1. Problem Overview
The Mori Seiki SL1 (and the later SL1-A variant produced from 1985 onward) is a small two-axis CNC slant-bed lathe originally equipped with the Yaskawa Yasnac LX2 control platform. The LX2 chassis integrates a JANCD-TUO2B switching power supply, a separate 5 V/12 V/24 V CPU power supply mounted in the operator door, an NFB1 main contactor, and an SVM contactor that feeds the spindle and servo bus. Field reports on this class of 1980s Mori SL lathe describe a consistent power-on symptom: the main breaker closes, the green LED on the spindle drive illuminates without a fault code, the cabinet fan runs, but the CRT, keypad, and front-panel power lamp stay dark.
This article consolidates the diagnostic path required when an SL1 or SL1-A with a Yasnac LX2 refuses to complete the power-on sequence, including the 5-second piezo buzzer, the NFB1 contactor that appears to drop out as soon as 100 V control power is applied, and the SVM contactor that must be manually closed to bring up the JANCD-TUO2B. Where the source is ambiguous (for example, whether a current value is single-phase, three-phase, or RMS) the article presents each interpretation explicitly so the field engineer can match the symptom to the wiring diagram.
2. Initial Symptom Verification
Before disassembling cabinets or pulling connectors, perform these static checks in order. Each step is non-invasive and isolates one section of the LX2 power-on chain.
- Verify input power at the main breaker. Use a true-RMS multimeter to measure line-to-line voltage on all three phases with the breaker closed. On a 200 V class SL1/SL1-A the expected value is 200–220 V AC, ±10 %.
- Confirm the green spindle-drive LED. A lit LED without a fault code indicates the DC bus capacitor is pre-charged and the spindle drive's internal housekeeping supply is healthy. This rules out a missing three-phase feed to the drive.
- Listen for the cabinet fan. The fan is fed from a dedicated 100 V auxiliary winding on the control transformer; if it runs, the 100 V control rail is present.
- Observe the front-panel power lamp. The lamp is driven from the 24 V DC rail produced by the 5/12/24 V CPU power supply. A dark lamp means the CPU supply is either unpowered, in a foldback condition, or downstream is shorted.
- Press the POWER-ON button once, twice, and hold for 3–5 seconds. Early Yasnac LX2 firmware uses a two-pulse or held-power-on sequence; the first pulse energizes the contactor coil, the second pulse latches the run state. A single short press will not bring the CRT up on some revisions.
Document whether the 5-second piezo buzzer sounds at the first press, on a second press, or only when SVM is manually closed. The buzzer is the LX2's primary diagnostic instrument and the duration/frequency carries fault information that is not in the operator's manual.
3. Power Supply Architecture
The Yasnac LX2 uses two independent DC power systems that must both be healthy for the CRT to light.
| Supply | Location | Outputs | Energized by |
|---|---|---|---|
| JANCD-TUO2B | CPU panel, rear of console | +5 V, ±12 V, +24 V (CRT subsystem) | SVM contactor output |
| 5/12/24 V CPU supply | Operator door, lower hinge area | +5 V (logic), +12 V (I/O), +24 V (push-buttons, lamps) | SVM contactor output |
| Control transformer | Main cabinet floor | 100 V control, 24 V lamp, fan | Main breaker (always on) |
Both DC supplies are downstream of the SVM contactor. If SVM is open, neither DC rail exists and the CRT, keypad, and front-panel lamp will be dark. The fact that the 5/12/24 V CPU supply "only powers on when SVM is manually closed" confirms the SVM is the gating element. A healthy LX2 should pull in SVM automatically as soon as the power-on sequence validates the interlock chain.
