Yasnac 2000GII Error 36 Encoder Cable Repair on Mori Seiki SL-1

On a 1983-vintage Mori Seiki SL-1 two-axis CNC turning center controlled by a Yasnac 2000GII, an Error 36 alarm with motion only on the X axis is almost always traced to the X-axis Feedback Unit (Yaskawa's term for the servo-mounted rotary encoder) or, more commonly in field experience, the cable harness between the Feedback Unit and the servo amplifier card cage in the electrical cabinet. This reference documents the diagnostic path that resolves Error 36 without replacing the encoder, plus the mechanical and electrical hazards encountered while working on a 40+ year old Yaskawa servo system.

1. System Architecture: Yasnac 2000GII and Yaskawa Servo Stack

The Yasnac 2000GII is Yaskawa's second-generation CNC controller released for the domestic Japanese machine-tool market in the early 1980s and exported as the OEM controller on Mori Seiki, Hitachi-Seiki, and several Takisawa lathes and machining centers. On the SL-1, the controller drives two brushless (BL) servo axes:

The X-axis Feedback Unit is mounted to the rear of the X-axis servo motor with the flexible clamp coupler; the same construction applies to Z. The motor and Feedback Unit part numbers are shared between axes in the Mori Seiki parts list, but the encoder ratio scaling is handled inside the Yasnac 2000GII parameter set (Prm 8xx-series axis constants in the Yasnac programming manual). As a consequence, swapping an X and Z Feedback Unit between axes is electrically permissible, and the controller will re-scale the position pulses. This is the basis for the swap-test diagnostic described below.

The Feedback Unit produces a 500 count per revolution quadrature output (±A, ±B, ±Z index, +5 V, 0 V) carried on a 9-pin cable bundle. The cable transitions through a Yaskawa flexible elbow strain-relief at the encoder end and terminates at a multi-pin header on the servo interface PCB in the cabinet. Pin count mismatch between the encoder-side connector (9 pins) and the cabinet-side header (significantly more) is normal — the extra conductors are shield drain, frame ground, and the +5 V supply return.

2. Error 36 Definition and Alarm Source

The Yasnac 2000GII alarm list groups Error 36 under the "Position Feedback" category. Per the OEM service documentation, the alarm is raised when:

• The controller detects loss of ±A or ±B quadrature transitions while the axis is commanded to move.
• The position-error counter (following error) exceeds the axis parameter Prm 8x4 limit without a corresponding pulse train from the Feedback Unit.
• The +5 V supply to the Feedback Unit drops below the encoder's brown-out threshold (typically 4.65 V at the encoder pins, not at the cabinet end).

The alarm is non-fatal in the sense that the controller drops axis-enable and applies dynamic braking through the servo amplifier's dump resistor, but it is persistent until the operator presses RESET on the MDI panel. Holding RESET while jogging will not clear the alarm if the underlying feedback is still missing.

3. Symptom Confirmation on This Machine

Observed behavior on the subject 1983 Mori Seiki SL-1:

1. Controller boots to READY with no alarm at power-on.
2. Z-axis manual jog responds normally, position counters update, no alarm.
3. X-axis manual jog (any feedrate, any direction) triggers Error 36 within < 50 ms of motion command.
4. Spindle, coolant, tool turret, and M-functions are unaffected.
5. Pressing RESET clears the alarm; the alarm re-arms on the next X-axis jog.

This symptom matrix isolates the fault to the X-axis feedback chain only. The Z-axis Feedback Unit and its cable are confirmed good by the absence of an alarm during Z motion.

4. Diagnostic Sequence

Follow this sequence before replacing any hardware. Each step is non-destructive and inexpensive.

4.1 Visual Inspection of the X-Axis Cable Run

With the machine at E-Stop and main breaker locked-out, remove the rear sheet-metal covers to expose the X-axis servo motor and Feedback Unit. Trace the 9-pin cable from the encoder to the cabinet entry. Look specifically at:

• The elbow strain-relief at the encoder end. On Yaskawa Feedback Units of this generation, the elbow is a cast aluminum hood with a rubber boot that flexes every time the axis traverses. Forty years of flex cycles concentrate stress at the boot-to-hood transition and at the boot-to-cable transition. This is the most common failure point.
• The cable carrier (drag chain). Inspect for cracked insulation, kinked conductors, and pinch points at the chain end-stops.
• The cabinet gland. Pull back the grommet and inspect for shield-drain fraying or conductor fatigue at the strain-relief clamp.

