10.
Reinstall right-side shield.
11.
Oil felt pads on fan motor while top cover is off.
12.
Install top shield cover.
13.
Test the amplifier using a dummy load.
14.
If OK, proceed to the next section.
surge protection
1.
Solder No. 14 (1.6 mm) bus wire 2 inches (5 cm)
long to pins 3 and 4 of relay K1 (fig.
2).
2.
Solder No. 14 (1.6 mm) bus wire 2 inches (5 cm)
long to pins 5 and
6
of relay K1.
3.
Bend the two wires and solder to a two-lug tie
strip.
4.
Connect pin 5 to
7
using No. 20 (0.8 mm) bare
wire.
5.
Connect a black insulated wire (rated for 220 Vac,
10 amperes) about 10 inches (25 cm) long to K1 pin
8.
6.
Stack the two current-limiting resistors (100 and
50
ohms) and connect in series. Solder this pair to
the lower holes in the tie strip.
7.
Mount the completed surge-protection into the
SB-220 using the center ground lug on the tie strip
and the existing chassis screw located about 2 inches
(51
mm) forward of terminal strip AE. The relay case
should rest against the chassis, being supported by
the bus wires.
8.
Connect the 10-inch (25-cm) black insulated wire
(trim as required) from relay
K1 pin 8 to terminal 213
on terminal strip AE of the linear.
9.
Remove existing black jumper wire between
power switch Z and front standoff AW.
10.
Connect Z to pins
3
and 4 of K1 using the tie strip.
Use insulated wire with (220
Vac, 10-ampere rating).
11.
Connect
Y
from standoff
AW
to pins 5 and 6
using the tie strip. Use insulated wire with 220-Vac,
10-amp rating.
12.
This completes the surge relay installation.
From the
Heathkit manual, these codes are used:
AE 1101220
Vac input terminal strip.
AW front-mounted standoff tie point.
AL front corner hole.
2 power switch.
operation
Checkout of the surge protection circuit can be
monitored each time the linear is fired up, assuming
the filter capacitors have discharged to a low level.
Place the selector switch in the
HV
position, while the
mode switch can be in either the
CWITUNE
or
SSB
position. After the power switch is pushed, there will
be a time period of
a
few seconds of dead silence.
This delay time is controlled by the value of the limit-
ing resistors. During this period the plate voltage
meter can be observed to slowly increase from zero
to about 1500 Vdc. Additionally, the meter illumina-
tion lamps will
slowly
energize to about half bril-
liance. Since the 3-5002 filaments are in parallel with
these lamps, they will be responding in the same
way. If in doubt, turn off your room lights while ener-
gizing the linear and peer down through the case top.
The cooling fan will be turning very slowly while
gradually building up speed. Therefore there will be
no noise from this source during the initial few
seconds.
After the five-second surge-delay period, adequate
voltage will be available for surge relay
K1 to pull in.
During a brief interval
K1 contacts will close and
hold, thus shorting the limiting resistors and applying
full line voltage to the transformers. Instantly the
plate voltage will increase from 1500 Vdc to its nor-
mal maximum value. The
3-5002 filaments will glow
with their normal brilliance, and the cooling fan will
attain maximum speed. Don't be alarmed when you
hear a brief buzzing sound as the relay closes. This
sound is caused by
K1 contacts bouncing (as all me-
chanical relays do) combined with slight inductive
arcing.
Although this article is written specifically for the
SB-220, other similar equipment could be surge pro-
tected using these mods.
For additional information on rectifier diode pro-
tection
I
suggest the April, 1980, edition of
Worldradio,
which has a fine article written by Joe
Carr,
K41PV.
Once you've installed the mods as shown in fig.
3,
you can place the problem of surge protection on the
shelf for a well-deserved rest. I've used these circuits
on two other
homebrew linear amplifiers with total
success. In addition I've used them on power sup-
plies for several transmitters using the lower line volt-
age. The only difference is the selection of the limit-
ing resistance for a satisfactory delay period.
Note:
K1
is a dpdt relay, 5000-ohm coil,
120
Vac. Comtacts are
rated at
10A, 125
Vac. Dimensions:
1-518
x
1
x
314
inches
(41
x
25.4~ 19
mm).
ham
radio
68
december
1980
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