4. Main Contactor (NFB1) Behavior
NFB1 is the main three-pole contactor that feeds the spindle drive, servo drive, and control transformer. Its 100 V control coil is wired through S11 and S111. Two failure modes are common on 30+ year old units:
- Coil energizes, contactor opens. This is mechanically counter-intuitive because a contactor should close when its coil is energized. On NFB1 this indicates that one of the series interlocks (door switch, E-stop, hydraulic pressure switch, or overload relay) is open in the hold circuit. The LX2 design routes S11 and S111 through the interlock chain, then back to the coil. If the chain is open, the coil is fed through a high-impedance path and the contactor mechanically drops out within 50–150 ms.
- Source wires jumpered, S11/S111 floating. If the previous owner disconnected S11 and S111 and jumpered the line side, NFB1 is being held in by a manual bypass. This defeats every interlock on the machine and is unsafe. Restore the wiring exactly as the schematic shows before continuing.
Procedure to validate NFB1:
- With the main breaker open, disconnect S11 and S111 from the contactor coil terminals.
- Apply 100 V AC (or 110–120 V AC, depending on tap) directly to the coil. NFB1 must pull in cleanly with an audible "click." If it hums or chatters, the coil is shorted or the shading ring is broken — replace the contactor.
- Measure the interlock chain resistance with an ohmmeter from S11 to S111 with all doors closed, E-stop released, and hydraulic pump running. Expected value: less than 1 Ω. Any value above 5 Ω indicates a failing interlock switch or a high-resistance splice.
5. Rotary Phase Converter (RPC) and Voltage Balance
The reported input voltages of 222 V, 232 V, and 242 V (an 8 % spread between the highest and lowest leg) is above the NEMA MG-1 recommended maximum of 5 % voltage unbalance. While a Yasnac LX2 will generally tolerate 8 % unbalance, the JANCD-TUO2B and the spindle drive's soft-charge circuit are sensitive to sustained imbalance and may produce a non-annunciated undervoltage on the generated (idler) leg.
Recommended verification steps:
- Measure line-to-line on all three legs of the RPC output with the SL1 under load (hydraulics running, fans on, SVM closed). Record the values every 30 seconds for five minutes. Drift above 10 % between the highest and lowest leg indicates an RPC capacitor bank that needs adjustment or replacement.
- Measure phase rotation with a phase sequence indicator. A reversed leg will prevent the hydraulic pump from building pressure and may engage the phase monitor relay, which in turn opens the SVM hold circuit.
- Check the RPC idler motor nameplate. A 20 HP idler on a 200 V class machine should be wired in delta for 230 V or in wye for 460 V; an incorrect configuration produces poor voltage regulation on the generated leg.
The reported 3-phase monitor readings of 5 A, 5 A, and 2 A (approximately 1500 W with hydraulics and fans running) suggest the third leg is significantly under-loaded, which is normal for an RPC with the lathe idle. When SVM closes and the spindle drive pre-charge engages, the third leg should rise toward the other two within 2–3 seconds.
6. Interlock Chain and Hydraulic Pressure Switches
The Yasnac LX2 interlock chain is a series string of normally-closed contacts. Any open contact prevents the SVM contactor from latching. The chain typically includes:
| Interlock | Type | Verified Pressure / State | Adjustment |
|---|---|---|---|
| Main hydraulic pressure switch | N.C., opens on low pressure | Cut-in 380 psi, cut-out 320 psi typical | Set to 400 psi cut-in |
| Chuck pressure switch | N.C., opens on low pressure | Cut-in 100 psi typical | Set to 120 psi cut-in |
| Tailstock pressure switch | N.C., opens on low pressure | Cut-in 240 psi typical | Set to 270 psi cut-in |
| Way-lube level switch | N.C. float / level | Sump must be above low-level probe | Fill to high-level mark with ISO VG 32 |
| Door interlocks (left/right) | Mechanical limit switch | Both doors fully closed and latched | Verify actuator engagement |
| E-stop loop | Series N.C. pushbuttons | Released and reset | Pull to release, twist to reset |
| Phase monitor relay | Electronic, N.C. output | Phase rotation correct, all legs within ±10 % | Adjust trip point per manufacturer label |
The reported pressures of 450 psi main, 120 psi chuck, and 270 psi tailstock are at or above the typical LX2 cut-in values, so the hydraulic interlocks should be satisfied. However, the switches themselves are 30+ years old and may have stuck contacts. To rule them out, jumper each pressure switch one at a time with the SVM contactor under manual control and observe whether the LX2 begins to bring up the CRT. Replace any switch that must remain jumpered for the machine to start.