4.2 Continuity and Resistance Check

With the Feedback Unit disconnected at the encoder end and the cabinet-end connector disconnected from the servo PCB, perform the following with a digital multimeter on the ohms scale:

4.3 Hot Check: +5 V at the Encoder Under Load

With the controller powered, E-Stop released, and the Feedback Unit reconnected, back-probe the +5 V pin at the encoder connector with an oscilloscope or a DMM on DC mV. Command an X-axis jog. The voltage should remain within 4.90 V to 5.10 V with less than 50 mV of ripple. A drop below 4.65 V or ripple exceeding 100 mV under motion indicates either cable voltage drop (high conductor resistance) or a failing Yaskawa SMC power supply. Substitute the +5 V supply with a bench-regulated 5.0 V / 500 mA source through the cabinet connector to isolate.

4.4 Swap Test (Axis-Swap Diagnostic)

Because the X and Z motors and Feedback Units are the same part number, swap the Feedback Units axis-to-axis to localize the fault:

1. Lock out the machine and tag out per OSHA 1910.147.
2. Remove the Z-axis Feedback Unit (no shaft-clamp orientation requirement on Z; the pulley can be rotated to align the clamp screw).
3. Remove the X-axis Feedback Unit (see Section 6 for the clamp-orientation procedure).
4. Mount the original X-axis Feedback Unit on the Z motor, and the original Z-axis Feedback Unit on the X motor.
5. Reconnect cables, restore power, and test.

Result interpretation:

• Alarm moves to Z axis: the original X Feedback Unit is bad. Replace it.
• Alarm remains on X axis: the original X Feedback Unit is good; the fault is in the X-axis cable harness or the servo amplifier card for X. Re-install the original Feedback Units.

On the subject machine, the alarm did not migrate. This rules out the encoder itself and points directly at the X-axis cable harness.

4.5 Cable Disassembly and Solder-Joint Inspection

This is the step that resolved the fault. Disconnect the Feedback Unit at the encoder side. Unscrew the elbow hood and slide it back along the cable to expose the solder cup connector inside the encoder. With a 10× loupe or a USB microscope:

1. Inspect each of the 9 solder joints for fractured solder, cold solder, or lifted pads.
2. Flex the cable gently near each joint while watching for movement at the pad or conductor.
3. Reflow any suspect joint with a temperature-controlled iron at 320 °C using leaded solder (Sn63/Pb37) for compatibility with the original 1983 solder.
4. Inspect the boot-to-cable transition for strands that have fractured inside the insulation — these will not be visible externally but will show up as intermittent continuity when the cable is flexed.

5. Repair Procedure: Reflowing a Broken Encoder Cable Joint

5.1 Tools and Materials

• Temperature-controlled soldering iron, 320 °C tip, 1.6 mm chisel
• Leaded solder, Sn63/Pb37, 0.5 mm or 0.7 mm diameter
• No-clean rosin flux pen
• Solder wick, 1.5 mm braid
• 10× loupe or USB inspection microscope
• Heat-shrink tubing, 1.5 mm and 2.5 mm diameter
• Strain-relief RTV (Dow Corning 3145 or equivalent)
• Multimeter with ohms and DC mV
• Oscilloscope (preferred for dynamic verification)

5.2 Procedure

1. Lock out and tag out the main breaker. Wait 5 minutes for the bus capacitors in the servo amplifier to discharge (> 300 VDC is present for several minutes after power removal).
2. Remove the X-axis Feedback Unit from the motor (see Section 6).
3. Remove the elbow hood from the Feedback Unit. Retain the rubber boot, grommet, and clamp.
4. Inspect every solder joint under magnification. Identify the broken joint by gently tugging each conductor with fine-tipped tweezers.
5. Apply a small amount of no-clean flux to the broken joint.
6. Touch the iron to the joint and feed a small amount of fresh solder. The original solder should reflow and wet both the pad and the conductor.
7. Remove the iron and allow the joint to cool without movement for 5 seconds.
8. Re-inspect under magnification; the joint should be concave with a visible fillet.
9. Repeat for any other suspect joints.
10. Slide heat-shrink tubing over each joint and shrink with a heat gun on low.
11. Re-assemble the elbow hood and clamp. Apply a small bead of RTV at the boot-to-cable transition to prevent re-fracture.

5.3 Verification

1. Re-install the Feedback Unit on the motor (Section 6).
2. Reconnect the cabinet-end connector.
3. Restore power, release E-Stop, press RESET. Confirm READY status.
4. Jog X axis positive and negative at low, medium, and rapid feedrates. Verify no Error 36.
5. Run a reference-return (G28) on both axes. Verify X and Z return to machine zero without alarm.
6. Run a part program with multiple X-axis cuts to apply thermal and mechanical stress. Monitor for any intermittent Error 36 over a 30-minute run.
7. Verify position counter agrees with the commanded position to within ± 0.001" (or 0.01 mm) at the end of the program.

