additions and edits to address recent
feedback from my posting on Antique Radio Forum.
----------------------------------------------------------------
The Atwater Kent Model 40 lamp socket radio should not be
plugged into an AC wall outlet directly, unless you are sure the
line voltage is within the range for which the model 40 set was
designed. As support for this assertion, here is the relevant
section from the Atwater Kent Service Manual;
See the photos of a line voltage test on the next page. Inresponse to Forum feedback, I used both a Digital Multimeter
(DMM) and a 1000/Volt Volt-Ohm-Meter (VOM) to measure our line
voltage in the early afternoon of March 19, 2012. Note that the
VOM measurement is essentially equal to the DMM measurement of
122.5 volts.
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Southern California Edison Line Voltage is 122.5, on 3/19/2012
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These measurements prove that modern-day AC line voltage often
exceeds the upper limit of the 10 volt permissible range for
which the Model 40 set was designed. According to the Atwater
Kent Company Service Manual, this situation could shorten tube
life.
Today, A New-Old-Stock (NOS) UX-280 costs $100 from Antique
Electronic Supply. An NOS UX-171A is $86. Therefore, I choose to
use a power rheostat to reduce the applied line voltage to no
more than 110 volts when the set is ON, exactly as recommended
by the Atwater Kent Company.
Also, an RFI filter is beneficial to stop 1) super-strong nearby
stations or 2) line noise from entering the set via the AC
mains. I had both of these interference problems, and the RFI
filter mitigated them during my breadboard tests.
A 1 amp fuse can protect the RFI filter, the rheostat, and the
radio from damage propagation due to a short circuit. See
additional fusing considerations at the end of this report.
Here is a suitable front panel layout (next page);
AC LINE VOLTAGE
CONTROLLER
DSEN333V
23 NOV.2011
50 RHEOSTAT
OHMITE #RJS50R
RFI FILTER
CURTIS INDUSTRIES
#F2700CA03
LINE
NEUTRAL
GROUND
FUSE
(SEE TEXT)
120 VOLT
AC LINE
INPUT
LAMP SOCKET
WITH
SCREW-INADAPTER
AC
VOLTMETERTEST POINT
FILTERED
120 VOLTS AC
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The lamp socket mounts nicely to the single-outlet wall plate on
the right. The rheostat and RFI filter are available from
various parts vendors, such as Digi-Key, Newark, Calco, etc. I
fabricated the test point terminals using a thick wood block,
5 mil brass shim stock, solder lugs, and wood screw fasteners.
Drill the test point holes in the wood block such that the
multimeter probes are held with reasonable tension, and engage
the brass terminals when fully inserted. A panel drill template
is shown below;
VARIABLE AC
TEST POINT
0
20
10
30
40
50
60
70
80
90
100
PANEL / HOUSING DIMENSIONS: 5 1/2 H X 10 W X AT LEAST 5 D
3 1/4" 2 3/8"
2 3/4"
1 1/8"
2 1/2"
2"
1 1/4"
10"
5 1/2"
3/8DIA
3/16DIA
1 1/8"
2 3/4"
1"
1 3/4"
5/8 DI AMETER
1 7/8"
1 7/8"
3 3/4"
3/4"3/4"
2 1/2"
DIAL PLATE
IS 3 X 3
CUTOUT
CUTOUT
3/4"
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A dial for the rheostat is shown below. Its printed dimension
should be exactly 4 x 4, including the flaps.
Make a 3 square dial plate from a piece of sheet metal, (tin-
plate, aluminum, copper, etc.). Install the dial cutout after
drilling the rheostat mounting holes and the corner holes. Fold
over and tape the flaps to rear of dial plate.
My homemade test point terminal block is fashioned from a 1 x
3 x 5/8 thick wooden block, two #6 x 3/8 sheet metal screws,
and 5 mil brass shim stock. I designed it to accept the probes
of my Fluke model 79 multimeter. Use two 1 long #8-32 machine
screws and nuts for panel mounting.
0
20
10
30
40
50
60
70
80
90
100
FOLDOVER FLAP
FOLDOVER FLAP
1"
3"
1" 3/4"3/4"
1"
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I cut the front panel and dial plate from soft steel sheet stock
using tin snips. In the photo below the drill template is
attached to the front panel piece. I made a separate drill
template for the dial plate.
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I fashioned an enclosure using 5 poplar hobby board and
square doweling from Home Depot.
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Adjustment for 110 Volts at Radio Set Start with the rheostat
set to maximum resistance (full counterclockwise for my wiring).
Plug radio into the lamp socket. After turning on the radio, the
voltage at the test point should quickly drop below 110 volts
(for Model 40 set). Slowly turn up the rheostat to bring the
test point voltage up to 110 volts. My Model 40 set draws about
34 watts at 110 volts. It is normal that the rheostat and front
panel of the controller will get quite warm after a while,
because they are dissipating approximately 3 watts after the
rheostat has been properly set.
Additional fusing considerations The RFI filter is rated at 3
amperes maximum, so be careful to not exceed 3 amps total
continuous load (dual AC outlet plus lamp socket). The rheostat
is rated at 1 ampere continuous maximum. The minimum short
circuit current at the lamp socket, 2.4 amps, occurs when the
rheostat is set to 50 (120 volts across the rheostat).
According to the data sheet a 1 amp fuse should then blow within
150 milliseconds.
---
While researching for this update, I came across a photo of an
antique light socket wall outlet and plug. The Atwater Kent
Parts List Manual has photos of several models with this
Edison-type plug on their power cord. It would be fun to have
such a plug for my set.
THE END
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