on or off as often as you like with no
problem-with one exception: If you have
just made a long RTTY or
FM
transmis-
sion, the glass-to-metal seals in the 3-500Zs
should be allowed to cool for about 1
minute before you switch the amplifier off.
HV-Rectifier Protection
In the early 1960s, silicon-rectifier
manufacturing technology was hit and
miss. There was considerable variation
between individual rectifiers of the same
type. This variation led designers to use
resistor-capacitor equalizer circuits in
parallel with each rectifier. Today, silicon-
rectifier manufacturing technology has im-
proved considerably; rectifiers of the same
type have very uniform parameters. Strings
of identical, modem silicon rectifiers do not
need to be equalized.'O Unfortunately, old
habits die a slow death, and many hams are
still using outdated design methods. Much
has been written about adding equalizing
resistor-capacitor piotection networks
across the rectifiers in the SB-220's HV
power supply. Unfortunately, these
"protection" circuits not only do not per-
form as advocated, but they can lead to
premature rectifier failure.
Here's why: The %-W resistors typically
used for voltage equalization are rated at
,250 V maximum. How can a 250-V-rated
resistor be trusted across a 600- or 1000-V
rectifier? If anything breaks down in a
series-rectifier circuit, it's like dominoes
falling. One resistor failure can wipe out
the remaining good parts in a series circuit.
The most frequent cause of failure in HV
power-supply rectifiers is excessive reverse
current. This problem can be eliminated if
the total peak-inverse-voltage capability of
the series-connected rectifiers substantially
exceeds the peak voltage encountered in the
circuit. In any series circuit, the current in
all of the elements is exactly equal. The rec-
tifiers are all in series, so, the reverse-
current burden is exactly the same for each
rectifier. How is it that things that are
exactly equal need to be equalized?
During the half-cycle application of
reverse voltage, it is important that all of
the rectifiers in a series leg have similar
junction capacitances. If they don't, then
the reverse voltage across the lower-
capacitance rectifiers will be greater than
the voltage across the higher-capacitance
rectifiers. Here's why: In a series circuit,
smaller capacitors charge faster-and to a
higher voltage-than larger capacitors.
Approximately 0.01
pF
of bypass capaci-
tance across each rectifier is probably a
good idea if, for example, 1-A rectifiers are
placed in series with 6-A rectifiers, because
of the wide difference in junction capaci-
tances between
I-
and 6-A rectifiers. If all
of the rectifiers in a series leg are similar,
they will all have similar junction capaci-
tances, so no external capacitors or resis-
tors are needed.
Long ago, before they knew better, some
commercial high-voltage silicon-rectifier-
stack manufacturers used internal
RC
equalizing networks. These manufacturers
stopped using these networks for the same
reasons that were previously outlined. I
don't know of any commercial HV-rectifier
manufacturer who has not abandoned this
malpractice.
Rectifier Failure
When a silicon rectifier fails from exces-
sive reverse-current, the rectifier will short-
circuit. This failure mode is very rare in
SB-220s because the per-leg total rectifier
PIV rating (more than 4.2 kV) is more than
1
kV higher than the actual PIV (3.1 kV)
in the circuit. This is a conservative design;
during a voltage surge, the chain of eight
electrolytic filter capacitors, which is rated
at 3.6 kV max, would likely fail before
a
4200-PIV rectifier string.
A much more common type of rectifier
failure in early production SB-220s is rec-
tifier opening. This is caused by a defec-
tive spot weld inside the silicon rectifier.
