Test Procedures - Adcom GFA-565 Service Manual

R158 and Cl 11 provide a load for the amplifier at high frequencies, stabilizing trie amplifier under varying load
conditions. D201 and 0301 provide a high-current return to the power supply lor backlash current from the load.
The output stage consists of two sets of 10 parallel transistors operated as emitter followers, driven by another pair of
emitter followers. This configuration minimizes distortion caused by varying load impedances. The output transistors
have 0.33-Ohm ballast resistors for current sharing and bias stability.
The AC input power supply board includes a power-on delay relay to reduce the turn-on current surge in the AC power
line, jumpers to set the supply voltage range, artd an amplifier bias delay.
CAUTION
DO NOT use any type of variable ACsupply, such as a variac. to slowly turnon the GFA-565 without first shorting
R506 (4.7 ohm/20 watt). R506 is an integral part of the turn-on, inrush-suppression circuit and failure to comply
will burn out n506.
TEST PROCEDURES
All tests are performed with a 120V, low-distortion (less than 2% THO), AC-power source, 8-ohm resistive load (except
slew rate), and a signal source of not more than 600 ohms.
Tests are performed after warming up the amplifier at 100 watts into an 8-ohm load for at least 10 minutes.
All grounds during testing are referred to the ground of the black output terminal.
80kHz low-pass tiller is employed during THO distortion measurements.
Signal-to-noise measurements are "A" weighted.
Damping factor Is measured by comparing the 20-watt-output voltage with and without an 8-ohm load.
Slew rate is measured with an inductive load, and is derived with a dual-time-based oscilloscope reading the slope of a
full-power (135V peak-to-peak) 5kHz square wave.
To
avoid damaging output network R158 and C121.
DO NOT
OPERATE THE AMPLIFIER AT FULL-POWER SINE WAVE ABOVE 22kHz OR FULL-POWER (135V PEAK-TO-
PEAK) SQUARE WAVE ABOVE SkHz.
IMPORTANT
BEFORE PROCEEDING WITH ADJUSTMENTS. MAKE SURE AMPLIFIER IS AT ROOM TEMPERATURE.
BIAS ALIGNMENT
1. With set-up as per the first paragraph of TEST PROCEDURES and with NO SIGNAL IN, set bias control (R143) to
midpoint.
2. Connect a millivolt meter across TP201 and TP301.
3. Turn amplifier on and allow a 3 to 5 minute settling period.
4. Adjust BIAS control to obtain either a
+
or— 24mV (±1 mV)
indication on the millivolt meter.
5. To check for proper bias setting, remove millivolt meter and apply input signal to obtain 100 watts into 8 ohms for 10
minutes with cover on.
6. Remove input signal and connect the millivolt meter as in Step 2. Let amplifier idle until bias stabilizes and readjust to
24mV(±1mV).
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