Harris MW-5 Reference manual

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. --
HARRIS w
COMMUNICATION AND
INFORMATION PROCESSING
MAINTENANCE AND
Broadcast Products Div ision
Bulletin No. AM-146-TLH
Equipment: MW-5/5A Date
April, 1980
SUBJECT: TROUBLESHOOTING TIPS AND UPDATE INFORMATION.
The contents of this bulletin are troubleshooting tips intended
to simplify problems that can require extensive troubleshooting.
Additional information concerning transmitter operation and prod-
uct improvements is also provided in this bulletin.
I. PROBLEM: DC overloads as soon as the high voltage is turned on.
A DC overload is sensed in the return lead of the high voltage
supply at 1Rl7 or in the step start circuitry of the high volt-
age transformer primary. In early MW-5 transmitters not hav-
ing the step start overload sensing circuit (A4Al K4), a DC
overload at turn-on may be characterized by blown step start
resistors. If your transmitter lacks the protection for the
step start resistors, some of the methods employed in the
following procedure may not be feasibly applied to your
situation.
Disconnect the audio lines from the transmitter.
Carefully observe the actions of the plate current and power
output meter while depressing the HIGH VOLTAGE ON button. If
the plate current meter in the isolated plate section momentarily
deflects upscale the overload current is being conducted through
the modulator section (the PA and modulator are in series, with
the plate current meter between the modulator plate and the PA
cathode). If the power output meter also deflects upscale, the
RF section is indicated to be functioning correctly. If neither
the plate current meter nor the power output meter deflect as
described, it is evident that the overload current is not through
the PA tube or modulator section. You will probably find that
the supply current meter deflects upscale when the high voltage
is applied.
Having made these important initial observations, you can quickly
arrive at the specific cause of the problem. Below are some spe-
cific causes, listed under some general headings. The sequence
of the steps listed under each heading is primarily based on the
most likely source of the problem. Each step assumes that the
preceding step has been done.
HARRIS CORPORATION P. 0. Box 4290 , Quincy, Il linois 62301 21 7 I 222-8200
1 10 -727-06 10

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2
A. DC Overloads - Plate current meter does not momentarily
deflect.
1. Bad DC blocker 1Cl2 or RF bypass capacitor lCll.
2.
Disconnect the plate of the PA tube and either of
the modulator screen circuit breakers (be sure to
tape the loose wire to avoid shorting to ground).
Turn the high voltage on. If a DC overload occurs,
then lCll and/or 1Cl2 are possibly the problem. Re-
move them from the transmitter and turn the high
voltage on. If the high voltage stays on, lCll and/or
1Cl2 are defective. Install one of them in 1c11~s
position to determine which is bad. You cannot ef-
fectively check them with an ohmeter or capacity
checker.
Shorted filter capacitor lCl and/or Cl5.
the positive output from the rectifiers.
mitter should not overload then if either
is at fault.
Disconnect
The trans-
capacitor
3. Shorted rectifiers. The high voltage rectifiers can
be checked for shorts with an ohmeter. It is important
to remember that high voltage rectifiers typically
measure open or a high resistance in the forward
direction. This is because the forward voltage drop
of the rectifier is often greater than the battery
voltage of the ohmeter. You should never measure
resistance less than infinity in the negative direction.
B. DC Overloads - Plate current meter deflects, power output
meter does not.
1. Complete loss of PA grid drive. The PA tube developes
its bias from the RF drive, so it will conduct heavily
without it. To check the PA grid drive, turn the fila-
ments on and ground terminal 5 of the RF oscillator
board. The PA grid current will generally be slightly
higher than its usual value when the high voltage is on.
If low or zero PA grid current is the cause for the
overloads, the overloads will not occur with the PDM
off (power controls set for minimum or terminal K of
the PDM control board grounded).
If there is no PA grid current, check the oscillator
board for output, RF DRIVER VOLTS, IPA I, RF DRIVER I,
and the condition of the PA grid circuit components.
Each output of the RF modules (Al-AS) should be a
squarewave whose duty cycle is 50 ± 5%. The ringing
on the squarewave is normal . The fault indicators

