230-01C4

Check External 24VDC Supply Voltage

Using a multimeter, measure voltage at the external 24VDC supply terminals. Verify output is between 22-26VDC under load.

Ensure proper lockout/tagout procedures before accessing electrical panels.

Verify Supply Current Capacity

Check the 24VDC supply current rating against total load requirements. Measure actual current draw with clamp meter.

TNC systems typically require 3-5A minimum for proper relay operation.

Inspect Power Supply Connections

Check all terminal connections at the 24VDC supply output, looking for loose, corroded, or burned connections.

Use thermal imaging if available to identify hot spots indicating poor connections.

Replace Power Supply if Defective

If voltage is low or unstable, replace the external 24VDC power supply with equivalent rated unit.

Ensure new supply meets TNC isolation and noise requirements.

Locate External DC Monitoring Relay

Identify the external DC voltage monitoring relay in the TNC cabinet, typically labeled K1 or K2 in the electrical schematic.

Consult machine-specific electrical drawings as relay designations vary by machine builder.

Test Relay Coil Resistance

With power off, measure relay coil resistance. Should typically read 200-2000 ohms depending on relay type. Open circuit indicates failed coil.

Disconnect relay from circuit before resistance testing to avoid false readings.

Check Relay Contact Continuity

Test normally open and normally closed contacts for proper operation. Contacts should switch cleanly when coil is energized.

Use contact cleaner spray if contacts show signs of oxidation or pitting.

Replace Relay if Defective

Install exact replacement relay with same coil voltage, contact rating, and pin configuration.

Verify relay socket connections are tight after replacement.

Trace External DC Wiring Path

Follow the 24VDC wiring from power supply through fuses, terminal blocks, and relay contacts to identify the complete circuit path.

Use electrical drawings to systematically check each connection point.

Perform Voltage Drop Test

Measure voltage at multiple points in the DC circuit under load to identify high resistance connections or wire damage.

More than 0.5V drop across any single connection indicates a problem.

Check Fuse and Circuit Protection

Verify all fuses in the external DC circuit are intact and properly rated. Check for blown fuses or loose fuse holders.

Even if fuse appears good, test with multimeter as internal element may be cracked.

Repair or Replace Faulty Wiring

Replace any damaged wires, tighten loose connections, and clean corroded terminals. Use proper gauge wire for current rating.

Ensure all connections are properly torqued to manufacturer specifications.

Access TNC Diagnostic Functions

Navigate to MOD function and select diagnostic mode to view input status. Check external DC voltage input state in real-time.

Use PLC status display if available to monitor input signal directly.

Verify Input Signal at TNC Terminals

Measure voltage at the TNC input terminals for external DC monitoring while relay should be energized.

Use proper ESD precautions when working near TNC control boards.

Check Input Circuit Components

Inspect input optocouplers, resistors, and filtering components on the TNC board for visible damage or burn marks.

Look for discolored components or unusual odors indicating component failure.

Contact Heidenhain Service

If TNC board input circuit is suspected, contact authorized Heidenhain service for board repair or replacement.

Do not attempt TNC board repairs without proper training and equipment.