Paia 4780 Guide
' ±` ' i
ELECTRONICS. INC.
USING
the
=EOUENCER
47=EJ
01975 PAIA Electronics, Inc.,1020W. WilshireBlvd., Oklahomacity, OK 73116
TESTING AND CALIBRATION
There is only one calibration point in the 4780 Sequencer so this procedure will be predominantly
concerned with establishing that the module is operating properly.
Begin by applying power to the 4 flea clips on the rear edge of the ''8" circuit board; + 18 volts
to the point marked " ++ ", +9 volts to the "+ ", -9 volts to the ''-'' and the ground clip to the
common ground of the supplies. Notice that the + 18 volt supply must be capable of supplying
a hefty ( for this equipment ) 50 rna. of current. Suitable power supplies are the PAIA 2720-7
or two 4770 modules.
Begin by setting the front panel controls as follows: Clock rate fully counter-clockwise (CCW).
Run/Stop switch fully down to "stop". Trigger width fully clockwise (CW). Glide fully CCW.
All pitch controls (small black knobs in the SEQUENCE box) fully CCW -note that these are
multi-turn controls that hav6\a built in ratchet at the extremes of their range. Set the ''duration
trim" trimmer on the upper 4780/A circuit board fully CW as viewed from the rear of the module.
Press the ''load" push button and observe that the Light Emitting Diode in the left-most position
of the upper row of LED's comes on and stays on. This should be the only LED lighted at this
point. Repeatedly press the "step" push button and observe that each of the LEDs light in turn
proceeding to the right in both of the two rows. When the last LED (right-most on bottom row)
extinguishes there should be no LEDs lighted. Observe that each time the "step" button is
pressed, the single LED in the Trig. box blinks momentarily.
Using a Volt-ohm meter. measure the voltage between the "Out" jack and ground. Observe
that as each stage is turned on , the pitch control for that stage can vary the output voltage
from essentially zero volts to over 5 volts. Return each pitch control to its fully CCW limit
after each measurement is made.
Slide the Run/Stop switch fully up to the "Run" position and observe that each of the LED's
light in sequence ending with no LEDs lit as the last indicator extinguishes. Note that the
single LED in the Trig. box continues to flash even though none of the LEDs in the SEQUENCE
hex are on. Slowly rotate the adjusting disk of the "duration trim" trimmer on the 4780/A
board in a COW direction (as viewed from the rear of the module) until the point is reached
at which the LED in the Tr.ig. box glows constantly, then back off in a CW'direction until the
indicator can be observed to wink off briefly during each clock cycle. Rotate the front panel
Width control in a CCW direction and observe that as this control is retarded the indicator LED
remains on for an increasingly brief portion of each clock cycle.
Slide the Run/Stop switch to its mid position between ''Run" and "Stop" and observe that the
trigger indicator LED stops flashing. Press the "load" push button and notice that the first
stage of the counter loads (as indicated by the LED associated with that stage) and that the
count progresses at a uniform I.ate through the counter. Observe that the trigger indicator
LED blinks while the counter is working but goes off as the last stage status indicator extin-
guishes.
Use a short jumper to connect the last (right-most in lower row) red stage output pin jack to
the "load" input jack. Slide the Run/Stop slide switch fully up to the "run" position and observe
that the count "circulates", i. e. that as the last stage indicator extinguishes the first stage
indicator comes on. Advance'the CLOCK Rate control in a clockwise direction and observe
that as this control is advanced the count progresses at an increasing rate. Slide the switch
to the "stop" position and observe that the clock stops (as indicated by the Trig. indicator
LED remaining off) and that none of the LEDs in the SEQUENCE box are lit.
Remove the jumper from the last stage of the counter and re-connect it to the second-to-last
jack in the chain. Switch the Run/Stop switch to ''Run" and observe that the count circulates
but that the last stage in the counter does not come on. Move the jumper back one more jack,
start the clock and observe that the last two stages do not activate.
Continue moving the jumper back one jack at a time and each time observe that the count progresses
no further than the stage correspondiog to the jack which is connected to the "load" input. NOTE:
connecting the first stage of the counter to the "load" input will produce no results.
USING THE PAIA 4780 SEQUENCER
CONTROL OPERATION
The front panel graphics of the 4780 divide the controls into four logical groupings:
1) Clock
2) Sequence
3) Trigger (Trig.)
4) Output (Out.)
