Marchand Electronics mb401 Manual Download

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Note

: The orientation of caps C11 and C12 is not

shown on the circuit board. They should be oriented
the same way as C10 and C16.

Diodes

: Diodes are installed in the usual way. Make

sure to observe polarity: the band indicated on the
circuit board must coincide with the band on the
device. The band indicates the cathode.

Transistors

: Transistors Q1…Q9, Q14, Q15 are the

small black parts with the three leads. Note that the
black part has a big flat side and a round side. Note
that Q1 and Q2 come as a matched pair in a separate
bag. Make sure to use these in the locations Q1 and
Q2. Install the transistors according to the marking on
the circuit board.
Power transistors Q9 and Q11 are mounted with a
heat sink. Q8 and Q10 need no heatsink.
Power transistors Q16 through Q18 are installed onto
the heat-sink bracket.
Apply a thin uniform layer of the white silicone
compound on the bottom of each transistor. Install the
transistors with the 4/40 screws, two split lockwashers,
flat washer and nut. See Figure 8. Orient the screws
so that the head of the screw is on the solder side and
the nut is on the component side. Solder the three
transistor pins only after all the mounting screws have
been tightened.

Header

: Install the two 1-pin headers at locations TP1

and TP2. These are the testpoint that are used when
adjusting bias.

Terminal Blocks

: Install the two small 3-pin and large

3-pin and 4-pin terminal blocks at the edge of the
circuit board.

TO220
MOSFET

Nut

Circuit board

Heat sink bracket

4/40 x 1/2”

Split lockwasher

Flat washer

Heatsink compound

Figure 8

Mounting of power transistors.

Assembly is now complete. Take a few minutes to
check all components and orientations. Also make
sure there are no solder bridges.

Bias current adjustment (Class AB) .

The bias current of the amplifier must be adjusted by
setting the potentiometer R22. First turn the
potentiometer fully counterclockwise. This will set the
bias current to zero. Hook the MB401 to a bipolar
power supply. The supply voltage should be between
+/- 30V and +/- 60 V. For doing this step it is best to
use a 30V supply. This lower voltage will reduce
chance of damage to the parts if there is an error in
the installation of the parts. It is best to mount the
MB401 onto a large heatsink during testing.
Connect a DVM or suitable voltmeter between the
testpoint TP1 and TP2. If the DMM indicates a voltage
of more than a few mV turn the power off

immediately

and check all parts placements. A very

safe way to do this step is to use a variac to increase
the power supply voltage slowly from zero to about
30V, while observing the DMM. Now slowly adjust R33
clockwise until a reading of 15 mV is shown on the
DMM. Precise adjustment is difficult. But a value
between 10 mV and 20 mV is acceptable. Note that
the unit will start heating up a little. The adjustment
should be made when cold. When the amplifier is hot,
the bias current will change a little. This is normal.

Offset adjustment.

The offset voltage of the amplifier must be adjusted by
setting the potentiometer R10. With no signal applied
to the inputs, adjust R10 for minimum DC voltage at
the outputs. A residual output voltage of a few mV is
normal.

The assembly and adjustment of MB401 circuit board
is now complete.

Power supply.

The power supply used is the PS112. It has two
separate sets of outputs, one for the preamp part of
the amplifier board and one set for the output stage.
The preamp voltage is about 20V higher than the
voltage for the output stage.

Output voltages of the PS112

(with 55V transformer)

V+

+80V

V-

-80V

V++

+100V

V- -

-100V