Open Circuit Test (contd.)
(-6dB). See Fig. 3 for a distorted signal. (See note regarding waveforms above. –Ed.) For a power amp (ML/P), check that turning down the
rear panel pot reduces the output to zero on each channel. Return the pot to fully on. Set the oscillator to be a –6dB, 1kHz signal. With both
loads set to 8 ohms, adjust R688A so that the output voltage from the left jack (R Channel) is 14.5Vrms, as indicated by the VOM. The output
should be a clean sine wave, as shown in Fig. 4 (see note above). The line current should be about 1.2A. Change the input signal to a square
wave and compare output with Fig.5. For a stereo unit, adjust R697 to obtain the same sine wave rms output voltage for the right output jack
(L Channel). Change the input to a square wave and check the output. For a mono unit, check that the output jumpers are connected by
viewing the signal on the right output jack, which should be the same as the left jack.
Resonance Check
With the loads at 8 ohms and a sine wave input, raise the input signal to 0dB and note that line current is 2A, which should send the output into
the rails. View the cathode of D537 with the 100MHz scope as mentioned above (Scope Setup). The S-curve due to resonant current should
stop about 0.5usec (deadtime) before the waveform rises, as shown in Fig.6. If this is not the case, the period must be readjusted (see Adjust
Period). The condition of not enough deadtime may be the result of a transformer core having too high a relative permeability, which makes
the magnetizing inductance too high. Therefore, the magnetizing current will be less, which will increase the ring-up time. In this case, add
more capacitance. If there is too much deadtime, decrease the period by decreasing capacitance, making sure deadtime is still at a safe level.
This will increase the operating frequency to a value closer to the transformer deign goal. Once the period is adjusted with respect to the
resonance, adjust the droop to 10V above zero. Securely glue the pot (R508).
Thermal Lift
With both sides connected to 8 ohms, set the input signal to be a –6dB, 1kHz sine wave. While observing the left output signal, short JU789
from the left outside pin to the middle with a screwdriver. After a short delay the output should go to zero. For a stereo unit, cycle the power
switch, waiting 2 seconds before turning back on. Repeat the above procedure for the right side output, this time shorting the right side of
JU789 to the center pin.
Overload
NOTE: in the following sections the amp will automatically shut off. To reset, the power switch must be turned off for 2 seconds before the
unit can again produce output when turned on.
Stereo- With the input set to 0dB, set both loads to 4 ohms. The amp should not trip off. Switch in the 8.2ohm load on the left side. The
automatic shutoff circuit should force the output to go to zero after a short delay (a few seconds max.). If this does not happen, R777 must be
adjusted. This component must be decreased in value (from 470 ohms) by paralleling it with either a 2.2k or a 1k resistor. If the unit still stays
on, R777 must be changed to a lower value. Settle on the value that trips first. Repeat the shutoff test on the right side, switching in the
8.2ohm load and adjusting R761 if necessary. Remove the 8.2 ohm load and switch the load box to 8 ohm on either side.
Mono-Remove the right jack from the unit and plug it into the left side of the load box, paralleling the loads. With a 0dB sine wave input and
both loads set to 4 ohms, the unit should not trip off. Switch in the 8.2ohm load. The output should go to zero after a short delay. If this does
not happen, R777 should be adjusted. This may be adjusted by paralleling a 2.2k or a 1k resistor, as noted in the above section. If this doesn’t
work, change R777 to a lower value, settling on the value that trips first. Switch loads back to 8 ohms and connect each output jack to a load.