Operation Of the controls within these groupings is as follows:
CI.OCK
Rate The rate control sets the tempo of the sequencer's
I=i=Trnal clock. Clockwise rotation of the control in,creases
the tempo.
Run/Stop switch This is a three position slide switoh.
When the switch bat is fully down to the "stop" position
the clock is stopped from free running. Changing the
Switch to "stop" from either of the other two positions
generates a short duration pulse that clears the counter.
When the switch is set fully up to the "run" position the clock runs unconditionally
at the tempo set by the RATE Control. Moving the switch to the run position generates
a short duration pulse that loads the first stage of the counter.
The middle position of the switch can be considered to be a "conditional run" setting.
With the switch in this position the clock will free run only when one or more of the
counter stages are loaded. Moving the switch to this position neither clears nor
loads the counter.
Run input The pin jack immediately below the Run/Stop switch provides an electrical input
that loads the first stage of the counter and duplicates the action of setting the Run/Stop switch
to the "conditional run" position (clock runs while counter is loaded but stops when counter clears).
g][j=g!= The Synch input allows the sequencer clock to be synchronized with other clocks in a
system (e. g. another sequencer). This input is active only when the Run/Stop switch is in the
"stop" position. If th6 RATE control is set such that the clock would be running as fast or
faster than the synchronization source then there will be one clock pulse for every synchron-
izing pulse. Setting the rate control such that the clock would be running slower than the
synchronizing source produces a dividing action that causes the sequencer clock to I)ulse
once for every two. three or more synchronization source pulses.
SEQUENCE
Load There are both electrical and
===lal provisions for loading the first
stage of the counter, some of which have
been covered in CI.OCK controls explan-
ation. The I-OAD push
button and pin jack
contained in the SEQUENCE
box of the panel graphics
allow the counter to be
loaded without changing
the clock status. It is
important to note that
the first stage of the
counter loads on the
trailing (falling) edge of
pulses applied to the
I.OAD pin jack.
©©©©©©
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§!£ii The step push button is essentially a manual substitute for the clock. Pressing trm
button causes the count to advance by one stage.
Ej±;s!i There are 12 unlabeled pitch controls (Small black knobs). While there is some 
USING THE SEQUENCER
Figure 1 shows what will be by far the most common patching arrangement between the 4780
and external processing modules. The control voltage output of the sequencer is driving a
VCO While either of the tl`igger outputs (step or pulse) are acting as trigger sources to a
fiHiction gener€itor which is in turn varying the gain of a Voltage Controlled Amplifier.
Fig`ul.c 1 -COMMONSEQUENCERPATCH
0ii. I)itcli that will I)e pl.oduced by ecach ,stage of the sequencer would ordinaril} be set by using
tl`it I,()AI) puslil)utton to lt)ad the counter then STEP ing through all the stages while setting
Llic I)ilt'li ttf the itscilliitor ns desired using the sequencer's pitch controls. Sliding the
i.ui` stiip switch to RUN will produce a single run of notes at the rztte set by the RATE control.
Fling the I.iili /stop switch set to the RUN position for single sequence run (non-recirculating)
I.i :I I)(i(I ide{i ft)r uno very important reason. The clttck continues to run whether the counter
stii.Lioli is iietuall.} counting or not. So what's so wrong' with that`? Just this, as long as the
clt)c`k sectitjn is riinning it is also triggering Lhe function generator which in turn is tuning
the VCA on flmd off. For some settings of the stage pitch controls there is a residual output
\olt.`tgi` froiii the sequericer which keeps the oscillator going at a very low pitch. This is
vci.y ;tm`o,\'ing if what you actually waiit is a single run of notes and then quiet, Even if there
is no residual tone from the oscill:`tor, most VCAs make a little noise (pops, hisses, etc. )
\\'hile thev7re working. As long as there is a tone input the little operating noises are not
olijectionable but, when there is no input to mask them they stand out like a topless dancer
ill a prayer meeting,
A better choice for single sequence runs is to set the run/stop switc'h to its intermediate
"conditional run" position and then start the counter by ljoADing tlie first stage. Now the
counter will go through a single twelve note sequence and then stop -- completely, not
even trigger pulses will be generated.
Re-circulating sequences do not present this problem since they are meant to run until the
run/stop switch is set to "stop" which both clears the counter and stops the clock anyway.
