should be limited to two times the normal
operating current. As the filaments pre-
sent almost a dead short to the trans-
former when they are cold, the initial cur-
rent passing through them can be far more
than two times their rated current.
Repeated fast heat-ups can ultimately
cause mechanical deformation of the fila-
ment and shorts between it and other tube
elements. Premature failure of
3-5002
tubes can be traced to this problem.
A relatively simple cure is to install 100
ohms of surge resistance in the primary
winding of the filament transformer and,
after approximately three to five seconds
after turn-on, short this resistor out using
a small time-delay relay. For reasons of
convenience, I selected a 200-ohm, 50-W
Clarostat wire-wound resistor which fit
diagonally across the capacitor
mounting-
bracket screws already available on the
bottom of the chassis. The slider should
be positioned in the middle of the resistor.
Solder a jumper between both end lugs. A
100-ohm, 50-W resistor will do just as
well, but finding a space to drill holes in
this area of the chassis is difficult.
I spotted a miniature
solid-state-con-
trolled time-delay relay in a Herbach and
Rademan catalog, their stock no.
21K233.
The plastic cover was removed and the ex-
isting 120-second time delay was reduced
to 5 seconds by installing an
82-kQ, 1/4-W
resistor in parallel with the gray, 1.2-Ma
timing resistor. There is a hole already
provided for mounting the resistor on the
small circuit board within this relay.
Drill a hole for a no. 6-32 screw 2 inches
from the front of the chassis and on a line
exactly between the loading and tuning
capacitors. Mount the relay with a 6-32
X
1/2-inch screw. A small rubber bushing or
grommet should be inserted between the
bottom of the relay and the chassis to
eliminate the possiblity of hum.
The black filament-transformer pri-
mary wire is removed from lug AW and
fastened to one end terminal of the
resistor. The slider is connected to lug
PUT
YOUR VOICE TO GOOD USE
Can you operate a tape recorder? Or
can you read aloud and explain advanced
scientific subjects? Could you learn to
operate a tape-duplicating machine? Such
volunteers are needed by the 29 units of
Recording for the Blind, an organization
that provides free textbooks to the blind
and to those physically unable to handle a
book. In addition to such subjects as
physics, math, computer technology and
The time-delay relay used in the surge-protection circuit. The arrow points to the resistor that
must be decreased in value to shorten the time delay.
AW, which effectively inserts the resistor
in a series with the primary.
Two leads are now connected between
the contacts of the time delay relay and
one end of the resistor and terminal AW.
One side of the relay coil is connected to
terminal block AE, terminal no. 2. The
other coil terminal is connected to ter-
minal AW.
If you plan to operate your SB-220 on
235 volts only, this completes the surge
protection project. If you are going to
operate on 117 volts, you
must
remove the
black-green wire from terminal 3 of block
AE; pull it out of the wiring harness and
connect it to the lower unused terminal on
the time delay relay. The center unused
terminal must now be connected back to
terminal 3 of block AE. This eliminates
one of the parallel windings of the fila-
ment transformer until the relay closes.
Reinstall the tubes and the inner chassis
cover. Turn on the power switch. The
filaments should take approximately three
seconds to come up to half temperature.
,
This is a bright red color. Two seconds
later the relay closes and the tubes almost
instantly reach normal operating tempera-
ture. In this hook-up the meter pilot lights
follow the same heat-up sequence. This
serves as a visual check that the time-delay
relay is functioning properly.
Color
I finally had my SB-220 performing to
my satisfaction. Now if only the cover and
panel could be changed to match the color
scheme of my Drake twins,
I
would be
happy. I fired off a letter to Heath asking
if they would consider making available a
case and panel that would be compatible
with most of the black boxes on the
market. I did get a very nice answer back,
but no encouragement. Maybe sometime
in the future, but not now. Those who feel
as I do should write to Heath.
The total time for completion of all the
modification projects, after the parts are
gathered together, should not exceed two
hours. These changes are very worth-
while, as you can expect much longer life,
reliability and convenience
-
and you'll
have the satisfaction of owning a
"custom-built" amplifier.
m
around 21.150 MHz at 1800 UTC. Ar-
mond Brattland, K6EA, 1135 Magnolia
Ave., Long Beach, CA 908 13.
chemistry, many Amateur Radio publica-
tions have been recorded and maintained
in the master library of Recording for the
Blind, 215 E. 58 St., New York, NY
10022. Check the phone book to see if
there is a Unit in your city. If so
-
volunteer!
I would like
to
get
in
touch with
.
.
.
Novices and experienced traffic men
interested in forming a Novice net on or
STATION NOW ABOARD
HMS
BELFAST
The Amateur Radio station aboard the
HMS
Belfast,
which is moored in the Pool
of London, has been granted the use of
the special call sign
GB2RN for use when
the ship is open to the public. The station
is interested in establishing schedules with
,
other museum and special-interest sta-
tions worldwide. Contact Don
Walmsley,
153 Worple Road, Isleworth, Middlesex,
TW7
7HT, England.
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