©1992 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Manual revised: 05/03/04 2:51:00 PM- Page 4 -
with ohmmeter, etc. Sometimes, you
can blow a transistor when you reverse
polarity of meter.
➋
Observe power and duty cycle
ratings in the specifications published
in our catalog. Keep heatsink fins in
free air, not closed in, and not upside-
down on solid surface. When tuning
on bench, allow for cooling periods to
avoid overheating while mis-tuned.
➌
Sometimes, transistors may be
destroyed by parasitic oscillations
occurring during tuning because of the
extremes of capacitors settings, etc. or
due to accidental shorting of
components. To protect against such
damage, turn power supply voltage
down to 9 or 10 Vdc when you first
apply rf until the unit is tuned. Of
course, bias adjustments and final
tuning should be done with full voltage
applied.
➍
Never exceed 13.6 Vdc. Be sure
you have a low-impedance connection
to power supply, i.e., short, heavy
cable.
➎
Any relay coil connected to the
same B+ line as solid state equipment
should have a reverse diode connected
across it to absorb the inductive
kickback which occurs when the coil is
de-energized.
➏
Be sure that your power supply
does not have an inductive surge when
you turn it on or off. If in doubt, borrow
an oscilloscope and watch the B+ line
when you turn the switch on and off.
Alignment is very simple. Connect
the input to a uhf exciter or
transmitting converter which has
previously been tuned into a 50
Ω
load
of sufficient power rating through a
power meter.
Preset variable capacitors as
follows:
•
Small red ceramic trimmers, mid-
range (1/4 turn from factory set-
ting).
•
Mica compression capacitor, mid
range
•
Piston trimmers, 3/8 inch of
piston exposed at top.
Apply moderate drive and B+.
Tune all trimmers alternately for
maximum output. Be careful not to
bottom out the tuning screw on the
piston capacitors.
Continue increasing drive slightly
and repeaking capacitors until
maximum output is achieved. At this
point, the current drain should not
exceed 8-9 Amp. Of course, during
ssb operation, you would not drive the
PA to its limit such as this, you would
stay in the linear region. However, for
alignment, you want to tune for
absolute maximum output to
establish the proper load for the pa
transistors for best linearity on ssb.
The exception is that the input circuit
of the first stage, C11 and C12,
should be repeaked with drive
reduced to just under full output
(25W output or so) to avoid saturation
effects from masking the peak when
tuning.
Mica compression trimmer C18 is
a loading capacitor, which normally
peaks near or at fully tight position.
Do not be concerned if it is all the way
tight when you are done tuning.
Full power output normally is 30-
35W in linear service for 1-1/2W of
drive, which is the normal full output
level of the XV4 Transmitting Con-
verter. In fm or cw mode, full output
should be about 35-40W with 2W of
drive from an exciter, such as the
TA451.
Notes:
➊
Do not retune exciter or transmit-
ting converter with PA connected. Once
it is tuned into a 50
Ω
load, it should
never be tuned again. Tuning the input
of the PA takes care of matching the PA
to the exciter.
➋
A small plastic tuning tool with a
metal bit is required to adjust the mini-
ature ceramic variable capacitors and
the piston capacitors. See the A2 Tool
in the catalog. A larger plastic tool with
a screwdriver bit should be used to
adjust the mica variable capacitor, but
the A2 Tool can be used if you are
careful not to apply too much torque.
➌
Never leave the B+ applied to the
PA if you suspect it is oscillating or if
an arc occurs somewhere. An audio or
low-frequency rf oscillation could
destroy a transistor or a capacitor if
sustained for any length of time. If the
unit draws excessive current, or there
is an output indication with no input, or
if you hear a high frequency frying
sound, turn off the power until you
discover the problem.
MOUNTING.
If desired, the PA can be mounted
to a panel or enclosure with angle
brackets at the left and right hand
edges of the heatsink. However the
unit is mounted, the fins should be in
free air to allow for good convection
cooling. Do not mount the PA with
the fins inside a cabinet. If the exciter
or transmitting converter is adjacent
to the PA, some shielding should be
provided between them to avoid
feedback.
OPERATION.
Operation is quite simple. B+ can
be applied either just during transmit
or all the time if desired. Merely apply
a signal to the PA when you want to
transmit.
It is necessary to avoid overdriving
the PA. Moderate overdrive will not
damage the unit, but it will cause
excessive intermodulation distortion of
ssb signals. About 1-1/2W of drive
should be sufficient to obtain the
rated 30-35W p.e.p. ssb output. Do
not drive the PA to the saturation
point on ssb to avoid flat-topping.
For cw or fm operation, 2W of drive
can be used for 35-40W output. Drive
levels over 2W should be avoided, as
severe overdrive might cause
transistor damage from overheating.
Note that exciters and power
amplifiers both run cleanest when
operated at full output. That is, if
drive is reduced considerably,
spurious levels may increase due to
under driving the unit. This is
especially true of class-C devices.
When the drive is reduced to the point
where the transistor is not fully
conducting, spurious outputs may
result. It is a common error to think
that running a PA at reduced drive
levels improves problems of this
nature.
TROUBLESHOOTING.
Since the unit has only two stages,
there isn't much which can go wrong.
It is helpful to know that the base
voltage in linear service should be
about +0.6 to +0.7 Vdc. Idle current
to each stage can be checked by
connecting an ammeter in series with
the choke for each stage as done in
the bias setup procedure earlier in the
manual. The rest of the circuitry is
straightforward, with shorted coax
cables or incorrect or shorted pc
board component connections being
the first things to suspect should
there be no output.
REPAIR.
Should it be necessary to replace
an rf power transistor, be sure to use
an exact replacement. See parts list
for types we have tested. There are all
sorts of transistors available on the
market which cost less or may be
easier to obtain, but they may not
operate properly in a uhf circuit,