Still. even re-circulating counts can be initiated by setting the R/S switch to ''conditional
run" and then "load"ing when ready for the sequence. This has some distinct advantages
when using multiple sequencers as we shall soon see.
As we saw in the unit'sverification procedure, producing a continuous sequence of notes
is simply a matter of determining how many notes you want in the sequence and then counting
down that many stages before jumpering the stage output back into the ''LOAD" input. For
example. to produce a continuing six note sequence, the sixth stage of the counter would
be jumpered back into the ''1oad" input as shown in figure 2.
Figure 2 - SEQUENCER SET FOR 6 STEP RECIRCUIATING COUNT
Even the simple connection shown in figure 1 presents a large number of variables that are
under your direct control and we heartily recommend playing for a time with this single patch
while making the count single run or re-circulate, varying the tempo from slow to fast. using
both the pulse trigger output of the sequencer and the step trigger at various settings of the
WmrH control, advancing or retarding the GLIDE. setting various pitches at each stage.
Also, don't neglect the controls that are external to the Sequencer, you've got Attack, Decay,
Sustain and Release of the Envelope Generator and a variety of oscillator waveforms to
listen to. All this playing could burn up a lot of time (you could get hooked and never want
to stop) but as you go along you'11 notice that very subtle changes in the configuration of this
patch's controls can make marked differences in the sounds that are produced.
The RUN and SYNCH inputs to the sequencer's clock are most useful with multiple sequencer
configurations but they also can be used to tie the sequencer to a keyboard. Let's look at
some examples.
Figure 3 shows the pulse output Of a keyboard comected to the sequencer's SYNCH input.
As is shown. the R/S switch in this application must be left at the STOP position to produce
the following result; as long as the counter is not LOADed, nothing happens and the VCO that
is counected to the sequencer is silent because there are no trigger pulses originating from
the sequencer. The keyboard, whose control voltage would go to a second oscillator, can
be played manually as usual. Once the counter is LOADed, the count will advanceby one
stage each time a key is pressed on the keyboard and each time the keys are pressed a
trigger pulse will be generated by the sequencer to activate its associated envelope generator
Figure `3 - SYNCI]RONIZING SEQUENCER TO KEYBOARD
:\ii(I Vl`^. [n other words, you have a pre-pi.ogrammagle harmony generatoI` th:lt will stay
in ,s\'nch with the keybo:ird and c:in be called up as requirecl by LOADing. As usual, the
•ctiu;l cnn \jc iinde to re-circulate by terminating the desired stage back into the LOAI)
iiir)ut i;ick but then the R/S switeh must be set to STOP to silence the patoh.
Thc.re are two ways that a keyboard or other manual controller can be used to initiate an
{iuLt>n`atic arpeggio using either the RUN or I.OAD inputs. In both instances the step
LrigLrcr pulsl` of the imnual controller is the best choice of trigger but pulse triggers
iilso p]`ti{]uce intercstinLr effects.
Using the slop trigger oulpul of the manual controller as an input to the RUN jack requires
lhnl the R/S switch be sat to STOP and results in a twelve step arpeggio each time a key
is prcsscd on t,he keybt]ard. In this case the arpeggio begins as soon as the key goes down.
Pel.htlps niorc useful is to have an arpeggio result from releasing a key so that the selected
note `)n the keyboard is played followed by the sequencer run. This effect can be produced
t]v sc`tting lt/S to "conditional run" and jumpering the step output of the keyboard into the
ljoAl) input of the sequencer (remember, we said earlier that the counter loads on the
LrwilinLr edge of pulses input to the I.OAD jack. (see figure 4)
Figure 4 - GENERATING ARPEGGIO AS EACH KEYBOARD KEY IS RELEASED
MULTIPLE SEQUENCERS
As with many other things, if one sequencer is neat -multiple sequencers are really terrific!
Two or more sequencers can be combined in ways that will produce long predictable sequences
or in other ways that will produce even longer more-or less random sequences.
Chaining two or more sequencers so that they act like one long sequencer is simply a matter
of jumpering` the output of one sequencer into the I.OAD input of the next as shown in figure 5 .
If you are not using synchronization between the two sequencers, then it is desirable that
both R/S switches be set to their CONDITIor\TAL RUN position. Under these conditions
the clock of each sequencel` is inactive until the counter associated with it is loaded so ea.c}i
sequencer should be triggeling its own function generator ( note that the two function generators
can be both driving the same VOA if desired). The big advantage here is that. the two sequencers
can be set to nin at entirely different rates so that the first sequencer plays (for example) at a
slow rate followed by a very rapid run when the second sequencer is activated.
