Dynaco QSA 300 Service Manual

Page 1

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comcrning (his equipment.

INSTRUCTIONS FOR
ASSEMBLY
OPERATION

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Page 9

19( )Select the four transistors, Part #561357 (2N6029 or
2N6030) , four metal insulating wafers, four transistor
sockets, the thermal compound, and eight of the #6
sheet metal screws (coarse threads with slightly
tapered ends). Do not confuse these transistors with
the four 571105 transistors, which look the same, but
will be mounted in the next step. First check to see
that the transistor pins are straight. These four tran-
sistors will be mounted from the outside at the BOT-
TOM of the four heat fins in positions QZA and Q2B.
Apply a thin film of compound to both sides of the
insulator, position it correctly, and install it over the
transistor pins. Wipe the pins clean, and plug each
assembly through the heat fin into the transistor
socket. Secure each with two sheet metal screws.

20( )Select the four transistors, Part #571105 (2N 5629 or
2N5630), the other four insulating wafers, the four
remaining sockets, and the last eight 3/4 sheet metal
screws. Check for straight transistor pins. Apply a
thin film of compound to the insulating wafers as
before, wipe the pins clean, install these from the
outside at the TOP of the heat fins in positions Q1A
and QlB, and snap them into the sockets. Secure
with the sheet metal screws.

This completes the need for the compound. Wipe off all
the excess, including the threads of the transistor mounting
screws. to avoid smears as you complete construction.

21( )Select the four perforated black transistor covers.
Their installation requires removal of the two sets
of #6 hardware used to attach the tops of the heat
fins to the back panel, and loosening the two sets of
hardware used to attach the bottoms of the heat fins.
Slip one end of a cover-they are made symmetri-
cally-under the hardware at the bottom, and re-
install the hardware at the top. Tighten all sixteen
screws very firmly.

WIRING THE HEAT SINK AND PC-43

Be particularly careful handling the preassembled audio
circuit boards, for they represent a substantial portion of
the cost of the QSA-300. Many of the components stand
upright on the board, and careless handling can unduly
flex and possibly break their leads. There are also four ad-
justable potentiometers on each circuit board whose values
(positions) have been precisely set under operational test
conditions for optimum performance. Be very careful that
none of these is disturbed when handling the boards.

In the following steps, two sets of brackets for checking
off, one below the other, are indicated: ( ) This means
( ).
that the same instruction is to be completed twice, first for
the right side of the assembly, and second for the left, before
continuing to the next step. The wire lengths, directions,
and identifying numbers are identical for both sides.

1( )Select the PC-43 audio circuit board, the two

( )Z-shaped metal mounting brackets, and four sets of

#6 hardware. Install the brackets on the [oil side

(not the components side) of the board, with the

long side against the board, and the first bend at

the row of holes at the bottom. Insert the screws

first through the board from the components side,

then through the bracket, with each nut against the
bracket. Firmly tighten the hardware.

2( )Place the back panel assembly in front of you, so

( )that it lies on its heat fins, and positioned so that

the top is toward you. Place PC-43, foil side up.

against the bottom of the back panel, so that holes

#12 and #15 on the board are adjacent to input

socket lugs #4 and #1, respectively. Support the

board about 2% off the work surface on a box, such

as a tissue box, so that the row of holes on P043 is
adjacent to the bottom flange of the back panel.

In succeeding steps connections will be made to plated
through holes on PC-43. A good connection is more certain
if the tip of the wire is first tinned by heating it and
applying a very small amount of solder before it is con-
nected to the hole. Insert the wire in the hole so that bare
wire is visible on both sides of the board. Let the soldering
iron contact the junction of wire and board circuitry as you
feed solder to the junction. Solder should flow smoothly
from the circuitry, around the hole to completely surround
the wire. Keep the wire steady while the connection cools,
and then wiggle it to make sure the connection is secure.
If in doubt, reheat the connection and add more solder.

Do not add solder to the holes before a wire is inserted.
If you do, it will be difficult to find the hole to clear it.
However, should you solder a wire in error to a wrong hole,
first remove the wire by using the iron, and while the solder
is still hot, push a toothpick through the hole to clear it.

The following connections will be made to the foil side of
the board.

