Specifications

ManualsBrandsDyna ManualsAmplifierSCA-35

Both

Channels

Not

Working

Should

neither

channel

function,

there

the

possibility

that

similar

mistake

has

been made in

the

wiring to both

sides.

This

unlikely situation,

but

should be checked

by visual comparison of

the

wiring with

the

diagrams.

Generally,

the

cause of

malfunction

both

channels

together lies

the

one section of

the

unit

which

com-

mon to both channels -

the

power supply.

The

checking of

this

section is covered earlier.

is also possible

that

the

unit

is working on

radio

other

high level

input,

but

not

on phono

tape

head.

this is

true

for

both

channels

together, then

the

power

supply

wiring which connects to

the

PC-ll

board should be

checked,

this is

the

only

element

common to both chan-

nels in

the

preamplifier section.

One

Channel

Not

Working

tubes

one

channel

not

light,

the

heater

circuits

must

be checked

described above.

Tubes

can

be inter-

changed between

the

two sides to see if

substitution

shows

faulty

tube.

this cures

the

bad

channel,

then

evi-

dently

the

faulty tube has been located.

not

tube

which causes

the

trouble,

then

necessary to determine whether

the

preamp

section

(the

PC-ll)

the

amplifier section

(the

PC-lO).

radio

input

satisfactory while

phono

is not,

then

the

trouble

the

PC-ll

section which

handles

phono

and

tapehead

inputs,

the

wiring associated with

the

selector switch.

inputs

function,

then

the

difficulty is

either

in the

front panel wiring which includes the tone control sections,

the

PC-lO

section

the

Z-565

output

trans-

former which connects to

that

channel.

The

wiring of the

front

panel

must

carefully

compared

on both left

and

right

channels.

is possible to re-do

the

wiring

and

to inter-

change

the

two PC-lO

boards

determine

whether

the

fault

one of these.

is also possible

interchange

the

Z-565

output

transformers

try

isolate

the

trouble.

However, a fault

the

output

transformer is most unlikely,

and

this should be tried

last

resort.

both

channels work

radio

other

high level

input,

but

only

one works

phono or tapehead

input,

then

the

trouble is in

either

the

PC-ll

board

the

selector

switch

the

front

panel

wiring from selector switch

through

the

front panel controls.

The

PC-ll

board has

both

preamplifier sections

a parallel

arrangement,

so com-

parison visually of wiring

and

parts

should

indicate

whether

there

any

difference between these sections.

Each

section of

the

selector switch should be identical

appearance

and

in wiring.

These

should be compared care-

fully.

The

12AX7's on

the

PC-ll

should be interchanged to

see if one is

fault.

Hum

and

Noise

the

unit

is working,

but

there

is excessive

hum

noise,

the

first

step

is to see whether

the

trouble is in

the

SCA-35

associated equipment.

The

SCA-35 should be

checked alone, with all

other

equipment

except

the

speakers

disconnected,

and

with

the

cover

and

bottom

plate

the

SCA

secured in place.

the

hum is reduced

without

the

other

equipment,

then

will be necessary to determine

what

kind

of ground connections

are

necessary between

equipment

reduce

the

hum. Sometimes connecting wires

from chassis to chassis

are

good, sometimes

they

are

harm-

ful. Reversing line plugs on associated

equipment

may

helpful.

must

understood

that

hum

which disappears

when associated

equipment

is disconnected

turned

off,

not

a problem of

the

SCA-35.

Hum

which persists

after

other

equipment

is eliminated

a source of trouble

generally a

result

either

faulty

wiring

a faulty tube. Wires which

are

not

positioned

shown

the

diagrams,

may

be a

cause

of hum.

This

particularly

true

for

the

wires which lead from

the

PA-7

transformer, since these wires

carry

currents

which

may

be a source of hum.

tube

causes

the

problem, then

not

likely to

appear

in both

channels

simultaneously.

the

hum

is loud

with

the

volume control

turned

.down,

then

is most likely

to be associated with

the

7199 tube.

occurs

phono

tapehead

inputs

with

the

volume

turned

a bit,

then

most

likely to be associated with

the

12AX7 tube for

the

channel which shows

the

hum.

These

tubes

can

checked

substituting

the

other

channel.

objectionable level of hiss is

most

likely caused by a

defective resistor

printed

circuit

board.

heard

with

the

volume

turned

down,

is likely to be

part

of the

7199

circuit

can

often be located by wiggling

the

body

the

resistor (using insu]ated pliers) to see if

the

hiss is

affected.

is on

phono

tapehead

input,

with volume

turned

up,

then

the

same possibility

applies

the

12AX7

circuit.

Some

hum

and

hiss will always exist -

depending

speaker

efficiency

and

other

factors. However,

should be

negligible

normal listening levels.

very

high

levels,

some noise will always be audible,

but

of course this will be

masked

the

loud sound.

With

the

volume control

minimum, noise with a

shorted

input

should be less

than

millivolts across

the

16 ohm

output;

with volume

maxi-

mum,

should

not

exceed 40 millivolts

this point.

Sometimes,

the

magnetic field

around

the

PA-774 trans-

former

may

cause some vibration of

the

cover which is

audible

a buzzing sound.

This

can

be e]iminated by

placing a' small wedge of fibreboard

wood

top of

transformer, between

and

the

cover.

Distorflon

distortion is encountered,

the

first thing to do is to

make sure

that

all

inputs

and

outputs

are

properly con-

nected

and

properly

functioning.

the

leads to a loud-

speaker

have

frayed cables

and

touch

several screws simul-

taneously,

this

can

be a source of distortion.

a ceramic

cartridge is

inserted

into

the

magnetic

cartridge

input

(or

if a

tape

amp

goes

the

tapehead

input)

there will be

overloading of

the

input

and

severe distortion.

the

trouble seems to be

the

SCA-35,

then

determine

one channel

both.

in both channels,

the

only

circuitry

common to

both

channels is

that

the

power

supply

and

the

ohm

resistor in

the

cathode circuits of

the

6BQ5 tubes.

only

one

channel

has

distortion, tests similar to those

described previously

should be used to localize

the

prob-

lem

and

tubes should be

interchanged

see if they

are

cause of it.

Inspection

parts

the

printed

circuit boards

should

made

to see if both sides of

the

circuit

are

identical.

Voltage

Measurements

The

voltage

chart

shown with

the

schematic diagram will

be of considerable

aid

in troubleshooting if

there

vacuum

tube

voltmeter

similar

test

instrument

available.

Voltages

through

the

circuit

should

be compared carefully

with this chart. Deviations

to 20%

are

not

signs 0

abnormality,

but

past

this

range

they

indicate a malfunc

tioning of

the

circuit

that

point.

Such

measurements

can

help

locate defective

parts,

miswiring,

bad

tubes.