If you want the two sequencers to run at the same rate you can arbitrarily designate one as the
''master" and the other(s) as slave units. In this case, the step (or pulse, whichever is not
being used to drive function generators) output of the ''master" ties back to the SYNCIl inputs
of the slave units. In this configuration, the R/S switch of the "master" unit must be set to
RUN so that its clock is working whether its counter is loaded or not - you ne:aiEose clock
pulses to drive the slave units whose R/S switch sho`ild be set to STOP. To be perfectly
in synch, the rate controls of the slave units should be set slightly faster than the rate of the
master. Retarding the slave units RATE controls will cause them to trigger on alternate
(or every third, fourth, etc. ) pulses from the "master" clock so that the two (or more)
sequencers can run at different - but still synchronized - rates.
Figure 5 - CHAINING SEQUENCERS
LOADing pulses to chained sequencers don't have to come from the last stage of either
''master" or ''slave" units and picking up a load pulse from the middle of a sequence
produces some interesting situations in which both - or all - Of the sequencers are
loaded and running for part of the cycle. This produces some particularly interesting effects
when_the sequencers are ±gi synchronized to one another.
We could go on explaining different interconnections between multiple sequencers for a good
many more pages but it would prove little because there would always be still more pages to
write. Anyway, you probably are beginning to see that. the modules can be patched together
however you like and the worst that can happen is nothing - there are no possible inter-
connections that will hurt anything. Some of the things that you might want to try are:
Connecting one of the stage outputs of one sequencer to the RUN input Of a second sequencer
so that when the selected stage of the first sequencer goes high the second sequencer produces
a rapid run.
T ry intercomecting stage outputs between synchronized or non-synchronized sequencers to
produce random - or pseudo-random sequences. When two stage outputs are tied together.
in this manner it produces the interesting effect of equivalency between the stage. anytime
one of them goes high the other also goes high. Varying Clock rates between units in this
type of situation produces interesting results.
The control voltage output Of the sequencer doesn't have to drive a VCO. It can also control
the parameters of a filter.
Melodic lines aren't the only thing that a sequencer carl generate. If you consider the module
to be a versatile function generator a number Of interesting uses come to mind such as the
possibility Of setting the pitch controls and glide controls such that the control voltage output
is an approximation of the dampled sinusoid shown in figure 6 . When this type of waveform
is capacitively coupled to one of the control voltage inputs Of a VCO it produces an interesting
decaying vibrato effect. Note that in this application the sequencer would be set up with R/S
to ''conditional run" and the manual controller's pulse output I-OADing the counter. Change
from the manual controller's pulse output to the step output and the counter won't load until
the key is released - which means that when the note is first played there will be no vibrato
but when the key is released, Bro-I-Nrd-I-N-G-I-N-G !
Sequencer generated approximation of
a damped sinusoid, produced by setting
"pitch" controls at alternating high and
low settings.
Figure 6
DESIGN ANALYSIS
C I.OCK
As is shown in the block diagraln of the sequencer (figure 7) and the schematic (figure 8),
the clock does a lot more than simply provide timing pulses for the ring counter.
At the heart of the clock is a simple relaxation oscillator built around a single Norton amplifier
stage (pins 1, 5 & 6 of IC-1) which is arranged in a Schmitt trigger configuration. Assuming
that the RUN/STOP switch is in the ''run" position, there are three major bias sources to the
inputs of this amplifier. The first is into the non-inverting input (pin 1) through resistor R18.
The second is also applied to the non-inverting input and is derived from the normally high
output of the amplifier through R16. The third current flows into the inverting input through
R34 as a result of the voltage that appears across the timing capacitor C4.
During the major portion of a clock cycle, the combined current flow into the non-inverting input
of the amplifier exceeds that which flows into the inverting input and as a result the output of the
amplifier is held very close to the positive supply voltage.
As C`4 charges through the fixed resistance R35 and the front panel rate control R135 the voltage
across it eventually becomes such that the resulting current flow through R34 exceeds the combined
current flow of R18 and R16 and at this point the output of the amplifier switch-es to a low state.
