Harris MW-5 Reference manual

www.SteamPoweredRadio.Com

.

--

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

www.SteamPoweredRadio.Com

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

www.SteamPoweredRadio.Com

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

3

of

the

oscillator

board

to

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.

www.SteamPoweredRadio.Com

•

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.

www.SteamPoweredRadio.Com

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.

www.SteamPoweredRadio.Com

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

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

www.SteamPoweredRadio.Com

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

www.SteamPoweredRadio.Com

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).

www.SteamPoweredRadio.Com

' -

9

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.

www.SteamPoweredRadio.Com

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

Harris MW-5 Reference manual

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