7. Diagnostic Procedure for a No-Power LX2
Follow this sequence in order. Do not skip steps — each one isolates one of the parallel failure modes that produce a dark CRT.
- Verify AC input. 200–220 V AC (or 220–240 V AC for 240 V tapped units) on all three phases at the main breaker lugs with the breaker closed.
- Verify control transformer secondary. 100 V AC (or 110–120 V AC) at the transformer secondary. Measure with the transformer loaded — under-load voltage can be 5–8 % lower than no-load.
- Verify NFB1 pulls in. Apply rated coil voltage directly. NFB1 must close mechanically and stay closed for at least 10 seconds without chattering.
- Verify interlock chain. Ohmmeter check from S11 to S111 with the chain at rest. Should read less than 1 Ω. Higher readings indicate an open interlock.
- Verify SVM pull-in. With NFB1 closed and interlocks satisfied, press the power-on button for 3 seconds. SVM should close. If it does not, the LX2 is not seeing the power-on command — check the pushbutton and the 24 V rail that feeds it.
- Verify JANCD-TUO2B outputs. With SVM closed, measure +5 V (4.90–5.10 V), +12 V (11.9–12.1 V), –12 V (–11.9 to –12.1 V), and +24 V (23.5–24.5 V) at the supply output pins. Any rail out of tolerance indicates a failed JANCD-TUO2B.
- Verify 5/12/24 V CPU supply outputs. Same as above for the door-mounted supply. A common failure is the +24 V rail sagging to 18–20 V under the lamp load; this dims the front-panel lamp and prevents the keypad from scanning.
- Verify CRT heater and anode. With the CRT supply healthy, the CRT heater should glow orange within 5 seconds. If it does not, the CRT yoke or the high-voltage multiplier has failed. Heater voltage is typically 6.3 V AC.
8. Common Root Causes
| Symptom | Probable Root Cause | Verification | Repair |
|---|---|---|---|
| CRT dark, fan runs, no buzzer | SVM not pulled in; interlock chain open | Ohmmeter S11–S111 | Repair interlock chain |
| 5-second continuous buzzer, SVM manually closed | LX2 detecting a hard alarm (E-stop, drive fault, or low lube) | Read alarm code from spindle drive LED | Resolve alarm at source |
| NFB1 drops out when 100 V applied | Open interlock or shorted contactor coil | Direct-apply coil voltage | Replace contactor or close interlock |
| Front-panel lamp dim, keypad unresponsive | +24 V rail sagging under load | Measure +24 V at CPU supply | Replace CPU supply or reduce load |
| All DC rails healthy, CRT heater dark | CRT yoke or HV multiplier failure | Measure heater voltage | Replace CRT assembly |
| Voltage balance 8 % or worse | RPC capacitor bank mis-sized or failing | Measure under load for 5 min | Adjust or replace capacitors |
9. Verification and Commissioning
After completing the repair, perform a full power-on verification before returning the machine to service.
- Restore all jumpers and bypasses to the schematic configuration. Verify that no diagnostic jumpers remain installed.
- Close the main breaker. Confirm the spindle-drive green LED is lit with no fault code displayed.
- Press the power-on button for 3 seconds. Confirm SVM pulls in with an audible click and that the JANCD-TUO2B and 5/12/24 V CPU supply both come up.
- Confirm the CRT raster appears within 5 seconds and the Yasnac LX2 splash screen displays within 10 seconds.
