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NOTE: Italics refer to stereo power units.
SETUP
Power switch off- Connect power cord.
1) Connect power 3-pin connector from power amp to P1 on board. Colors should read (L-R): Black, Violet, Brown. P1 is furthest to the
back.
2) Connect signal 3-pin connector from power amp to P2. These colors should read (L-R): White, Black, Red. P2 is furthest the front.
3) Connect outputs to load box- Left to Load A, Right to Load B.
4) Resistance loads open (switch in center).
5) Load box to scope-B and AC-VM.
6) Set scope switch on load box to look at load A (down).
7) Probe (1:1) to scope-A and DVM. (No gnd lead required).
8) Connect oscillator to unit.
9) Set oscillator on 100Hz sine wave @ 1.6Vrms (+4dBV).
10) DVM on 20V DC range.
11) AC Voltmeter on 30V range.
12) Scope-A on 5V/cm, scope-B on 10V/cm.
13) Scope sweep on 1ms/cm, scope trigger on A.
14) Output level to 0, switches out.
15) Boost, shape, gain, lead master to 0.
16) Clean volume, tones to 10.
INPUT SETUP AND LEAD CHANNEL TEST
1) Turn on power switch- clean channel LED only should light.
2) Engage channel switch- lead channel LED only should light.
3) Look at U1/1 with probe. Adjust R5 so that signal hits the rails evenly on both sides. Signal should be about 22Vp-p.
4) Look at drain of Q48 (square pad-J113): +7.3VDC.
5) Adjust R63 (1k trim) for 4.0-4.5VDC on drain of Q59 (J113).
6) Adjust R86 (1k trim) for 5.8-6.2VDC on drain of Q69 (J113).
7) Increase gain to 10 and watch the signal become a square wave- it should clip fairly evenly on both sides and reach 9Vp-p.
8) Decrease oscillator level to –66dBV (.5mV).
9) Change DVM to AC (20V) scale.
10) Adjust R70 (50k trim) for 2VAC at drain of Q69.
CLEAN CHANNEL TEST
1) Change scope trigger to B.
2) Disengage channel switch so that clean channel LED lights.
3) Set oscillator to –46dBV (5mV) in at 100Hz.
4) Look at tip of Send jack (J2) with probe: 0.47Vrms.
5) Tip and ring of Line Out jack (J4) should be 0.92Vrms.
6) With output level at 0, there should be no output. Gradually increase output and it should hit the rails at about 1o'clock and =30Vrms.
7) Flip scope switch to look at load B (up) and see that it also is about 30Vrms, and hits the rails at about 1 o'clock.
8) Look at tip of Headphone Output jack (J5) and continue increasing the output level. Headphone output should hit the rails at about 4
o'clock (it actually may not hit the rails even when all the way up). It should be about 4.4Vrms.
9) Repeat 8) looking at ring of headphone jack.
TONE CONTROLS TEST
1) Set all large knobs to center (12 o'clock), switches out.
2) Set scope-B to 2V/cm @ 1ms/cm.
3) Set oscillator to 100Hz square wave at –46dBV.
4) Look at output, compare to Fig. 1.
5) One at a time, turn tone control knobs and compare outputs, resetting after finishing:
A) Treble to 10- Fig.2; treble to 0- Fig.3.
Microamplifier Series-Lead
Standard Preamp Turn-On Procedure
GK Document #420-0073-A
Preamp Board # 206-0073-D- ML/S board
Model #'s- ML120S, ML260S-All Options
5/18/90 Rev. 9/15/99-SW
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