Luxman 5 M 20 5 M 21 Service Manual

Page 1

Exam?" 5M21/5M20
SERVICE MANUAL

Page 2

SMZI/SMZO Alignment Procedure

The

51421 and the SMZO are of DC amplifier configuration unlike conventional AC amplifiers.

Therefore, the following items should be noted in case of the alignment procedure. Don't think

the same

(1)

(2)

(3)

(4)

(5]

[6)

Position

(1)
(2]

(3]

(4)

Position

(1)
(Z)
(3)

procedure as that for the AC amplifier can be followed.

To apply AC mains power source with loads connected will cause blow-off of power
transistors if connections or wirings are imperfectly soldered. Therefore, check all
the wirings to printed circuit boards before turning on the AC mains power.

This power amplifier amplifies even DC. Slight DC ingredients are often present in
old testing equipments or in DC amplified ones. In case such DC voltage is fed to the
amplifier, the DC is amplified enough to give damage to the amplifier. Therefore, set
the "INPUT CAPACITOR" switch provided at the back panel to the "IN" position before
operating the amplifier.

:GOV - t7OV DC voltage is applied at the final stage amplifier board, the driver stage
board and the constant voltage board. Especially at the "B" power supply, giant elect-
rolytic capacitors are employed, and it takes a long time to finish discharging. When
you proceed to disconnect printed circuit board, do it after perfect discharge by use

of a resistor or an electric bulb.

The power transistors' section at the final stage is driven in class "AB" operation,
which is different from our conventional amplifiers driven purely in class "5"
operation. Therefore, check the idling current as soon as possible after turning the

power switch on, to protect power transistors from breakage by excessive current.

Our original FET hybrid I.C. (Dual Monolithic Linear LC.) is used in the pre-driver
stage. This I.C. contains differential stage, constant current circuit and cascode
circuit. Check if the pins of the LC. are correctly and perfectly connected before

turning the AC mains power on.

The output terminals for Peak Indicator are provided in this amplifier. These are
taken from the Speaker Terminals through lk ohm resistor, therefore it might be possible
that the resistor burns out or the amplifier itself is damaged, in case short-pins etc.
are mistakenly inserted to these terminals to cause short-circuit. Note that these are

not input terminals.

of knobs before alignment

Attenuator Volume: MIN.

Speaker Terminals: Connect oscilloscope and milivoltmeter. Do not connect
loads.

AC Mains Switch, Meter

Sensitivity Selector Switch: OFF
Input Capacitor Switch: 0N

of knobs at the time of delivery

Attenuator Volume: MAX.
Input Capacitor Switch: ON
AC Mains Switch, Meter

Sensitivity Selector Switch: OFF