Philips PW 1720 manual 7

3-6

6. Change

the

filament

voltage

supply,

if necessary,

depending

on

the

type of

focus.

A

fine

focus

tube should have a filament

voltage

of

8

volts,

and broad

and

normal

focus

tubes

require

10

volts.

On

later

tubes the

type of

focus

is

indicated,

and in

other

cases consult

the

tube instruction

card.

To

change

the filament voltage, switch off

the

unit,

open

the

right-hand

side

panel

by

loosening

the

two

Allen

screws.

The

panel

is

hinged at the bottom. Behind

the

front

panel

is

a

switch

marked BF

(broad

focus)

and

NF FF (normal/fine focus). Select as required.

Close

the

cover.

7. Close the air inlet valve, and turn on the water supply.

8. Turn the kV and mA controls to the minimum position, and operate the GENERATOR I push-button.

9. When an X-ray

tube

is

new, or

has

not

been used for a

week or

more, the following procedure

should be

followed:

Check

that the

mA

meter reading is

stable, then

slowly increase the kV control

to

30

kV,

over

a period

of one

minute.

Slowly

increase

the

mA

control

watching

for signs

of instability

of

the

mA

reading.

DO NOT EXCEED THE RATED kV AND mA VALUES, as shown in Appendix 1; disregard the instruction card values (X-ray diffraction tubes).

Slowly increase the kV to	the	required operating high-voltage,	at	the	same	time	adjusting		the	mA reading
so that the maximum power	to	the tube does not exceed the	rated		limits.	If the	mA	meter		reading shows
any sign of instability the kV		should be reduced, and held at	a	lower value		for	about	a	minute before
attempting to advance the	kV	control again.

3.7.POSITIONING THE TUBE SHIELD

The tube shield can be placed in one of

three

orientations on

top

of the generator, i.e. so that

the

X-ray

beams make angles of either 90° or 45°

or

0°

to the

front

of

the

generator.

However, the rotation of the tube shield

must

not

be

more

than

90°

in

either

direction from its

original

position. Since the line and point focus beams

are

90°

apart

it

is

possible to place the tube shield with

either

beam

in any position

depending

on

the

use

of

the

generator,

or

personal

preference.

The tube shield is fixed to the table

top

by means of four Allen screws. Remove the screws and

rotate

the

tube

shield carefully, simultaneously ensuring that the cables and

water

hoses

inside

the

cabinet

are

not

unduly

tensioned. For this purpose open the left-hand

side

panel

of

the

generator cabinet, and

observe

the

hoses

and

cable.

Refix

the

tube

shield on the table

top

with

the

screws,

and

close

the

side

panel.

3.8.

DIFRACTOMETRY

OPERATION

The generator will normally be used to obtain

the

simultaneous (photographic)

registration

of

reflections

from

a

specimen,

therefore,

stabilisation

of

the

X-ray tube voltage

is

not

necessary.

A

PE

1612

a.c.

voltage

stabiliser

can

be

used

in

the

mains

supply

to

the

generator,

when

an

improved

stabilisation

is required for more accurate powder diffractometry.

This voltage

stabiliser

is

specially

designed

to provide a stable mains voltage to small units, such

as

the

PW

1720

X-ray generator. Variations in the mains

voltage

from

-15% to + 10% will be stabilised so that the output

voltage variation is better than 0,1%. A

full

specification

is

given

in

the

operation

manual of

the

PE 1612

a.c.

voltage

stabiliser.

A PW 1050 vertical goniometer with stepping motor or with synchronous motor can be installed on top of the generator, and two mounting holes are provided in the top cover, see figure 3.4. Refer to the PW 1349 “Diffractometer kit” instruction manual, for alignment information.