3( )Prepare a 2% (7 cm) black wire, but remove 3/3

( )(1 cm) of insulation from one end and the conven-

tional 1/4 (.6 cm) of insulation from the other. Pass

the longer prepared end through input socket lug #3

to lug #2, and solder both lugs. Connect the short-
prepared end to hole #13 on the board (S).

4( )Prepare a 2 (5 cm) yellow wire, by removing the

( )conventional IA (.6 cm) of insulation from both

ends. Connect one end to hole #4 (S). Connect the
other end to speaker fuse holder lug #3 (S).

5( )Prepare another 2 (5 cm) yellow wire. Connect one
( )end to hole #23 (S). Connect the other end to
speaker fuse holder lug #2 (S).

These wires will have stabilized the position of PC-43
relative to the back panel assembly.

6( )Prepare a 2% (5.5 cm) blue wire. Connect one end
( )to hole #12 (S). Connect the other end to input
socket lug M (S).

7 ( )Prepare a 2% (5.5 cm) yellow wire. Connect one
( )end to hole #15 (S). Connect the other end to input
sockct lug #1 (S).

8( )Prepare a 13/4 (4.5 cm) blue wire. Connect one end
( )to speaker fuse holder lug #4. Connect the other
end to red binding post lug #4.

9( )Select a .27 ohm, 5 watt resistor, and cut its leads
( )to % (1.6 cm) each. Do not confuse it with the
.33 ohm resistors, which look the same. Connect one
lead to speaker fuse holder lug #4 (S-2). Connect

the other lead to red binding post lug #4 (S-2).

9

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Page 14

CIRCUIT DESCRIPTION

The components in the QSA-300 have been selected to
protect against failure, and all parts are operated conserva-
tively to assure unaltered performance and proper operation
for many years. The semiconductors have been thoroughly
researched and selected for minimum noise and distortion
in sustained use, and the printed circuit boards associated
with audio signals have been pretested in the circuit to insure
that this unit will meet or exceed all specifications.

The amplifier stages are fully DC coupled, with a differ-
ential pair for the input and a fully complementary output
stage. Each power supply has a full wave bridge with
10,000 mfd capacitors on both the plus + and minus -
supplies to provide excellent filtering and dynamic load
stability. Protection circuitry includes two separate power
supply fuses for each channel, volt-amp limiting, individual
thermal breakers for each two channels, output fuses to pro-
tect the amplifier and speakers, and an AC line fuse.

DIFFERENTIAL AMPLIFIER

Transistors Q101 and Q102, the input differential pair,
are driven from a constant current source, transistor Q103,
to optimize parameters for the input pair. Potentiometer
P101 adjusts the reference voltage for the base of Q10],-
which in turn balances transistors Q101 and Q102 to control
the DC offset of the amplifier.

BIAS ADJUSTMENT

An adjustable electronic equivalent of a zener diode with
negligible dynamic resistance is formed by transistors Q106
and Q107, resistors R111, R112 and R113, and poten-

tiometer P102. Thus the signal sees the bases of transistors
Q110 and Q111 tied together. Potentiometer P102 adjusts
the zener voltage to bias the amplifier for Class AB opera-
tion, and therefore determines the quiescent (idle) current
in the driver and output stages. Complete circuit stability is
assured by mounting transistor Q106 to thermally track the
driver and output transistors, and to provide temperature
compensation for the amplifier.

DRIVER AND OUTPUT STAGES

The driver and ouput stages of the QSA-300 are of the
full complementary configuration. Transistors Q110 and
Q201 form a Darlington pair for the positive half of the
signal, and transistors Qlll and Q202 form another Darling-
ton pair for the negative half of the signal.

PROTECTION

A volt-amp limiter circuit protects both the positive and
negative halves of the driver and output stages. When the
current through resistor R201 exceeds a preset limit, its
resulting voltage drop turns on transistor Q108, and limits
the drive to transistor Q110 until the current is reduced for
the positive half of the circuit. The same operation takes
place to protect the negative amplifier circuit through re-
sistor R202 and transistor Q109, and limits the drive to
transistor Q111. Diodes D201 and D202 across the output
devices limit possible reverse voltage from inductive loads
to 700 millivolts maximum. If the heat sink temperature of a
pair of channels rises to 75° C, a thermal breaker will
shut down that power supply until that section cools down.

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