Hints and
Kinks
IMPROVING THE HEATHKIT SB-220-
AMPLIFIER
The life of some of the components in
the SB-220 amplifier
can
be
prolonged with
simple circuit modifications. These
modifications concern:
The 3-5002s: If 3-5002s possessing
above-average gain are used in a stock
SB-220, the amplifier may occasionally
oscillate near 110 MHz. (This problem is
not unique to Heathkit@ amplifiers.) The
presence of this condition is indicated by
occasional arcing at the
T~ME
capacitor
and/or band switch. If a full-blown
parasitic oscillation occurs, the result is
usually a loud bang. Sometimes this results
in a grid-to-filament-shorted 3-5002, a
shorted Zener bias diode, exploded grid
bypass capacitors, open grid-to-ground RF
chokes (RFC4 and/or RFCS in the SB-220
circuit), or any combination of these
effects. A full SB-220 parasitic cure
includes: (1) installation of Q-damping
resistors (RIA and R2A in Fig 1A) in the
tube cathodes (necessary because the
coaxial cable between the SB-220's band
switch and the 3-5002 cathodes happens to
resonate near the SB-220's parasitic-
oscillation frequency!); (2) installation of
low-Q parasitic suppressors in the 3-5002
anodes; (3) installation of a 10-Q,
7-
to
10-W, wirewound resistor in series with the
anode-supply lead (RSA in Fig 1B) to serve
as an HV fuse should a full-blown parasit-
ic oscillation occur; and (4) replacement of
the 3-5002 grid RF chokes (RFC4 and
RFCS) with
24-
to 304, %-W resistors
(R3A and R4A in Fig 1A) to protect the
tubes from grid-to-filament shorts. Full
information on steps 1 and
2,
and a dis-
cussion of how and why VHF parasitics
can
cause component failures, can be found in
my article, "Improving Anode Parasitic
Suppression for Modern Amplifier
Tubes," QST, October 1988, pp 36-39,66
and 89.
Heat reduction: The eight 30-kQ, 7-W
resistors (R12 through R19, inclusive) that
equalize the voltage drops across the
SB-220's electrolytic
HV
filter capacitors
(C10 through C17, inclusive) are a major
source of heat: They dissipate about 38
W.
The filter capacitors are subjected to this
heat. Problem: Over a period of time, this
heating
can
cause the filter capacitors to fail
prematurely, and can also cause the
capacitors' molded-plastic holders to melt.
This problem
can
be corrected by replacing
each of the 30-kQ equalization resistors
with a 120-kQ, 2-W, 2%-tolerance Sprague
Q-line@ resistor. This modification reduces
the power dissipation of the equalization-
resistor string by 75% and greatly extends
the life of the HV filter capacitors. (Don't
use carbon-composition resistors here; they
tend to change value unpredictably with
Conducted By David Newkirk, AK7M
Assistant Technical Editor
I
41
41
4b
::
C21
-
Y\
FROM
FILAMENT
+
HV
BAND SWITCH,
(B)
(A)
TERMINAL
7
Fig 1-Part of the SB-220
VHF-parasiticoscillation
cure
(A)
consists of installing
Qdamping resistors (RIA,
R2A)
in
the amplifier cathode circuit and replacing the 3-5002
grid chokes with fuse resistors (R3A, R4A). Note that the installation of RIA and R2A also
entails the addition of a second filament blocking capacitor (C32A).
Whether or not you apply parasitic-oscillation fixes to your SB-220, the installation of an
HV
fuse resistor (MA, at
B)
is
strongly recommended. The resistor protects the amplifier
tubes by limiting, and opening
in
response to, the huge anode current pulse that occurs
when the SB-220's 3-500Zs "take off" at VHF.
C32A-0.01 pF, 1 kV, disc ceramic.
R3A, R4A-24 to 30
n,
1/2
W.
RIA, R2A-10
n,
2
W, metal
film.
R5A-10
Q,
7 to 10 W, wirewound.
r
ANT
RELAY
90.
RLi
Fig 2-Relay contacts tend to be chewed
up after several years of controlling an
SB-220 because of the current pulse that
occurs when C52 (the bypass capacitor at
the SB-220's
ANT
RELAY
jack) is discharged
through the contacts. The addition of a
current-limiting resistor
(R)
solves this
problem.
use. This trait could result in [potentially
destructive] unequal voltage division across
the SB-220's HV filter capacitors.)
Pitted contacts in the amplifier-control
relay: A common problem with the SB-220
is that it pits the contacts of the control
relay in .its associated transceiver after
several years' operation. The contact
pitting is caused by the repeated short-
circuiting of C52 (the 0.02-pF bypass
capacitor at the SB-220's
ANT RELAY),
limit the capacitor discharge current (see
Fig 2).
Fan lubrication: The fan-motor
bearings on early-production SB-220s did
not have lubrication holes, and lack of
lubrication sometimes led to premature
failure of the fan motor. Small lubrication
holes can be drilled into the top of the
castings that hold the front and rear oilite
bearings. This can be done without
removing the fan motor.
Ordinary, SF-grade 20w motor oil is a
satisfactory fan lubricant; 0.1 cc of oil in
each of the two holes once each year is
adequate. More oil is not better, just
messier.
The SB-220 can be modified for
160-meter operation without sacrificing any
of its HF coverage. For details, see
"Adding 160-Meter Coverage to HF
Amplifiers," QST, January 1989, pp
23-28.-Richard L. Measures, AG6K,
6455
La Cumbre Rd, Somis, CA 93066
BAND-PASS
FILTERS
FOR
80
AND
160
METERS
Using the 80- and 160-m preamplifier
described by Doug DeMaw in August 1988
QST1 with a Beverage antenna, I
encountered intermodulation from strong
which charges to
+
115 V during receiving
periods. This problem can be solved by
ID.
DeMaw, "preamplifier for
80-
and 160-M
placing a U)O-Q,
%-W
resistor in series with
Loop
and Beverage Antennas,"
QST,
Aug
1988.
the center pin of the
ANT
RELAY
jack to
PP
22-24.
To Next Page
Loading...
