Alto SR500A, Service Manual

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Instruments required:
1) Sinusoidal signal generator, 5) Resistive load 4
Ω
800W,
2) Dual trace scope, with probe, 6) Resistive load 8
Ω 30
0W
3) Digital multimeter, 7) Pink noise from the GM “AUDIO CD”.
4) Variac 0-250 Vac,
P.S: Isolate the oscilloscope from earth and adjust the two traces on center of screen in GND
position.
Par. 1: visual check
- Make a visual check if the amplifier is perfectly done and especially verify:
a) the right value of fuses 8A 250V *4PCS
b) the right earth main connection on rear surface of front panel.
c) the insulation between the heat-sink and the case of TR1-15, IC1, 2, 3, 4 -then,
- Set the VOLUME control full clockwise.
Par. 2: connections
- Switch off the amplifier module,
- Set the ENCODER pot. on preset “NORMAL”.
- Connect the mains of amplifier module to VARIAC and set it at zero AC voltage.
- Connect the signal generator to Line Input (XLR socket) and set it at 1000Hz /-1.5dBu,.
- Connect the scope CH1 probe to amplifier low output and CH2 probe to amplifier high output-
Par. 3: Initial test
- Increase slowly the VARIAC voltage up to the nominal value (230Vac).
- Check the transformer sec. voltages with multimeter, (without signal on input):
F3-F4: 54V ±1.5Vac
F1-F2: 106V ±2Vac
- Check the +/-15Vdc supply on relative voltage regulator outputs (IC3 & IC4 devices).
- Switch ON the signal generator and verify that the output signals must be symmetrical referring
to GND, without detectable distortion and oscillation.
- Check that the low amplifier power output signal is 35V
RMS.
- -Check of SIGNAL led function, moving the volume potentiometer of amplifier;
when the output signal is <0.5V
peak
the led must be OFF,
when the output signal is > 0.5V
peak
, the led must be GREEN lighted.
-Then, rotate the volume potentiometer knob full clockwise
Par. 4: BOOSTER check of LOW Amplifier
- Connect alternatively the CH2 probe tip of scope on D2 cathode and D3 anode and
verify the following conditions:
when the output signal is < 18Vrms the ±VCC1 voltage must remain linear at 35V,
when the output signal is > 18Vrms the ±VCC1 voltage must follow the output signal wave with
6Vdc offset, as shown in the following picture
05-1