Continental Electronics 816R - PA Grid Adjustment Procedure

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81

6R

INFORMATION

LETTER

January

18,1992

POWER

AMPLIFIER

GRID

ADJUSTMENT

PROCEDURE

There

may

be

several

reasons

for

readjustment

of

the

PA

Grid

Tuning

and

Coupling

circuits.

When

an

IPA

Module

fails

and

is

replaced,

the

following

procedure

should

be

used

to

make

certain

that

some

failure

within

the

PA

Grid

circuit

does

not

cause

another

IPA

failure.

Tube

replacement

is

another

reason

for

using

this

procedure.

In

general,

the

following

procedure

should

be

used

if

there

is

any

reason

to

suspect

that

there

may

be

a

bad

load

on

the

IPA

or

if

changes

have

been

made

in

the

PA

Grid

circuit.

When

the

IPA

internal

VSWR

protection

circuits

are

utilized,

the

IPA

will

turn

OFF

if

the

VSWR

presented

by

the

PA

input

exceeds

3:1.

This

can

make

grid

tuning

difficult

if

the

grid

is

far

out

of

tune.

The

802A

Exciter

can

be

used

to

adjust

the

Grid

Circuit

to

approximately

correct

tuning

and

coupling

positions.

The

802A

Exciter

has

a

Reflected

Power

Foldback

circuit

that

limits

the

reflected

power

output

to

a

safe

level

for

the

exciter

regardless

of

the

VSWR

presented

to

the

Exciter

by

the

PA

Grid

Circuit.

The

first

six

steps

of

the

following

procedure

allows

you

to

tune

the

grid

circuit

using

the

output

of

the

Exciter.

1.

Use

a

coax

cable

and

BNC

to

Type

N

adapter

to

connect

the

Exciter

RF

output

directly

to

the

Grid

input

connector.

This

bypasses

the

Driver

and

the

IPA.

2.

Turn

the

Transmitter

Plate

Breaker,

CBS,

OFF.

3.

Turn

Filament

and

Plate

Control

ON.

After

the

filament

time

delay,

there

will

be

IPA

and

Driver

Collector

voltage

but

no

IPA

or

Driver

Collector

current.

There

will

be

no

PA

plate

voltage

or

Screen

Voltage

since

the

Plate

Breaker

is

OFF.

4.

Set

the

Exciter

RF

output

level

to

approximately

15

watts.

5.

Adjust

the

PA

Grid

TUNING

and

COUPLING

controls

for

minimum

indication

on

the

Exciter

Reflected

Power

Meter.

6.

Increase

the

Exciter

RF

output

to

approximately

40

watts

and

repeat

step

5.

7.

Turn

Plate

and

Filament

Control

OFF

and

remove

Primary

Power

from

transmitter.

Restore

the

Exciter,

IPA,

and

Driver

to

original

configuration.

Turn

Plate

Breaker,

CB5,

ON.

8.

Remove

the

thyrectors

(voltage

absorbers)

that

may

be

in

parallel

with

the

Plate

Power

Supply

choke

LI.

This

is

necessary

to

prevent

damage

to

the

thyrectors

when

the

power

control

is

used

to

reduce

the

plate

voltage

to

zero

in

later

steps.

NOTE:

The

PA

Tube

input

impedance

decreases

as

the

Grid

input

power

is

increased.

The

Grid

Tuning

and

Coupling

must

be

readjusted

when

the

IPA

and

Driver

are

reconnected

and

when

IPA

output

power

is

increased

to

normal.

26