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MARSHALL SERVICE BULLETIN
 
SUBJECT: Bias Info/Procedure for JCM2000 series Marshall amplifiers
 
THEORY OF OPERATION: We are setting the bias using the current method instead of cross over distortion method. The molex connector CON2 pin out as follows:
PR1: Bias mini-pot adjustment for pin1 and for two output tubes(100w) or one output tube(50w) or one side of the push/pull amp.
Pin1: Connected to the cathode of the output tube which then goes through a 1 ohm resistor then to ground.
Pin2: ground reference
Pin3: Same as pin1 but for the other side of the push/pull amp.
PR2: Bias mini-pot adjustment for pin3 and other side of the push/pull amp
 
PROCEDURE:
Make sure amplifier is connected to a load with the proper impedance selected.
Power up amplifier on STANDBY and let the circuit stabilize for a couple of minutes.
 
Locate the male three pin molex connector (CON2) with the two mini-pots (PR1 and PR2) on
both ends found on the bottom of the tube bay.
 
Connect DMM (set to read mV) with alligator leads, reference common lead to center pin (pin2)
on molex connector CON2 and positive lead to pin1 on CON2.
 
Take amplifier off of STANDBY with no signal, adjust mini-pot (PR1) closest to pin that the positive lead from
your DMM is connected to and set it to the mV voltage that is listed in the chart below.
Repeat steps 4 & 5 for pin3 and until both pin1 and pin3 mV are the same.
 
DSL50 45mV TSL60/1/2 80mV
DSL100 90mV DSL401 1.375V
TSL100/122 90mV DSL201 .675V
 
EDIT:
 
Ja tässä vielä rautalankaa
 
I just checked mine - use the first 'V' next to your off position (The V with the line and the dots in black) ---
_____________ ...
For the leads use the black - Common, red to the middle jack
 
Ok for the biasing - simply connect the black lead to the middle pin and the red lead to the outer pin & adjust the bias via the trim pot on that side.