Heathkit W-5M User Manual

UNUSUAL
TROUBLES
AND
CORRECTIVE
PROCEDURES
Amplifier oscillation
is
usually
indicated when
the performance
seems
"strained”
or
"muddy.
"
Intermittent
performance of
this
type
is
usually a
definite
symptom,
as
is sudden
change
in
ap-
parent output level.
To
check for
oscillation,
it is usually
a good idea to connect
an
oscilloscope
or
wide
range
AC VTVM
across
the
output with
the normal
speaker load
left
connected.
If
these
instruments are not
available,
checks
can
be made with a
NE-2
neon
bulb
or
a fluorescent
bulb
from a
desk lamp.
While
holding
the
glass envelope of the neon bulb
in one
hand, touch
either
of the
leads
to pin
3
of
either
socket
V3
or
V4.
Keep your other
hand
in your
pocket or
behind
your back
when making
this check,
since
dangerous
voltages
are
present
at this
point.
If
the
bulb glows, the amplifier
is oscillating.
If a fluorescent
bulb
is used, touch
one
of
the pins
at
one
end to
pin
3
of
either socket,
touching
the pins
at
the
opposite
end
of the
bulb
with a finger.
Observe this type
of bulb in
a
dimly
lit
room, since the
glow will
be quite
dull.
Any type of glow
indicates
oscillation.
High frequency
instability
is usually
indicative
of
high wiring
capacity within
the amplifier
.
Check the wiring
over
carefully,
making
sure
that none of
the leads
are any longer
than
they
ab-
solutely
have
to be to
make
the
connection.
Special attention
should
be
given
to
the output trans-
former
leads;
they must
be as short
as possible
and dressed
tightly
to the chassis.
The
"Tweet-
er
Saver"
should
be
checked
to make
sure
connections
are correct
and the parts
are
all
right.
This
circuit
consists
of
a
.1
/ifd condenser
and
a
47
£2
resistor
connected
across the
outside
ter-
minals of
the speaker
terminal
strip.
If everything within
the amplifier
appears
to be all
right,
it
is
quite likely
that there
is too
much
capacity in the
speaker or input
leads.
Shielded
wire
should
never
be used for
speaker
leads
,
the
capacity
is
much
too high.
Best results
are
usually
obtained
with
heavy
duty lamp
cord,
ob-
tainable in
any dime
store.
If the
leads must
be
quite long,
it
might
be advisable
to
separate
the
two
wires
to reduce
capacity.
For the
best frequency
response
and
stability
characteristics
it
is
recommended
that the
preamplifier
be
connected
to
the
main
amplifier
with
leads
just long
enough
to
satisfactorily
meet the
requirements
of
the
individual
installation.
Excessively
long
audio
cables will
attenuate
high
frequencies
and the
high
capacity
may
cause oscillation.
Length
limits
are
usually
specified
for the
preamplifier
by the
manufacturer.
Motorboating
or
low frequency
instability,
may
be caused
by
high
frequency
oscillation
and
the
steps outlined
above
should
be checked. Make
sure the
output tubes are
properly balanced
and
that the filter
condensers in the
power supply are
wired correctly.
In
rare
cases, the
feedback
might be
marginal and
it will
be
necessary
to increase the
value
of the
feedback resistor slightly
to
approximately
6800
£2.
Feedback
reduction will
be
very slight and
overall
performance will
remain
substantially
unaltered
with
a
resultant
improvement in low
frequency
stability.
Output
tubes
will not balance.
This condition
will
be
caused
by
one
of two things;
a
leaky
1
/xfd
coupling
capacitor or a seriously
unbalanced
pair of
tubes. First the
coupling
condensers should
be checked
by connecting a high
sensitivity meter
such
as
a
vacuum type voltmeter
across
each
of the
100
K£2
grid resistors for
the
output
tubes,
V3
and
V4.
Any
potential
across
this resistor
indicates one
of
the following
conditions:
(a) If the voltage
appearing
at the grid
end is positive,
either the
coupling condenser
is
leaky
or
the
tube
is
gassy.
(b) If the
voltage
appearing
at
the grid
end
is negative, the
amplifier
is
probably
in
oscillation.
Note that
the
meter is connected
directly
across
the resistor,
not from
grid
to
ground.
The
grid
is normally
positive
with
respect
to ground.
If
the
tube
socket potentials
are
normal but
balance cannot
be
achieved,
one of
the
output
tubes
can
be assumed to
be weak.
A
replacement
tube
balanced against
each
of
the
original
tubes
should
allow
a
satisfactory
pair
to
be located.
Balance
near
the end of rotation
of
the
balance
control
is not
abnormal
and need
not
be
cause for
concern.
As
long
as
the
tubes
can
be balanced
the
distortion
figures
will meet
specifications.
Page
26

Heathkit W-5M Specifications

General

Brand	Heathkit
Model	W-5M
Category	Amplifier
Language	English