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3
for
the
RF
DRIVER
modules
(DS2-DS5)
sense
differences
between
the
module
output
signals,
therefore,
will
illuminate
only
if
there
is
output
from
at
least
one
of
the
RF
modules.
No
output
(22-35
volts
peak
to
peak)
from
the
RF
oscillator
could
be
caused
by:
a.
+30V
not
present
on
the
board.
Check
fuse.
b.
Defective
crystal.
Switch
to
the
other
oscillator.
c.
Defective
oscillator
transistor
1A2Al
Ql
or
Q2.
Switch
to
the
other
oscillator.
d.
1A2Al
CRl
shorted.
Measure
for
15V
at
the
cathode.
e.
1A2Al
CR2
shorted.
Measure
for
5.lV
at
the
cathode.
f.
1A2Al
Q3
defective.
Check
for
the
5V
squarewave
at
the
collector.
g.
1A2Al
Ul
defective
.
Check
for
squarewave
output
at
the
carrier
frequency
at
terminal
8
of
the
oscillator
board.
h.
1A2Al
Q4
shorted.
Check
1A2Al
R23
and
R24
for
over-
dissipation
.
Causes
for
zero
IPA
current
would
be:
a.
No
drive
to
the
IPA
input.
Check
continuity
from
pin
3
of
the
oscillator
board
to
pin
1
of
the
IPA
module.
b.
1A2A3
Fl
blown
and
probably
Ql
and
Q2
on
the
IPA
module
are
shorted.
Other
than
a
failed
RF
DRIVER
module,
the
cause
for
low
RF
DRIVER
current
is
most
likely
in
the
PA
grid
circuit,
resulting
in
an
unloaded
condition
for
the
RF
DRIVER.
Check
RF
DRIVER
volts.
If
this
is
normal,
proceed
to
examine
the
PA
grid
circuit
for
broken
connections
.
Check
for
open
grid
resistors.
Change
the
PA
tube
if
nothing
is
found.
C.
DC
Overloads
-
Plate
current
and
power
output
meter
deflect.
This
is
indicative
of
a
modulator
problem.

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•
4
1.
Disconnect
the
plate
of
the
modulator
and
either
of
the
modulator
screen
circuit
breakers
(be
sure
to
tape
the
loose
wire
to
avoid
shorting
to
ground).
Check
to
see
if
the
overload
still
occurs.
If
it
does,
1C5,
lCB,
lCl0,
or
1CR15/1CR16
are
shorted.
The
best
method
of
pinpointing
the
fault
y
component
is
by
disconnecting
each
of
them
one
at
a
time
until
the
bad
component
is
found.
Be
sure
to
leave
the
modulator
plate
and
modulator
screen
circuit
breaker
disconnected
until
that
time.
You
cannot
effectively
check
these
capacitors
with
an
ohmeter
or
capacity
meter.
The
high
voltage
test
in
the
transmitter
is
most
effective.
2.
If
the
DC
overloads
will
not
occur
with
the
modulator
plate
and
modulator
screen
circuit
breaker
disconnected,
reconnect
only
the
plate.
A
DC
overload
under
these
conditions
would
indicate
a
bad
modulator
tube
or
a
loss
of
modulator
grid
bias.
Check
the
modulator
grid
voltage
with
just
the
filaments
on.
It
should
be
approxi-
mately
-230V.
If
this
is
not
so
,
it
could
be
a
fault
of
the
bias
supply
on
1A5,
Q2
on
the
Audio
Driver,
or
CRB-
CRll
on
the
audio
driver.
Measure
the
voltage
at
lAlTBl-
21
to
determine
if
the
bias
supply
is
working.
Q2
on
the
Audio
Driver
may
be
removed
at
this
time
since
its
only
purpose
is
to
boost
positive
modulation
capabilities.
The
voltage
drop
across
1AlA3
CR8-CR11
should
be
about
230
volts
with
the
filaments
on.
Normally
the
modulator
grid
voltage
will
remain
at
-230
volts
with
the
modulator
screen
circuit
breaker
disconnected
and
the
high
voltage
on.
3.
If
the
DC
overloads
happen
only
when
you
reconnect
the
modulator
screen
circuit
breaker,
this
indicates
the
PDM
signal
is
always
in
a
full-on
state.
This
is
a
fault
of
the
circuitry
driving
the
modulator
tube.
a.
Reconnect
the
modulator
sc
reen
circuit
breaker.
Ground
terminal
K
of
the
PDM
control
board.
Test
the
transmitter
again
for
overloads.
If
no
over-
loads
occur,
then
the
damper
diodes,
PDM
oscillator,
and
negative
feedback
are
suspect.
The
damper
diodes
lCRl
-1CR13 may
be
checked
individually
with
an
ohmeter.
The
PDM
oscillator
output
can
be
checked
with
a
scope
at
the
collector
of
lAlAl
Q2
while
the
filaments
are
on.
The
first
step
towards
fixing
a
possible
negative
feedback
problem
is
replacing
1A6
CRl.
Negative
feedback
will
not
be
present
unless
there
is
RF
power
output.
Other
checks
of
the
feedback
circuit
may
be
made
with
an
ohmeter.