Eventually the weld breaks and the recti-
fier opens. The forward voltage jumps the
gap at the open weld. When this happens,
the heat generated by the arc blows a hole
in the rectifier and a 60-Hz arc can usually
be heard from inside the amplifier when
current is being drawn from the HV sup-
ply. It is important to switch off the
amplifier immediately when this noise is
heard. Here's why: In a full-wave, voltage-
doubler rectifier circuit, there are two
series-connected filter capacitors.ll One
capacitor charges during the positive half
of the cycle; the other charges during the
negative half of the cycle. The two capaci-
tors discharge in series. If one of the filter
capacitors is not being fully charged by its
rectifiers, when current is being drawn
from the supply, the capacitor that is being,
charged may force reverse current through
the capacitor that is not being fully
charged. If unchecked, reverse current will
cause electrolytic capacitors to discharge
their corrosive electrolyte through their
safety vents. In other words, reverse Cur-
rent will destroy polarized electrolytic capa-
citors in short order. Here's another
measure of protection against this cause of
capacitor failure: Place a reverse-biased
rectifier diode across each capacitor. This
allows reverse current to flow through the
diodes, not the capacitors.
The Antenna and Bias Relay
A single three-pole relay switches the
amplifier in and out of the coaxial l~ne
during operation, and handles tube-bias
switching as well.
A
few improvements are
in order in this area. See Fig
4.
Add a diode across the relay coil to
absorb the reverse-voltage spike that.occurs
when current stops flowing in the cod.
This
prolongs relay-coil-insulation life and
quenches the magnetic pulse generated
by the coil when it's switched off. If the
magnetic pulse is unchecked, it can
trigger the transceiver's
VOX
circuit and
cause other problems-
*
In the stock wiring configuration,
+
110
V
is connected
to
a terminal of the
relay. During receive, the relay connects
this voltage to the
center
tap of the filament
transformer, which
is
the dc cathode-
current path to the
3-5002
filaments. The
positive cathode volwe causes the tubes
to cut off during receive
by
pulling the grids
110 V more negative
rhan
the cathodes.
A sticky problem
arises
if one of the
tubes develops a filament-to.-grid short
(which,
as
mentioned earlier, is frequently
the result of VHF parasitic oscillation).
Because each grid
b
grounded for dc, a
shorted tube also short-circuits the
+
1 10-V
,
antenna-relay power supply, which is
derived from the un-d filament trans-
former. Thus, if
a
filament-to-grid short
occurs and the amplifm
is
not switched off
promptly, the filament transformer will
literally melt down andshort out, and the
black tar that comes Out of the overheated
transformer makes
an
unpleasant mess
.
inside the amplifier. There are more
pleasant ways to spend
a
Saturday morning
than changing a smoked filament trans-
former!
This potential source of grief can be
eliminated if the relay
is
rewired
as
shown
in Fig
4.
This circuit
uses
resistor-cutoff
bias, using the existing 100-kQ resistor
(R27),
which is rewired
to
another relay ter-
minal. The current through this resistor
,
during receive is usually less than 0.25 mA
-
(R27 dissipates less than
7
mW), so its
%-W rating is more than adequate.
The antenna relay
is
mounted on a
.
rubber grommet.
This
was intended to
2
reduce the vibration that the relay trans-
mits to the chassis, which would otherwise
'$
act as a sounding board. Over time, the
-.
grommet hardens, increasing the acoustic
f
noise generated by relay operation. This
;
problem can be corrected by removing the
?
mounting screw and the grommet from the
top of the chassis and applying a small dab
of silicone-rubber adhesive through the
'
hole.
After the silicone rubber cures, an addi-
tional noise reduction can be gained by in-
.
stalling U-shaped strips of thin, flexible
*.
copper ribbon near the relay
in
series with
the stiff wires soldered to relay terminals
.;
4,
6, 7 and
9.
The stiff wires should be
shortened by about
'/4
inch before the
U
,
links are soldered in. The flexible
U
links
:
act as shock absorbers, and keep the stiff
'
copper wires from transmitting libration
.
from the relay to the chassis. This simple
,
modification results in a substantial noise
;
reduction.
During "barefoot" operation on 10
meters, when the amplifier
is
snitched off,
the SWR presented to the rig by &e ampli-
fier is less than wonderful. is due to
the inductive reactance in the amplifier's
:
TR relay. The relay's inductive reactance
can be canceled by adding reac-
tance between a relay terminal and chassis
ground (see Fig
4).
The required apacitor
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