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E L E C T R I C A L
P R I N C I P L E S
37
the main transformer is protected by fusetrons.
HD3 points open the ground connection when the
switch is
OFF.
The tube power supply transformer primary
connections are made across posts 9 and 10. The
drive motor is directly across the main line switch
and hence operates on the commercial A.C. supply
under the control of H D I relay. I t is protected
by the 10.9 ampere element in the sentinel switch.
A split plug allows the motor to be disconnected
from rhe machine.
Note that the test lamp outlet is "hot" as long
as the attachment cord is plugged in, regardless of
the seting of the sentinel switch. The test lamp
outlet is protected by 3 ampere glass fuses 12 and
13.
As soon as the main line switch is turned
ON,
40
volt ,D.C. supplied by the selenium rectifier is
available across posts I and 3. Post 1 is the posi-
tive side of the line and post 3 is the negative side.
For sake of consistency, wherever possible, all cir-
cuits will be traced fronz ~zegativc ternzi?znls to
positive.
Time Delay Circuit
Owing to the necessity of heating the cathodes
of the rectifier tubes and the vacuum tubes to op-
erating temperature before putting them in opera-
tion, it is necessary to allow an interval of time
before starting rhe machine. T o insure full oper-
ating temperature, approximately 50 seconds is al-
lowed for heating the tube cathodes. The rectifier
tubes in the power supply chassis are gas-filled
tubes and will be damaged if the load is applied
before their filaments have reached operating tem-
perature. If vacuum tubes are placed in operation
before they have been fully heated, the tubes will
not be damaged, but erraric operations will result.
The time delay is effected by a thermal delay re-
lay (RF in machine).
One contact strap of this
relay is a bi-metal strip, i.e., it consists of two
strips of different metal placed one on top of the
other and bonded together.
These two metals ex-
pand at a different ratio when heated and hence the
strap will bend. A coil of high resistance wire is
wound around this strap to serve as the heating
element. The lower strip of metal has the greater
expansion rate; therefore, the strap will bend up-
ward when heated, and the contact points will
touch to close a circuit to the coil and complete
circuits for the operation of the machine.
The
upper contact point is in the form of a screw
tipped with contact metal.
This screw provides
a means of varying the time interval required for
the relay points to close by varying the distance
between the contact points.
The adjusting screw
is normally set to provide 50 seconds delay. An
air gap of approximately
A"
will provide a good
starting point in setting this adjustment. This time
is applicable when starting with a cold machine.
Naturally, the time delay will vary if the machine
is turned
ON
and
OFF
several times in a few min-
utes because the heating unit will not have time
to cool.
The time delay relay operates on the regular 40
volt supply of the p ~ n c h unit furnished by the
selenium rectifier. With the main line switch
ON,
a circuit is complete to .the time delay heating ele-
ment as follows: post 3, terminal CNp5, through
ground cable in the electronic unit and back to
CNpl3, through heater element,
N/C
points of
R5BL, to R2A N/C, P16, P I , post 1. Also, as soon
as the main line switch is closed, R2 coil will be
energized in parallel with the heating element. The
purpose of R2 and the cable jumper will be ex-
plained shortly.
When the heating element has su&ciently heated
the bimetal conract strap, which is one strap of
R5A points, the R5A points will close, and a cir-
cuit will be completed to the coil of RF as follows:
post 3, terminal CNp5, through ground cable to
CNpl3, through R5 coil, RFA contact, and out
to post 1.
Relay F will then pick up and its BL and BU
points will transfer.
When RFBL transfers, the
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