Two things happen simultaneously. The current that was flowing through R16 is removed so that
the curl`ent through R34 will have to decrease significantly before the amplifier can switch again and
diode Dl becomes forward biased and begins to discharge C4.
As C4 discharges, its voltage soon reaches the point at which the current through R34 is less than
that throug.h R18 causing the amplifier's output to once again switch high. This simultaneously
reverse biases Dl and re-establishes the feed-back current through R16. From this point the
cycle repeats.
The minimum width of the negative going clock pulses that appear at pin 5 of IC-1 is limited by
the dynamic impedance Of diode Dl and while they are relatively short (about 1% of the total
pericid at the slowest clock rate) they are far too long to use directly on the clock line. The
clt>ck pulses arc differentiated by Cl and R15 and used to s`\'itch a second aniplifier section uf
IC-i (pins 10, 11, and 12). The output of this amplifier stage is once again high ridiiig with
niicro-second range pulses to ground.
Switching the RUN/STOP switch to the "stop" position introduces a fourth bias current to the
amplifier in the relaxation oscillator by raising the ungrounded side of R30 to the positive suppl}`
voltage. The resulting current flow through R31 and R19 is such that the total current flow into
the lion-inverting input of the amplifier is much greater than the current produced by the highest
voltage that C4 can charge to. The output of the amplifier cannot switch to its low state and the
clock can be considered off .
The fourth bias current can be removed and the clock started either by returning Sl to the "run"
position or by turning on transistor Q2, thereby shorting the junction of R19 and R3l to ground.
Q2 can be turned on and held on by applying a positive voltage to the ''run" input jack on the front
panel or it can be turned on for very short periods of time for synchronization purposes by
applying pulses to the "synch" input pin jack. Pulses applied to the "synch" input are differentiated
by capacitor C3 with the negative going spikes at the trailing edges of the pulses clamped to ground
by D2.
To examine the operation of this comparator, assume that the relaxation oscillator is at the point
whet.e it is just ready to fire and produce a clock pulse. At this point the output of the comparator
is low because there is greater current flow into pin 3 than pin 2. As soon as the output of the
relaxation oscillator (pin 5) goes low the current that was flowing through R39 is removed. Without
the current contribution of R39, there is greater current flow into the non-inverting input than the
inverting input and the output of the comparator switches to its high state causing a voltage step to
appear at the step trigger output.
The output of the relaxation oscillator quickly returns to its normal high state; but at the time that it
does, the voltage appearing across C4 is considerably lower than it was at the point Of firing. Even
through the current flow through R39 is restored, there is still greater current flow into pin 2 of the
comparator than pin 3 because of the reduced cui`rent flow through R38. As the voltage across C4
increases, the point is eventually reached at which the currents through R38 and R39 exceed that
through R136 and the output of the comparator returns to its low state removing the voltage step
'0
12
Figure 8
at the output. The point at which this transition to a low state occurs is a function of how high the
voltage across C4 has to rise before the invei.ting input current exceeds the non-inverting input
current and this in turn is dependent on the setting of the front panel DURATION control.
When the output of the comparatol` switches high, the leading edge of the step is differentiated to
a pulse by C7 and appears at the front panel pulse trigger output jack. The negative spike tbat would
appear at the trailing edge of the step is clamped to ground by D3.
The step trigger also directly drives the light emitting diode LED-13 which indicates to the user the
duration of the step trigger output. In order to ease power supply regulation requirements, the
relativel.v heavy periodic current flow required to light the LED is balanced by switching the dummy
load R24 ''on" anytime that the LED is off. The inversion required to perform this is provided by
the fourth stage of the amplifier in the IC-1 package, pins 8, 9 and 13.
RING COU_\TER
The design of the ring counter section of the sequencer is a common concept employing 12 serially
coupled latches. A typical stage is shown below. Like any other latch, this circuit has two stable
states. When the (iutput voltage of the amplifier is low there is no current flow through the feed-
I)rtek resistor Rf and the greater current into the inverting input through Rb than through Rc into the
non-inverting input causes the output to stay low.
T0 NEXT
STAGE
Oniu the circuit ch:inges slate to a, high output a current begins to flo\\' thrttugh Rf whicli wheii Lidclc(I
to the current alrcad\' flowing through Rc results in n greater total flo\\' into the noii-in\'erting ini)ul
than is provided to the in`'erting iiiput thr(>ugh Rb. The result is that the outpiit sta}'s high.
