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:
  1. Make sure amplifier is connected to a load with the proper impedance selected.
  2. Power up amplifier on STANDBY and let the circuit stabilize for a couple of minutes.
  3. 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.
  4. 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.
  5. 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.
  6. Repeat steps 4 & 5 for pin3 and until both pin1 and pin3 mV are the same.
BIAS CHART mV SETTINGS
(pin1 and pin3)
DSL201: 675mV
DSL401: 1.375V
DSL50: 45mV
DSL100: 90mV
TSL60/1/2: 80mV
TSL100/122:

90mV

VM 2466:

74mV - 80mV

Basic Trouble Shooting Tips: The following routines can be used for basic amp trouble shooting:
  1. To check out if the problem is in the pre-amp section:
    • Connect the send jack of the effects loop to a line-level capable device (poweramp, recorder, etc.)
    • If the line-level capable device does not get a signal from the amp, then the problem is likely in the pre-amp section
    Possible causes:
    • Bad input jack Soldering
    • Electrolytic cap has dried out
    • Other soldering problems
    • Bad pre-amp tube
  2. To check out if the problem is in the power amp section:
    • Connect the output of a line-level capable device to the return jack of the effects loop
      CAUTION: Make sure the volumes are down on both devices before doing this
    • If you get no sound through the amp, then the problem is likely in the power amp section
    Some things to check:
    • Are the powertubes all lit equally? If not, you could have a dying powertube