Heathkit SB-220 Assembly And Operation Manual page 122

Upgrading Your SB-220
Linear Amplifier
A modest outlay for parts and a few hours on the workbench
. .
.
ingredients-for "customizing" this Heath workhorse. The
results will be longer life, highel reliability and more operating
fl
In' "Upgrading the SB-220 Linear
convenience.
~rnplifier," February 1979 QsT, R1
~ h o w n in Fig. 1 has an effectiveresistance
By
K e n n e t h
M. Gleszer,* W I K A Y
-
of 50 ohms, not 100 ohms as is indicated
L ,
in the text. The 50-ohm value works
satisfactorily when the amplifier is being
C O R ~ ~ " '
operated
011
117-V house current, but on
234 V ac a value of 200 ohms at R1 pro-
vides better in-rush current control. It is
also necessary to use contacts K l B for
both 117 V a i d 234 V ac.
1
he continued popularity of the Heath
As a group, they seemed pleased w i t r m w i t c h i v a s m t t o be desirable t o enable
SB-220 linear amplifier after eight years in the unit's performance; however, most on-the-air tests and facilitate tune-up and
production is not surprising, considering
felt that there were a few areas that could band changing without constantly turning
its price and good reputation. Since I was be improved upon. A few experienced the high-amperage 3-5002 filaments o n
in the market for an amplifier I decided to failure of one or more diodes in the high- and off. A few experienced what they felt
do a bit of research on the SB-220. I voltage power supply. Many found the was premature failure of the now fairly
decided to ask some on-the-air questions cooling fan to be excessively noisy. Some expensive 3-5002 power tubes. Most
of present owners. mentioned occasional arcing between the owners expressed the desire for a color
top inner shield of the case and the plate that would match equipment other than
*P. 0.
BOX
2234, Stamford, CT 06906
connections on the tubes. A STANDBY Heathkit.
As none of these problems seemed dif-
ficult to correct, I began by purchasing
Fig 1
-
Schemat~c d~agrarn of the mod~f~cations for standby sw~tch, two-speed fan, and fllament
and a kit. It worked quite
protect~on.
and I began to use it on the air. After a
K1
-
T ~ m e d e l a y relay, dpdt 10-A contacts, Ph~ladelph~a, PA, stock no. 21K233. short time operating with it, I was con-
Potter and Brumf~eld. Available from Her- R1
-
W~re-wound resistor, 200 ohms, 50 watts;
,,inced that the suggeStions made to me
bach and Rademan Inc., 401 East Erie Ave.,
Clarostat type VP50KA.
were worthwhile. I began to modify the
unit one step a t a time.
The Power Supply
The power supply circuit board did in-
deed look sparse with 14 I-A, 600-PIV
diodes, unprotected by equalizing
resistors and capacitors. Every time I
turned the power switch on, I expected
K1 C
r: "
A E - 2
E
STANDBY
I
SWITCH
I
RELAY
FIL
a F
E
x J +
(B)
noise and smoke t o appear because of the
high current surge which occurs as the
capacitor bank charges.
The fix was relatively simple. I removed
the circuit board and the 14 diodes. Each
diode lead hole was drilled out slightly
larger from the foil side, just enough to
accommodate two more component leads
of about the same diameter. One by one
each diode was replaced with a 1000 PIV,
2.5-A silicon diode and bypassed with a
0.01-pF, 1000-V ceramic disk capacitor
and a 470-kQ, half-watt resistor.
There was n o difficulty in fitting in
these extra components; they may be
mounted o n the top surface of the circuit
board. I then reconnected the board t o the
appropriate color-coded leads and re-
placed it in its original position.
It is not possible t o reinstall the lower