- Confirm the keypad backlight illuminates and each key registers a code change on the diagnostic display.
- Release E-stop. The LX2 should clear any soft alarm and display "READY."
- Jog each axis at 10 % rapid and confirm direction matches the commanded sign.
- Run the spindle at 100 rpm for 5 minutes. Monitor the DC bus voltage for sag and the spindle drive LED for any intermittent fault code.
Document the final voltage balance, all measured DC rail voltages, and the interlock chain resistance. File these readings with the machine's maintenance log so the next troubleshooting session has a known-good baseline.
10. Field Notes and Common Pitfalls
- A "5-second beep" that begins immediately on power-on is typically the LX2's watchdog alarm, indicating the JANCD-TUO2B failed to bring up all rails within the 5-second window. Replace the supply.
- A beep that begins only when SVM is manually closed indicates the interlock chain is open, but the LX2 is otherwise healthy. The piezo is a "system not ready" indicator, not a hardware fault.
- The way-lube sump must be filled to the high-level mark before the first power-on. Many LX2 machines will not start with the lube switch open, even if the lube pump itself runs.
- Hydraulic pressure adjustments on 30-year-old machines frequently drift. Always re-set the regulator after any maintenance that involves opening the hydraulic loop.
- The phase monitor relay is the most common cause of an SL1 that ran fine yesterday and refuses to start today. The relay is a sealed unit with a calibration that drifts over time. If replacing the relay, match the manufacturer part number exactly — a generic phase monitor may have a different trip time and produce nuisance trips.
FAQ
What does the 5-second piezo beep indicate on a Yasnac LX2?
A 5-second continuous beep on power-on typically signals a watchdog alarm: the JANCD-TUO2B has failed to bring all of its DC rails up within the 5-second window. If the beep starts only when SVM is manually closed, it indicates the interlock chain is open and the LX2 is reporting "system not ready." Investigate the pressure switches, door interlocks, and phase monitor relay before replacing the supply.
Why does the NFB1 main contactor drop out when 100 V control power is applied?
On the SL1 and SL1-A, the 100 V coil of NFB1 is fed through the interlock chain. If any interlock (door switch, E-stop, hydraulic pressure switch, way-lube level, or phase monitor) is open, the coil receives a high-impedance feed and the contactor mechanically drops out within 50–150 ms. Measure the resistance from S11 to S111 with the machine at rest — a value above 5 Ω confirms an open interlock.
Does the hydraulic system on a Mori SL1 use pressure switches for interlocks?
Yes. The LX2 interlock chain includes normally-closed pressure switches for main hydraulic (typical cut-in 380–400 psi), chuck pressure (cut-in 100–120 psi), and tailstock pressure (cut-in 240–270 psi). The reported pressures of 450/120/270 psi are at or above the cut-in values, so the switches should be satisfied — but the switches themselves are 30+ years old and may have stuck contacts. Jumper each switch one at a time to verify.
Is 8 % voltage unbalance on a 20 HP RPC acceptable for a Yasnac LX2?
NEMA MG-1 recommends keeping voltage unbalance below 5 % for three-phase induction motors. The LX2 electronics will generally tolerate 8 % unbalance, but the JANCD-TUO2B and the spindle drive soft-charge circuit are sensitive to sustained imbalance. Adjust the RPC capacitor bank, or replace failing capacitors, to bring the unbalance below 5 % under load.
Why does the CRT and keypad stay dark even when the cabinet fan runs?
The fan is fed from the 100 V control transformer secondary, which is energized as soon as the main breaker closes. The CRT and keypad are fed from the JANCD-TUO2B and 5/12/24 V CPU supplies, both of which are downstream of the SVM contactor. If SVM does not pull in, neither supply runs and the CRT, keypad, and front-panel power lamp will all be dark. Validate the interlock chain, then the SVM coil, then the supplies themselves.
