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III. Peripheral PC Boards
In this chapter, we will consider PC boards which are not
directly involved in tone generation or processing but rather
serve as connection or interface points to the "'outside
world" (peripheral) devices such as the swell pedal, a
microphone, a tape recorder, headphones, M.I.D.I. or a
personal computer.
1. PCBoards CB24 and STl5
PC board CB 24 (Fig. 28) contains the interface circuitry for
the Musical Instrument Digital Interface (M.I.D.I.), R5232
(home computer) and the audio output. (The BETA CP has
the ST 15 instead of the CB 24). The M.I.D.I. function
allows 'the organ to exchange data with other organ models
or with other musical instruments which have M.I.D.I., a
standard interface.
The organ transmits keying and registration information via
"M.I.D.I. Out" jack, pin 5. The voltage which exists on pin 4
drives the LED in the optocoupler of the receiving device
(cf., "M.I.D.I. In" jack). The optocoupler assures complete
electrical isolation between the instruments. The received
data goes to the organ electronics via IC1 - The data from
the home computer passes to and from the organ electronics
via the RS232 connector -
On ST 15 (Fig. 29), the BETA CP's swell pedal connects to
the "Volume" jack. As explained under AF 12, the overall
volume of all the organ signals is controlled by a varying
control current from the swell pedal. Zero cur-rent means the
volume is down completely; increasing current (up to 0.6
mA) means increasing volume. The BETA CP is
transportable and the swell pedal is a separate unit. What do
you do if you have forgotten to bring the swell pedal? Well,
the ST 15 contains a missing swell pedal detection switch"
circuit that will save you from total embarrassment.
If nothing is connected to the "Volume" jack, QS is
nonconductive because its base is held virtually at emitter
potential by R22 and R24;this makes QS look like a high
resistance. In this case, R23 and D4 supply an auxilliary
current to the
1
abc line; this current goes to the AF 12 board,
maintaining the amplifiers at full volume.
If the swell pedal (PC board OS 1) is plugged in, the LED
current of the optocoupler (on O51) flows through P1, R3,
pin 4 of the jack and R24 on ST 15, causing a voltage drop
across R24 sufficient to turn on as. This biases D4 off,
stopping the auxiliary current. The swell pedal now has sole
control over the volume.
2. PC Boards MP 2 thru MP 6
These boards (Fig's 30 thru 34) contain the interfaces for
microphone, headphones and tape recorder. They differ by
organ model:
MP 2: DX 400 - Microphone amplifier
MP 3: DX 400 - Microphone connector panel
MP 4: DX 500 - Microphone amplifier and
connector panel
MP5:DX4OOCP - Microphone, tape volume
MP6:DX4OOCP - Headphone amplifier,
interface panel for tape
recorder and microphone
3. PC Boards TS Sand PA 10
The TS 5 triac switch and the PA 10 power amplifier are
both in the power chassis LE 30. The LE 30 also contains
the power transformer, which, along with the line voltage
components and wiring, is enclosed in a protective metal
cage.
a) Triac Switch (Fig. 35)
The line voltage c9mes in via the AC panel plug and goes
first through a noise/static suppressor. It then goes through
one of the two fuses, depending upon the line voltage being
wed, through the transformer primary winding and through
the triac switch.
The triac switch TS 5 is comparable to a relay: a low-level,
non-dangerous voltage is used to activate a relay, which then
switches through the high line voltage. In this way, the
operator is not exposed to the line voltage. In this case, triac
BT139 functions as an electronic relay. Its trigger voltage
comes from winding W2 of transformer NT1 -
Since NTI is permanently connected across the AC line via
R1, the triac is triggered on each half cycle of line voltage,
so that the circuit between pins 3 and 8(7) of PC board TS5
is continuously switched through. However, if winding W3
of NT1 (only a minute voltage is present here) is shorted out
by a closure across pins 9 and 10 of TS 5, there is no longer
any voltage on any of NT1's windings and the triac cannot
be triggered on. It can only be switched through again when
the short across W3---present when the organ's AC line
switch is closed-is removed. Hence the AC line switch
works just the reverse of the way a power switch normally
works: when it's closed, the organ is off; when it's open, the
organ is on!
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