Adjust Reset
Caution: The following procedure calls for increasing the variac from 25Vac to 117Vac in less than 5 seconds. The startup circuit is designed
as such that Q518 turns on when line voltage is below 90V. This causes it to dissipate a large amount of power. Q518 will fail after about 10
seconds if line is set at 50Vac. During normal turn-on, when power is switched with 117Vac applied to the line, the transistor will dissipate
for only a few milliseconds. Set the variac to zero. Connect the power cord and turn power on. Set the voltmeter to the 100V scale. With an
insulating adjuster inserted into R508, slowly raise the variac to 25V. The line current should be less than 200mA. Compare the waveform
with Fig. 1 (As of 9/99- picture available only as poor copy, see accompanying wave form pages- Ed.). The top part of the waveform should
be starting to break over and droop towards the centerline. Adjust R508 until the scope looks like the photo. Counterclockwise increases
droop.
Full Line Voltage
Bring the variac up to 117Vac within 5 seconds, making sure that droop increases with voltage. During this period, the supplies will turn on
and the presence of current may be noticed in the bottom of the waveform. Obtain the droop as shown in Fig. 2 (see above note regarding
waveform pictures-Ed.) by adjusting R508 so that it stops 10V (1/2 div) below centerline. NOTE: If there is a problem with shorted supplies, it
can be detected by the shape at the bottom of this waveform as the variac is increased. It will show as excessive ringing and may look similar
to the bottom of the waveform of Fig. 6. If this occurs, do not increase the variac to 117Vac and refer to the Service Manual.
Adjust Period
At this stage the period is set such that the bottom flat part of the waveform is 4usec/ wide, +or-0.2usec. This may cause the period to be about
10.5usec, which can be adjusted by connecting or omitting C503, 504, & 505 with the shorting bock JU502. As a shorting clip is added from
the board edge towards the interior,, the capacitance added to the oscillator increases, and the period will increase. Always turn the unit off
when adding or removing clips. Adjust the reset as above as necessary. The waveform should now be similar to Fig. 2. The period and droop
will be finalized below. Leave the unit on.
IMPORTANT! – Only a polyester or polypropylene capacitor may be used for the oscillator, which is used for temperature stability. If it
is necessary, change one of the caps to get a waveform like Fig. 2. However, the caps provided will usually be sufficient.
Power Amp Start-Up
Preparation
Check that the emitter jumpers connect the transistors to the outputs. P612A, 655A and 683A should be toward the rear panel and P602A
should be towards the edge of the board. Connect the loads and set their toggle switches toward the middle, open circuiting them. Set the rear
panel pot of a power amp (ML/P) to be fully on. Set the 35MHz scope to 10/div. @ 200usec/div.
Continuity Test
With the DVM check that there is a short circuit between the grounding screw and the rear panel. The grounding screw is between Q824 and
Q808, and its washer must be touching the board. If an open circuit is measured, check that the screw hole in the heat sink is not anodized. It
is crucial that this connection is made.
Adjust Bias
With the loads open and the DVM on the 200mV DC scale, adjust R652 so that 8.5mV (+ or – 0.5mV) appears between the emitters of Q808
and Q824. (The emitter is the leftmost pin on TIP33 and TIP34). This voltage will increase as the unit warms up, and 8.5mVis recommended
when the unit is cool. For a stereo amp, repeat this procedure with R711, Q759 and Q783. For a mono amp, check above Q’s to see if base
connections have been made. Usually, a smaller voltage will appear between the left channel emitters of a mono power amp.
Load Tests
Open Circuit Test
Set the voltmeter to 30V full scale. Set the 35MHz scope to 10 div @ 200usec/div. Connect a 1kHz signal from the oscillator into the Stereo
Line Out (stereo), Right Line Out (mono), or Line In (power amp ML/P). This jack is next to the output jack on the right. Use a mono-to-
stereo adapter from the oscillator. With the loads open circuited, look at the output signal on each output jack with the 35MHz scope. The
waveform should look sinusoidal at low input levels (-16dB) and distorted at high input levels
