Metal Samples MS2500L, Operator'S Manual

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Test the operation of the current loop control circuit (AR5-B and Q1). The inputs of AR5-B pins 5 and
6 should be equal. The voltage drop across R23 will represent the total loop current. The difference
between the voltage drop across R40 and the voltage drop across R23 will reflect the amount of current
required to power the circuitry. This current cannot exceed 4mA.
Erratic or Intermittent Operation
This problem is most often caused by faulty switch contacts. Again, if there is visual evidence of corro-
sion attack or moisture on the boards, then the switch contacts are most likely damaged. It will be
necessary to replace the switches or the boards.
Check the operation of the power on reset circuit. cycle the power off and on and check the output of
U17 pin 3. There should be a high pulse for one second after the power is applied to the circuit.
Check the operation of the system clock. U16 pin 3 should show a 2750 Hz square wave. This is
divided down to a .1666 Hz square wave by U12 pin 3.
Check the timer circuits (U8, U10, and U11). There should be a pulse each the time the counter
reaches 0 at U8 pin 12 and the counters reloaded with the BCD value of the switch settings.
Check the control circuits. U2-B outputs (pin 13 and 12) will alternate between polarization mode and
relaxation mode upon each countdown of the timer circuits. U3-A controls the current gate (pin 1), U2-
A controls the OCP sample trigger (pin 1), and U3-B controls the output sample and hold (pin 13)
Excessive Current
This symptom (current > 30mA) is usually caused by a faulty or damaged component which draws an
excessive amount of supply power. Test for a voltage drop across R4.
If a voltage drop appears, current is drawn by the loop control circuit. Test the operation of Q1 and
AR5. Test to see that R23 is not damaged.
If no voltage drop appears, current is drawn by a faulty or damaged component. Check the +V and
+V1 rails. They should be about +6 volts with reference to ground. Also check COM and COM1
these common reference points should be at + 1 volt with reference to ground. If left operating for 30
minutes or more, the faulty component may overheat. If this occurs, the component can then be easily
located and replaced. If not, then the component must be located by substitution or the board replaced.
If the cause of the problem cannot be resolved, send unit back to Metal Samples for repair. Use form in
the Appendix E of this manual.