Setup; Power And Limiter Test; Input Set-Up And System Gain Test - Gallien-Krueger MB150S Service Manual

MB150S Preamp Turn-On Procedure
GK Document # 420-0061-A / Preamp Board #206-0061-C
Model- MB150S- All Options

SETUP

1) Power switch off- connect power cord.
2) Connect "power" 3-pin connector to P1 on MB-S board so that it clicks into position.
Colors should read (L-R)- black, violet, brown. (P1 is furthest from front of panel.)
3) Connect "signal" 3-pin connector to P2 on MB-S board. Colors should read (L-R)- white, black and
red. (P2 is closer to front of panel.)
4) Connect either output to Load Box (Load A).
5) Resistance loads open (switch in center).
6) Load Box to scope-B and AC-VM.
7) Set scope switch on Load Box to look at Load A(down).
8) Probe (1:1) to scope-A and DVM. – No Gnd lead required.
9) Connect oscillator to input.
10) Set oscillator on 100Hz sine wave @ 0.5 Vrms (-6dBV).
11) DVM on 20V range.
12) AC voltmeter on 30V range.
13) Scope-A on 5V/cm, scope-B on 10V/cm.
14) Scope sweep on 2ms/cm, scope trigger on A. On MB-S Preamp set:
15) Output level to 0, switches out.
16) Contour, hi-boost to 0.
17) Volume, tones and limiter level to 10.

POWER AND LIMITER TEST

1) Turn on power switch.
2) Power LED on front panel should be lit.
3) Look at output- should be a clean sine wave w/ no crossover distortion, and be about 1.2Vp-p.
4) Flip scope switch to look at Load B (up) and see it is also 1.2Vp-p w/ no crossover distortion.
5) For Combo units: Switch internal speaker on, make sure disconnected. Switch off.
6) Change scope to 10V/cm scale.
7) Increase oscillator output to 160mVrms (-16dBV). Output = 14Vrms.
8) Engage limiter. Output falls to 11Vrms.
9) Decrease limiter level to 0. Output falls to 7.5Vrms.
10) Increase output level to 10. Output goes to 10Vrms.
11) Disengage limiter. Output hits rails hard.
12) Switch both loads to 4 ohms- signal should clip on top and bottom and then shut off after a few seconds. (May have to add 8.2 ohms
resistance).
13) Remove loads (switches to center) and set output level to 12 o'clock.
14) Turn unit off and then on again. Note how long it takes before output reappears on scope- should be no more than 2-3 seconds.

INPUT SET-UP AND SYSTEM GAIN TEST

1) Turn on power switch- power LED only should light.
2) Look at U1-U7 with probe. Adjust R13 (1K trim pot) so the signal hits the rails evenly on both sides. Signal should be about 22 V p-p.
3) Engage –10dB switch. Signal becomes 1.7Vrms.
4) Direct Out pins 2 & 3= 1.7Vrms. Disengage –10dB switch.
5) Oscillator to –46dBV (500mVrms).
6) Scope trigger to B.
7) Increase output level. Output should hit rails at about 3-4 o'clock.
8) Engage limiter. Limiter LED should light ( output raises slightly).
Revised 7/26/99