ac line voltage controller

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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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