MTU K-1002-2 - Page 6

. Ihg
audio
power
arpliflef is-a
spegia]
high
efficiency desigrr
that
allo,'ls
a
significant
power
output
with
only a single
5 vo'lt
power
supply-and
no output
transf
ormer. Three
para'l1e'l
connected
CiOS
gates provide
most
of the
voltage
anplification
in the
circujt as
well
as
presenting
a high input
impedance
and
having a moderately 1or
output
impedance.
The
output
stage is
fully
comp'lenentary
and
incorporates
considerable local
feedback
of its
orn.
The
voltage
gain
of
the
outptt stage is
approximately
18.
0verall
feedback
from the
output back to
the
vo'ltage anplifier input comp'letes
the loop
and
gives
a closed
loop
gain
of
about
3.5
with
maximum
volume.
The 92P1,01
(NPN)
and 92PU51.
(Pl'lP)
output
transistors
actually contain a one-anp capability
transjstor
chip
in
a
modified
T0-92
plast'ic
case.
Thjs is
ttp sane chip as used in
the
Motorola
MPSttrL and
MPStlSl
plast'ic
power
transistors. Wjth an 8
ohn
'load,
the
anp'l
if ier
outprt can
swing
to
within
.3 volts of the
5
volt supply or
gnound
before
saturating.
The
2.7
otm
resistor
and
.luF capacitor
(R46
and C9) accross
the outpt
prevent poss'ible
osci'llation
w'ith'induct'ive'loads.
R39, R40, 01,
and
D2 form
a
bjas
network
wh'ich allols
a
couple
of mjlliamps
to
flol in
the
output trans'istors at
all
times
to minimize
class-B
crossover distortion. R45
mixes the CB2
s'igna1
from the
connector
in
with
the
filtered DAC
output
to
allor continued
use of
this
s'igrra1 with
existing
game
and simple
music
prograns.
TROUB
LESHOOT
ING
D'iagnosing
problems
with the K-1002-2 board
is
fairly
simp'le because
of
the
un'id'irectional
s'igna1
flor
and
minimal
interaction anong circujt
components.
If
the
board is
completely'inoperative, first do a thorough
visual inspect'ion
of the
board. Look on the sol
der
si de of the board for
component
I
eads that may be
bent
and
shorting out to adjacent
pads
or
PC
runs. Check for loose
components
and the
possj
bi 1 ity
of a col
d
sol
der
joi
nt.
If careful inspection fails to turn
up anything,
enter and
run
the
prognam
given
earl
ir. Then
using a scope or
anpl
if ien/speaker
if a
scope
is
unavajl
ab]e,
find out where
the
sigal is being lost.
The first
point
to
check
is
the node
where all of
the DAC resistors
are
tied together.
The signral should be
a square
wave
modulated at a 60Hz rate
(ca'used
by
timr
interrupts in
the PET)
with
ground
and
+5
voltage
levels.
Next
check
the
output of the first filter
section which'is
U3-8.
The
waveform should
be
sorneylhat rounded wjth
voltage
levels
of
about
L.25
and
3.75 volts.
The
next stage output
(U3{)
should
be
ssns{hat less rounded
w'ith
a hint of
ringing
on
the
square wave
edges.
Again the
signa'l
levels
(1ess
ringing) should
be 1.25
snd 3.75 volts. The
'last
stage output
(U4-6)
has
cons'iderable
ringing
at a
little
over 3kHz but
still
the sane
voltage levels. If
the sigrtal
js
lost
anylhere
jn
the fjlter
or
the
voltage
levels
are
cons'iderably
offset
(greater
than
.5 volt
deviat'ion)
from thejr
proper
valrcs first
wiggle
all
associ ated components
to see
'if
anythi
ng
changes. If
thi
s
f
a'i1s, the associ
ated
CD4069
(the
74C04
is
an equivalent) should
be
replaced.
Be
sure to
use a CD4069
or
a
7464;
other varieties oF inverters may
not
be
suitable
f
or
I
inear operation
or
may have too h'igh
an
internal
impedance.
If
the
problsn
is in the
power
amp'l
'if
ier, first measure the
output
trans'istor
collector
voltage
(tfte
little tabs
sticking out the top of the transistors
are the
collectors)w'ith
no sigrra'|. This voltage
should
be
within a
quarter
volt
of
2.5
volts.
If it is off
consjderably, one of the
4
transistors in the output
stage
may be shorted or open or the CttOS voltage
anpl
if ier may be bad. Temporarily
cut
the
l
jne
running
from
U4€, 10,
and
12 to the
junction
of
Dl
and
D2
and measure
the
col
I
ector
vol tage agai n. If it i
s
now
centered, the CI'OS is
bad;
otherw'ise
one
or
more transistors
are
bad
or one of the
biming resistors
(R38-R43)
'is
open.
The
anplifier
is only
partially protected
fnom outout shorts so exercise care
in
connecting
the speaker
and
don't run the
unit with
the volune
full
up
when there
'is
an obvious
probl
sn such as a speaker l'ine short.
If
the
custoner
is
unable to f ind the
prob'lan,
return
the
un'it to the factory
f
or
servicing
a'long w'ith a description of the malfunct'ion.
This is
a
rel
at'ive1y
simple
board and
factory
repair
can usually
be
accomplished
in
a couple of days.
4