Th].s st€ige can l)e "set" or "loacled" (t>utput changed to a high state) b}-in(>mentfii-il}' decreasiiig lhc
current flow Into the in\ erting iiiput. This is the case when the series capacitaJice ancl i.esistaiicc
(Ri/'Ci) are connected to the output of another lz`tch whose output Ls transitioning from a high to cl
lo\\i state. Once set, the stage can be reset by momentaril} dropping the clt)ck line from positi\ e
supply to g`round.
If the t,ime constant of Ri/Ci is chosen sucli that it is long when compared to the duration of the clocl`
pulse, a I'ing of these stages will count l)y advancing the high state front one stage to the next e:`ch tinie
there is ft clock pulse. In the process of being reset, each stage passes the count to the stage Immed-
iately following. it in the chain.
The entire chain can be cleared by holding the clock line at ground for a period of time gi`etiter than
the time constant of the Ri/Ci combination. W'hen the RUN/STOP switch is set to the "stop/clenr"
position, the series combination of R29 ancl C5 on the clock board generate a relatively long ( . 5 scc.
approximately) current pulse that turns on transistor Ql which in turn shorts the clock line to gi`oiind.
It is undesirable to have more than a single stage of the counter set at an}' given time, yet this
situation would naturally occurr if the output of an interniediate stag.e of the counter was fed biick
into the input. 'I`o prevent this, the output of stage one serves as a reset for the resL of tile st£`ges.
13
When stage one goes high, current supplied to the inverting inputs of the rest Of the stages in the
counter by way Of Rill -R121 causes each stage to reset.
The first stage of the counter can be set in a variety of ways. Pressing the front panel LOAD push
button causes a voltage to appear across R133 on the clock board which is coupled to the non- invert-
ing input of the first counter stage by means of C23 and R134. Applying a voltage step to the front
panel RUN input jack produces an identical result because of diode D5 (but note that pressing the
LOAD button reverse biases D5 and will not cause the clock to run). Setting the RUN/STOP switch
to the ''run" position cuases a voltage to appear acl.oss R62 which when coupled through C8 and
R134 produces the same results.
Taking the first as typical, the output of each of the counter stages is connected through a current
limiting resistor (R7) to a light emitting diode (LED-1). All of the LEDs in turn connect to a com-
mom line that returns to ground through the zener diode D4. These LEDs and the zener perform a
number of functions.
First, the LEDs light to indicate to the user which stage is currently loaded. Second, the LED
and zenei together serve to clamp the voltage that appears across the voltage divider at the
output of each stage (R41 in stage 1) to a reference level of approximately 6 vc>1ts and thereby
assists in eliminating voltage drift at the control voltage output of the 8equencer. Third, the
LEDs function as normal diodes to prevent interaction between the twelve counter stages.
Finally, the voltage that appears across the zener any time that any of the ring counter stages
are set serves as a signal back to the clock that the counter is loaded.
The multi-turn potentiometers at the output of each stage of the counter are resistance coupled
(Rl in stage 1) to a common line that feeds non-inverting summing amplifier IC-5. The output of
the summing amplifier is connected to the glide circuit consisting of the front panel GLIDE
control R137 and capacitor C22. The emitter follower Q4 serves as an output buffer to minimize
unwanted control voltage level changes that could otherwise be a side effect of changing the glide
rate.
In the "conditional run" (middle) setting of the clock's RUN/STOP switch, the clock runs only
when there is a ''1" loaded into the ring counter (see USING section). This is accomplished by
using the voltage that appears across the zener diode D4 when any stage of the counter is high
(see ring counter analysis) to turn on transistor Q3 on the clock board. As long as Q3 is on,
the voltage divider R21 and R20 in the clock is connected between positive Supply and ground
causing a normal bias current to flow through R18. When Q3 turns off, the voltage divider is
no longer grounded causing the junction of R21 and R20 to rise to essentially supply thereby
forcing 4 times more current to flow through R18 than previously. As explained previously,
this stops the action of the relaxation oscillator. When the RUN/STOP switch is in the "run"
position the current to hold Q3 on is supplied through RI41.
A third amplifier in IC 1 (pins 2, 3 and 4) takes care Of generating the step and pulse trigger
outputs of the sequencer. This amplifier is wired as a comparator so that its output voltage
(at pin 4) is low until the sum of the currents into the non-inverting input through R38 and R39
falls below the level of the reference current through R136.
14
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