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5
b. If t he transmitter still overloads with termi n al
K of the PDM control board grounded, there i s a
pr o blem on either the PDM or Audio Driver bo ar ds .
At t h is point, it is important to remember t hat
t he mo dulator screen supply provides the positiv e
vo
ltage for the Audio Driver 1AlA3, and is on l y
e
ne rgized when the high voltage contactors are
cl o se d . Disconnect at least 2 of the 3 wires
fr om the primary of T4, the high voltage trans-
fo rmer . Ground terminal Kon the PDM control
board . Turn the transmitter on . A modulator
screen overload may occur due to the fact the
plate is disconnected . If so , turn the modulator
screen voltage down. Note the action of the
m odulator grid voltage when depressing the plate
on button . Normally the voltage will not ch a ng e,
bu t in this case will swing towards zero volts.
Ca use s for this are as follows:
l Al CRl shorted. Check for 39V at cathode with
th e filaments on .
1AlA3 Q2 shorted . It should already have been
re mo ved.
lAl Al Q5 shorted. The collector voltage of QS
is a pproximately 4 volts in its normal on sta t e
an d 8 volts in its off state with just the filam ents
on. T he off state voltage increases to 10 volts
wi t h the screen supply energized. Q5 should be
o ff with terminal K grounded .
lAlAl Q3 shorted . Measure the voltage drops of
lAlAl CR3 and the base emitter junction of Q4.
Each sh o uld be about 0.6 volts normally, indi-
c at ing that Q3 is turned off. 1AlA2 terminal K
sh ou ld still be gro u nded for this measurement.
II. PROBLEM : P oo r positive peak capabilit y .
Drive the transmi t ter directly with an audio signal generat or .
The enhancer in th e MW-SA should be bypassed. Ver i fy the
positive pea k limit with a scope connected to the modulatio n
monitor out put of the transmitter . Remove audio from the
transmitte r.
Causes for poor positive modulation capability are as follo ws :
1. Low s u pply voltage. Compare your present supply voltag e
with t hat on the facto ry test data. 13 to 14.5 KV i s
t y
pica l.

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6
2. Low PA grid current. Grid current of greater than or
equal to 225 MA is usually sufficient for positive peak
performance. A bad PA tube, an open grid resistor, a
failed RF DRIVER module, or low output from the oscillator
board can cause low PA grid current. A downward deflecti on
of PA grid current with modulation may indicate that the
PA tube is bad or that the neutralization circuit is
broken or severly misadjusted.
3. Poor PA efficiency. Compare the efficiency with the
factory test data. Check the tuning of the third har-
monic resonators. The grid resonator should be adjusted
for a dip in plate voltage. The plate resonator should
be adjusted for a peak in the efficiency reading on the
output meter or by adjusting for the correct PA plate
waveform on a scope.
4. Poor modulator efficiency. Compare the supply current
with that on the factory test data. High supply current
confirms that the modulator efficiency is poor. Causes
for poor modulator efficiency are as follows:
a. Inefficient drive to the modulator. Check the PDM
signal at pin 10 of the audio driver with a scope.
The signal should closely resemble that shown on
the PDM and audio driver schematic. If it does not,
trace the signal back through the audio driver and
PDM boards. Components that typically degrade the
slope of the PDM signal are leaky zener diodes coupling
the PDM signal and the transistors which amplify it.
The slope of the PDM signal is important with respect
to modulator efficiency and audio distortion.
Once you have located the area where the signal differs
in shape from that shown on the board and schematic,
try substituting new components for the suspected ones.
You may otherwise find it difficult to determine which
component is at fault. In this case, a component has
developed an unacceptable amount of leakage.
b. A bad modulator tube. Substitute a different one.
c. PDM frequenc y grossly out of its normal range. Typi-
cally the range of adjustment is 70-90 Khz. The great-
est sensitivity to PDM frequency is 10 Khz distortion.
Secondary effects are noise and frequency response.
d. Grossly improper PDM filtering. Place a scope probe
through the screen mesh above the modulator tube to sam-
ple the modulator plate waveform. Except for a small a-
mount of ringing on the top of the signal, it should

