Hafler 945 manual AM/FM Tuner, General Trouble Shooting Hints Ground Loops

HEADPHONE AMPLIFIER

AM/FM TUNER

The

Headphone

Amplifier

utilizes

a separate special purpose operational

amplifier

to

drive the

high

current demands

of headphone

operation.

This

provides

the

extra power

necessary

for

driving

difficult

headphone

loads,

and leaves

the

main

signal

path uncompromised for signals to

power

amplifiers.

The

Headphone jack is equipped with a sensing

switch

that

automatically turns off

the

Line

Output

when

headphones

are

plugged

in.

Antenna signal input to the FM Tuner section is via either balanced 300 ohm thumb-screws, or via an unbalanced 75 ohm F-Type connector. Antenna signal input to the AM Tuner section is via unbalanced thumb-screws.

The

FM front end features

dual-gate

FET’s

for

exceptional rejection of spurious noises

and the

reduction

of IM distortion in the presence of

high level

signals.

Tuning is

accomplished

via a

voltage

controlled

varactor diode, driven by a

digitally

synthesized

Phase

Lock Loop

system.

Twin

ceramic filters

in the IF

stage

have

flat

group delay characteristics,

improving

selectivity,

phase

linearity,

and

lowering

distortion.

The

audio

output

features

multi-stage

multiplex

filters

for

removing

the

19

kHz

pilot

tone.

ADDITIONAL INFORMATION

GENERAL

TROUBLE-

SHOOTING

HINTS

GROUND LOOPS

Operation of the 945 Preamp/Tuner is straightforward, and few operational problems, should be encountered after a thorough reading and understanding of the owner’s manual. The Functional Block Diagram provides an excellent “picture” of the signal flow, and should be referred to along with the descriptive text.

If all controls are in the intended position and the Ready light is green, check all system power

connections,

interconnecting

and

speaker cables,

and

fuses.

The 945

does

contain one internal power

fuse, but this fuse should

not

generally

blow

unless

a

malfunction

has

occurred.

This

fuse

should be

replaced only

by aqualified

technician, and

only

with the

exact type

and

rating

of

fuse

originally

supplied.

If

this fuse

is

replaced and

blows again

within

a short

time,

disconnect

all

power

immediately

and

return

for

service.

If all controls, fuses, cables, etc. seem to be functioning properly, a process of one-at-a-time component substitution should be employed until the defective unit is identified. If only one channel is not functioning properly, a one-at-a-time reversal of interconnect and speaker cables from left to right should reveal the malfunctioning component.

Ground loops are characterized by a low level hum or buzz in the system. Loops are caused by a voltage potential difference between two points in a ground circuit, and aggravated when multiple paths for a given circuit exist. Noise-free audio performance is dependent upon all grounds being at the same potential, with a single path for each ground connection. Ground loops can exist in two forms: 1) loops created in audio interconnects, and 2) loops created between earth grounded chassis.

Mounting components to a rack with metallic rails may introduce ground loops between associated equipment, because the rails can introduce a second ground path. The extent of this problem will depend on the grounding arrangements of associated equipment. Ground loops can occur in non-rackmounted equipment, though it is less common.

If

ground

loops occur, and

any other

component

in

the

system has a

three wire grounded power cord,

the

first step

should

be to use a ground

adaptor

(with

the

ground tab

or

wire

of the

adaptor notconnected)

on the power cord plug of the preamplifier. DO NOT cut

off the grounding pin on the plug! It may be

necessary

to

use

additional

adaptors

on

other

grounded

components

if

more

than

two components are

earth grounded. (In other words, only one earth ground per system should exist.) Another potential

source of multiple earth grounds	is from coaxial antenna or	cable service feeds for FM or video sources,
which usually are (and should	be) earth grounded. The	ground adaptor(s) should cure this grounding
problem as well.

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