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A concise diagnosis: SV5136 means the control’s FSSB sees fewer amplifier nodes than expected, typically due to FSSB optical link issues, amplifier configuration/parameter mismatch, or a failed servo/spindle module on the fiber ring, while SP1220(S) means the control cannot recognize any serial spindle amplifier on the FSSB line for the spindle channel when the spindle drive is a separate module, usually due to power, fiber, or connector faults to that spindle amp or a failed amp/control FSSB interface .

### What these alarms mean

– SV5136 “FSSB: number of amplifiers is insufficient” is raised when the number of amplifier nodes detected on the FANUC Serial Servo Bus does not match the configured controlled axes or the expected topology, pointing to wrong axis/amp settings or an interruption on the fiber chain between the control and amplifiers .

– SP1220(S) “No spindle amplifier” is raised when the CNC cannot recognize a serial spindle amplifier on the spindle channel; common causes include no power to the spindle amp, wrong or broken fiber/cable, or a faulty control/amp PCB on the serial link, with the spindle drive being a separate module from the servo amplifier chain in this scenario .

### Safety first

– Lock out/tag out main machine power, verify DC bus has discharged below safe voltage before touching any drive terminals or removing boards, and mark all connectors before removal; spindle/servo DC link can retain charge, so confirm bleed-down before service .

– Never hot-plug FSSB optical fibers or move COP10A/COP10B links with power applied; perform all fiber re-seating and topology checks only after full power-down and discharge confirmation .

### Quick differentiation guide

– If axes and spindle both disappear, suspect an early break in the FSSB chain near the control or first module; if axes are present but spindle is missing, focus on the segment feeding the separate spindle amplifier module and its power/fiber .

– If all modules are powered and linked but SV5136 persists, suspect mismatch of auto FSSB settings vs. physical modules, or a failed module that drops downstream nodes from enumeration .

### Structured diagnosis for SV5136

– Physical FSSB chain check: With power off, verify fiber from axis PCB to the first module COP10B, then from that module COP10A to the next module COP10B, continuing daisy-chain order; a wrong port or inverted first link is a common cause on new installs .  

– Fiber integrity: Inspect and clean fiber ends, ensure no kinks or sharp bends; if needed, temporarily substitute with a known-good fiber to isolate a bad cable section causing missing nodes on FSSB .  

– Module power/LEDs: Confirm each servo/spindle module has DC bus and status LEDs normal; a failed module in the chain can prevent downstream enumeration and produce “amps insufficient” .

– FSSB automatic setting: Perform FSSB auto-setting from the control to rebuild the amplifier/axis mapping when hardware has changed; ensure parameter workflow per platform where auto-setting is supported so the detected node count matches configuration .

– Segment isolation test: Reduce expected axis count temporarily and move the control’s COP10A fiber output directly to the COP10B of a target drive to test modules one-by-one; this technique isolates a bad module/cable or control-side FSSB card when every drive tested still alarms .  

– Control-side hardware: If topology, fibers, and modules check out, suspect axis/FSSB interface card or motherboard FSSB interface as a root cause of incorrect node recognition .

### Structured diagnosis for SP1220(S) with separate spindle drive

– Spindle amp power: Verify the separate serial spindle amplifier is powered and ready; if unpowered, the CNC cannot enumerate it and will report SP1220 .  

– Correct connector/fiber: Confirm the spindle amplifier’s serial/fiber connector is on the correct COP10 port in the FSSB chain and properly seated; reseat with power off and verify no broken or misrouted cable . 

– Cable condition: Check the fiber/cable continuity to the spindle amp; a break or wrong port yields “no spindle amplifier” even if the amp powers up .  

– Drive-local status: Observe the spindle amp seven-segment and LEDs on power-up; normal “—” boot then ready vs. local alarms (e.g., AL-24 vs. none) influences whether to power-cycle the amp and CNC per guidance to re-enumerate after non-AL-24 alarms .  

– Control/amp PCB fault: If cabling and power are correct and SP1220 persists, suspect the spindle amp control PCB or the CNC printed circuit board handling the serial link as defective .

### Common root causes and fixes

– Miswired or incomplete FSSB chain: Correct the fiber order from control axis PCB → first module COP10B, then first COP10A → next COP10B, repeating for all modules; fix any skipped or reversed link segments .  

– Bad fiber segment: Replace the suspect optical cable section; even minor damage can drop modules from the bus and trigger SV5136 or make the spindle vanish for SP1220 . 

– Failed amplifier control section: Replace or repair the module that, when inserted, causes downstream nodes to disappear in isolation testing; this restores proper node count and clears SV5136 . 

– Parameter/auto-setting mismatch: Run FSSB auto-setup to rescan and reassign amplifier/axis after hardware replacement; align configured axes to detected modules to resolve enumeration mismatch . 

– Spindle-only absence: Restore power to the separate spindle amp, confirm its fiber link, or repair the spindle amp/control PCB on the serial link to clear SP1220 .

### Practical test sequence

– Power down, discharge, and inspect all FSSB fibers and ports end-to-end from the control through servo modules to the separate spindle amp; correct any porting/order issues first .  

– Power up and watch module LEDs; if SV5136 appears, perform the parameter-limited isolation test by lowering the temporary expected axes and moving the control fiber directly to each module’s COP10B to identify a failing module vs. cable vs. control FSSB interface .

– If physical and isolation tests pass, run FSSB auto-setting and verify amplifier/axis mapping screens reflect the actual modules; then cycle power to apply settings and confirm alarms clear . 

### Notes on resets and parameters

– Some field guides suggest temporary parameter changes to limit expected axis count during isolation or to clear alarms, but these must be done with full understanding of the control series and only to aid diagnosis; follow platform-specific steps and return parameters after tests, using the official auto-setting where available .

– Always record original parameters before changes and ensure PWE is disabled after service; improper parameter states can mask real hardware faults or create additional alarms .

### When to suspect the control

– If every module still triggers SV5136 when connected directly to the control’s FSSB output with a known-good fiber and correct temporary expected axis setting, the axis PCB or motherboard FSSB interface is a likely fault path .

– If SP1220 persists with a known-good spindle amp, proper power, and known-good fiber, the CNC side PCB handling the spindle serial link may be defective .

### Final verification

– After repairs, confirm that the FSSB page enumerates all expected modules and that the spindle amplifier appears as a node with normal status before enabling servos/spindle; run auto-setting once more if hardware was swapped .

– Conduct a low-speed spindle orient test and a single-axis jog to validate stable communication and absence of recurring FSSB errors before resuming production .

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These blogs should not be considered as official repair or service manuals. For detailed instructions, critical repairs, or advanced troubleshooting, it is always necessary to contact and work under the guidance of the respective *machine manufacturer* or *CNC controller support team*.  

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