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7
look
very
much
like
the
grid
signal
(Pin
10
of
the
Audio
Driver).
If
this
signal
is
distorted
at
the
plate,
but
not
at
the
grid,
there
is
a
filtering
problem.
A
physical
inspection
of
lLl,
1L2,
and
1L3
is
recommended.
A
change
in
the
inductor
will
most
likely
be
visible.
Capacitors
1C5,
lCB,
and
lCl0
should
be
substituted,
but
a
capacity
checker
may
be
able
to
locate
the
bad
capacitor.
e.
Bad
tube
socket.
Check
the
modulator
screen
current
and
make
continuity
checks
to
verify
that
the
screen
voltage
is
actually
getting
to
the
tube.
III.
Poor
Frequency
Response
1.
Check
the
negative
feedback
voltage
at
terminal
Non
the
PDM
control
board.
It
should
be
-12
to
-16
volts
with
5
Kw
output.
2.
If
the
frequency
response
rolls
off
several
DB
at
10
Khz,
examine
the
PDM
filter
coils
1Ll-1L3.
A
change
in
these
coils
is
usually
visible.
3.
Check
the
PDM
frequency.
It
should
be
in
the
range
of
70-85
Khz.
Changes
within
this
range
have
an
effect
on
10
Khz
distortion,
noise,
and
a
relatively
small
effect
on
response.
4.
Make
substitutions
in
the
PDM
filter
for
1C5,
lCB,
and
lCl0.
Improper
PDM
filtering
will
affect
high
frequency
response.
5.
Poor
antenna
sidebands
can
also
affect
high
frequency
response.
Test
the
transmitter
into
a dummy
load
and/or
make
an
impedance
sweep
of
the
antenna.
6.
Components
which
could
affect
low
frequency
response
are
1AlA2
Tl,
C3,
and
C5.
IV.
High
Distortion,
Several
Percent
at
All
Frequencies
1.
Troubleshoot
the
shape
of
the
PDM
waveform
without
audio
as
in
4a
under
Poor
Positive
Peak
Capability
in
this
bulletin.
V.
PROBLEM:
No
power
output,
no
plate
voltage,
no
plate
current
with
high
voltage
circuitry
activated.
1.
Check
the
multimeter
for
the
presence
of
supply
voltage
and
screen
voltage.
Each
supply
should
be
energized

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8
when the high voltage circuitry is on. Absence of either
voltage will cause this symptom except that a modulator
screen overload may occur if the 14 Kv supply voltage is
missing.
2. Note the modulator bias reading with the high voltage on.
a. If it is at or near zero, the circuitry driving the
modulator is probably working properly. Check the
modulator screen current reading. If it is high,
the modulator plate circuit may be open. Low or
zero screen current under these conditions would
indicate a bad modulator tube, open screen circuit,
or possibly an open cathode circuit. Remove all
primary power and check continuity from the modu-
lator plate to the PA filament. It should measure
less than 100 ohms. Ohmeter checks of the screen
and cathode circuits may also be made.
b. If the modulator bias is over 200 volts negative
when the high voltage is on, the PDM and Audio
Driver are holding the modulator tube cut off.
Remove any connection at 1TB2-9 (EBS/Phasor Off)
which may be holding the PDM Off.
Aside from the EBS/Phasor Off and the adjustment
of the power controls, components likely to cause
the modulator bias to be fully negative with the
high voltage on are as follows:
1A2A2 K6 The contacts may be sticking in a closed
condition or the relay coil is not being de-energized
by 1A4 K4 when the high voltage is turned on.
1AlA3 Ql shorted.
1AlA3 CR15 Measure 47 volts at the cathode of CR15
with the high voltage circuitry activated.
1AlA3 CR1-CR6 shorted.
lAlAl Q4 shorted.
lAlAl CR4 shorted. Measure 15 volts at the cathode.
1AlA2 R22, R23 open. Check with an ohmeter, or
measure forward bias on the base emitter junction
of lAlAl Q3 (approximately 0.6 volts).

