Roland TR-808 Service Notes page 5

JUN.
15,
1981
TR-808
START ON
ANT
I
LOG
Q16
POWER ON RESET
SWITCH
FIGURE
8
START/STOP & TEMPO CLOCK
CIRCUITS
Start Sto
F
oiAiti/ oi
Uir
oiAlUo
Q12
collector
TEMPO CLOCK
Q15
collector
ICl pin
1
ICl pin
2
IC2 pin
4
IC2 pin
6
ICl pin
13
ICl pin 12
X
Q16 ON (charge)
FIGURE
9
TEMPO CLOCK
TIMING
DIAGRAM
START/STOP & Tempo
Clock
When
the power supply
for
TR-808
is
turned on, the
TEMPO
clock
continues oscillation regardless of the operation mode of TR-808.
However,
when
the
START
button
is
pressed
in
the
STOP
nnode,
oscfllation stops once for
9ms
to provide
a
mode
change preparation
time to
CPU.
In this way, the
START/STOP
circuit
and the
TEMPO
circuit are closely related with each other.
When
the
SYNC IN/OUT
switch
is
set to IN, both circuits
become
ineffective and externaf
signals
from
the
DIN
socket duplicate the both circuits.
When
the_
START/STOP
switch
is
pressed (closed) with
rhythm
stopped,
Q
of
F/F IC2B becomes L,nhe
collector of
01
2
becomes
H, of IC2B becomes H and IC2A
is
reset.
Q
of
IC2A becomes H
and the collector of 015 becomes
L.
Then,
since
O
of
IC2B becomes
H, pin 2 of ICl becomes L
to turn
on 016. As
a result, the
TEMPO
GENERATOR
of 2/4
ICl (D, E) stops oscillation (details are de-
scribed
later).
After
9m$
later, pin
1
of IC1A drops below
the thresh-
old
level
and pin 2
is
reversed.
The
rising
edge
reverses
of IC2A
to
L and the collector of
015 (TEMPO CLOCK
output) becomes
H.
At the same time,
016
is
cut
off,
and CIO
starts discharging
through
the
ANTI-LOG 014
to continue
oscillation.
This discharging speed of CIO determines the
oscillation
frequency
of
the
TEMPO
clock. The
variation
range
is
between 8.3ms and 65ms.
With TR-808,
J is
defined to have 24
clocks,
and thus
I
is
approximately equal to 400—300.
When
the
level
of CIO exceeds the threshold
level
of
pin
13
of ICl
due
to discharging,
the output
of pin
10
is
reversed,
016
turns on,
and CIO
is
charged. The output
of pin
12 of
ICl
is
divided into 1/2
byT-FFof
IC2A.
^
=
rn
21LJ R.l-e2-
C/-C2.
FIGURE
11
REPRESENTATIVE BRIDGED T-NETWORK
E^y ELOPE
zz
OUT
-o
r
FIGURE
10
POWER ON/OFF DETECTOR
HUTIK)^
Muting, Reset
The circuit
composed
of
010—012
detects
power
on/off or sharp
voltage drops in
TR-808 DC
lines and feeds forward bias (0 volts) to
FET
switches
connected
to point A. These
FETs
are for resetting
CPU
(064), preventing writing into
RAMs (075) and muting Master
Out
(013).
Power on
:
OV
1
-2sec
-
1
5
V
Power
off:
-15VtoOV
If this circuit
is
defective, the
CPU
may be kept
reset. (Detail in
TROUBLESHOOTING
on page
14.)
D
C3
^
FIGURE
12
REPRESENTATIVE SWING TYPE VCA
Sound
Generators
The
bridged T-network
filter
shown
in Fig. 11 is
used
to
generate
periodic damping
drum
sound. This configuration has
variations
according to application (instrument sound). Values of R and C
can be changed. With this
circuit,
the decay time becomes longer
as
O
increases.
The swing type
VCA
shown
in Fig.
12
is
used to generate metalic
sound
(noise).
This
circuit features
its
output
waveform
having
many
high harmonic components
to
provide
ringing metalic sound.
Major
features
of each sound
generator are described below.
Bass
Drum
This
sound
generator
is
composed of
a
multi-feedback,
bridged
T-network
including 1/2 IC12 (pins 1-3)
as
an
active element.
The
decay time
of the resonating waveforms can be controlled by
ad-
justing feedback
amount by VR6.
Immediately
after a trigger pulse
is
fed into the generator, the
filter's
time constant
-
when
ACCENT
is
present
-
is
halved and has
a
resonance on twice
its
inherent frequency
for a half cycle period,
then on the fixed frequency with decaying amplitude.
This changing
frequencies
will
sound
a
punchier
crisp bass. This trick
is
performed
by the
circuit
composed of 041—043.
When
a trigger signal
is
outputted
from
the collector of Q40, 041
turns on, 042
turns off,
043
turns
on and R165
is
shorted. This
halves the time constant of
this network. The
ON
period of
043
is
determined by R156 and C38 and
equals
4ms
which
is
1/2 x 1/2
of
16ms
of the inherent
oscillation period of the filter.
When 042
tgrns on after 4ms,
current discharging
from C39
via
R161
produces a retriggering pulse. At
this
time
the generator oscil-
lates
on
the inherent frequency.