6. Mechanical Procedure: Removing and Re-installing the Feedback Unit

6.1 Z-Axis Feedback Unit

The Z-axis Feedback Unit is the easier of the two to remove because the shaft-clamp orientation is not constrained:

1. Loosen the shaft clamp screw on the flexible coupler between the motor and the ball screw. Do not remove the screw.
2. Rotate the motor pulley (and therefore the encoder shaft) by hand until the clamp screw is accessible through the access port in the sheet metal.
3. Slide the Feedback Unit off the motor pilot.

6.2 X-Axis Feedback Unit

The X-axis Feedback Unit requires the slide to be jogged to a specific position before the clamp screw is accessible:

1. Power on the controller in fully assembled state (all Feedback Units connected).
2. Jog the X axis in manual mode until the flexible coupler's clamp screw is positioned through the access port. This is typically near mid-travel.
3. E-Stop, lock out, and tag out.
4. Loosen the clamp screw.
5. Slide the Feedback Unit off the motor pilot. Do not pry against the encoder body; pull straight back.

7. Transient Alarm 5482 and Recovery

During this repair, an alarm 5482 appeared after the controller was powered with only one Feedback Unit connected and the Z axis was jogged. The alarm manifested as a sustained beep and a "Collating" message on the MDI display.

Error 5482 on the Yasnac 2000GII is a non-latched parameter-corruption alarm raised when the controller detects a CRC mismatch in its CMOS parameter backup during a hot-reset. It is not a hardware fault. The alarm self-clears on the next power-down/power-up cycle provided the parameter backup battery (Yaskawa YEC-BAT-3.6V, mounted on the CPU board) is healthy and the parameter checksum in CMOS matches the checksum in the EPROM.

Recovery steps:

1. Power down the controller completely.
2. Wait 30 seconds for the CMOS to fully reset.
3. Power up. The alarm should be absent.
4. Verify all Yasnac parameters (Prm 1xx through Prm 9xx) against the machine documentation.
5. If the alarm persists, replace the parameter backup battery and reload parameters from the printed parameter sheet.

8. Lessons Learned: First-Timer Checklist for Yasnac 2000GII Service

9. Preventive Maintenance Schedule

For any Yasnac 2000GII-controlled machine, add the following to the annual PM:

1. Visual cable inspection at the encoder elbow and the cable carrier end-stops. Look for cracked insulation, exposed shield, and boot hardening.
2. Continuity check on every Feedback Unit cable with the machine locked out. Record the resistance of +5V and 0V return; trending upward indicates joint fatigue.
3. Dynamic +5V check at each encoder pin under axis motion. Voltage must remain above 4.85 V.
4. Coupler inspection on the flexible shaft coupler. Look for cracking in the rubber element and for any axial play between the motor and ball screw.
5. Battery replacement on the parameter backup battery every 3 years, regardless of measured voltage.
6. EPROM checksum verification annually to detect EPROM bit rot in the Yasnac firmware.

10. When to Replace vs. Repair

11. Quick-Reference Error 36 Decision Matrix

12. Related Alarms and What They Tell You

The Yasnac 2000GII raises a family of feedback-related alarms. The full list is in the Yasnac 2000GII Maintenance Manual (Yaskawa publication YEC-MNT-2000GII), but the operator-relevant subset:

The Yaskawa Yasnac 2000GII Maintenance Manual is available through Yaskawa America's documentation portal at the official Yaskawa product page for legacy CNC controllers. For machine-specific wiring, refer to the Mori Seiki SL-1 electrical diagram set, typically supplied as a separate binder with the machine. The Mori Seiki SL-1 parts list identifies the motor part numbers but consolidates the encoder under the motor assembly, which is why the Feedback Unit is often overlooked in the documentation.

13. Summary

The Error 36 alarm on a Yasnac 2000GII-controlled Mori Seiki SL-1 was traced not to the encoder itself but to a fractured solder joint inside the elbow strain-relief of the X-axis Feedback Unit cable. The fracture was hidden inside the elbow hood and was not visible without disassembly. The repair — reflowing the broken joint and re-insulating with heat-shrink — restored reliable continuity, and the machine returned to service with both axes zeroing correctly.

Two safety lessons are worth restating:

1. Never power the Yasnac 2000GII with any Feedback Unit disconnected. The resulting servo runaway is destructive.
2. Always inspect the cable elbow and connector solder joints before assuming the encoder is bad. The swap test will confirm whether the fault follows the Feedback Unit or stays on the axis.

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