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' -
9
Neutralization
Neutralization
is
not
particularly
critical
on
this
trans-
mitter.
Tube
replacements
may
be
made
without
re-neutralizing.
Should
you
desire
to
check
neutralization,
follow
either
of
these
procedures.
Disconnect
all
primary
power.
Connect
an
RF
signal
generator
to
the
bottom
side
of
1A3
Cl2
in
the
PA
section.
Disconnect
the
RG-58
coaxial
cable
at
Cl2.
Set
the
signal
generator
to
your
carr
i
er
frequency.
Connect
a
scope
to
the
plate
of
the
PA
tube.
Adjust
the
neutralization
ca-
pacitor
for
minimum
deflection
on
the
scope.
Another
method
of
neutralizing
is
by
using
the
RF
driver
as
the
signal
source.
Ground
terminal
5
of
the
RF
oscillator
board
with
just
the
filaments
on.
Loosely
couple
the
PA
plate
to
an
oscilloscope
by
placing
a
scope
probe
through
the
screen
above
the
PA
.
Adjust
the
neutralization
capaci-
tor
for
minimum
deflection
on
the
scope.
VSWR
Trip
Adjustment
Turn
the
high
and
low
power
cont
r
ols
to
minimum.
Turn
the
transmitter
off
and
r
emove,
then
invert
the
directional
coupler.
The
reflected
portion
will
now
sample
forward
power.
Switch
the
power
output
meter
to
reflected.
Turn
the
transmitter
on
and
gradually
turn
the
power
output
up
until
400
watts
is
reg
i
stered
on
the
meter.
The
VSWR
circuit
should
trip
at
this
point.
If
adjustment
is
necessary,
turn
1A2A2 R26
fully
clockwise
.
Turn
1A2A2 R32
3/4
of
its
rotation
counter-clockwise
.
With
the
power
output
meter
reading
400
watts
reflected,
adjust
R26
counter-clockwise
until
a
VSWR
trip
occurs.
Turn
the
power
controls
to
minimum,
then
gradu-
ally
bring
the
transmitter
power
output
up.
Observe
where
the
VSWR
trip
point
occurs.
If
necessary,
adjust
R32
to
obtain
the
correct
threshold
of
400
watts.
Tube
Life
Generally
this
has
been
3- 4
years
for
both
the
PA
and
modu-
lator.
Signs
of
aging
could
be:
1.
A
gradual
increase
in
disto
r
tion
.
2.
A
gradual
reduction
in
positive
peak
capability.
3.
Steadily
decreasing
PA
grid
current.
4.
Filament
voltage
having
an
effect
on
distortion,
positive
peaks,
and
grid
current.

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Multiturn
Power
Controls
A
multiturn
replacement
is
now
available
for
the
high
and
low
power
controls.
It
is
a
direct
replacement
for
CTS
type
305,
Bourn
3861C-122-253A,
and
Allen
Bradley
GP2604
OS
253
MA.
It
is
not
a
direct
replacement
for
the
flat,
blue
CTS
or
Mallory
potentiometers
as
are
used
for
the
dissipation
10
and
carrier
shift
controls.
The
flat
CTS
and
Mallory
potentiometers
were
used
for
high
and
low
power
controls
in
early
productions,
therefore,
these
potentiometers
will
not
be
printed
circuit
board
compatible
with
some
older
MW-5
transmitters.
Check
to
see
what
your
transmitter
uses
before
ordering
this
part.
Part
Number
550-0976-000
Description
25K ohm
3/4
watt
multiturn
cermide
thick
film
CTS
type
VA305