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'68 johnson 65 amplifier

Without special equipment (DVA/peak reading voltmeter), best way is by the process of elimination. If everything else checks out ok, suspect amplifier. And I don't mean replace everything else...most things can be tested with a multimeter and knowledge of how the system works.
 
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Testing imput:
Insert your red lead from meter into red wire to amp, turn key on voltage shoul not be below 9.5 volts.Crank motor and watch meter and should not drop below 9.5 volts
Testing output:Connect a spark gap tester to the high tension lead coming from the ignition coil and set it to approximately ½". If it fires when
you crank the engine over, there is a problem in the distributor cap, rotor button or spark plug wires.
2. Check the voltage present on the red wire (from the pack) at cranking. It MUST be at least 9½ volts. If not, the problem's in the
harness, key switch, starter or battery.
3. Check DVA voltage on the blue wire going to the coil. It should be approximately 200 volts at cranking.
4. While monitoring the DC voltage on the Red wire going to the CD, disconnect the black/white trigger wire and strike it against
engine ground. The unit should fire each time. If it does, the CD module is usually good and the points should be checked. If the
CD does not fire, verify the DC voltage is not dropping below 9-1/2 volts when you tap the black/white trigger wire against engine
ground.
Thank you

 
(Voltage Drop To Battery Capacitance Discharge)
( Pulsepack When Electric Starter Is Engaged)
(J. Reeves)

On the older Battery Capacitance Discharge ignition systems (1968-1972), the electric starter reaches a point, even with a top notch battery, whereas the starter will draw excess voltage/current/whatever which results in a voltage drop to the pulsepack. The cure is to purchase a diode which is capable of handling 12 volts and installing it between the starter terminal of the starter solenoid (NOT the battery cable terminal) and the wiring terminal that supplies voltage to the pulsepack.

The diode must be installed so that the current flows from the starter terminal of the solenoid to the pulsepack... NOT vice versa. When that diode is installed in this manner, when the key is turned to the start position, the voltage that is applied to the starter is also applied directly to the pulsepack via the diode effectively eliminating the voltage drop and energizing the pulsepack with the required voltage needed for its proper operation.
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(Battery Capacitance Dischage Powerpack Test)
Various OMC Engines - 1968 to 1972)
(J. Reeves)

Purchase a small 12v bulb at your local automotive parts store (the 12v bulb is to look like a flashlight bulb, not a headlight bulb). Solder two wires to that bulb, one to the side of the bulb (ground), and the other to the positive point. You might use a bulb of a somewhat lower voltage to obtain a brighter glow... just a suggestion.

Remove the spark plugs. With the key in the on position, make sure that you have 12v going to the pack at the terminal block (purple wire). Now, connect the ground wire from the bulb to any powerhead ground. Connect the wire from the positive point of that bulb to the powerpack wire that is connected to the coil wire on the terminal board (blue wire).

Crank the engine and observe that bulb closely (CLOSELY!). If that bulb glows even the slightest bit, the powerpack is okay. It may be a very dim glow... just so it glows! If it doesn't glow, the pack has failed.

Keep in mind, that type powerpack (Battery Capacitance Discharge) demands a top notch battery of at least 70 amp hours. Any less will, in time, cause powerpack failure.

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HINT: The wire lead that connects the amplifier to the sensor under the flywheel usually has a connector plug somewhere between the two. disconnecting that lead, then tapping it against the powerhead simulates the action of ignition points or the simulation of the sensor, whichever might apply in your case.
 
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