Repair record: Swiss Tornos deco sigma 20 machine
A Tornos deco sigma 20 with FANUC 31i-MODEL A would not start and had no display. The original record located a short circuit in the mainboard power section. This revision separates observations from suspected supply instability and explains why historical fuse trials are not selection advice.
Contents
The faulty machine was a Swiss Tornos deco sigma 20 with FANUC 31i-MODEL A control. It would not start and the display was blank. The original repair record is organized below in inspection order, retaining the site photographs and recorded results.
From a blank display to the mainboard power supply
Repair of a Swiss Tornos deco sigma 20 machineThe mainboard fuse was found blown; its recorded marking was 5A. The external 24V supply voltage measured normally. A 3.2A fuse was tried at the time, and the fuse still blew when the mainboard’s external interfaces were disconnected for checking, prompting further investigation of the board. These values document historical actions; they are not a fuse-selection table for this model or advice to repeat the trials.
The original article states a total of “twelve” amplifiers in the cabinet, while its listed power-supply, spindle and servo amplifier counts add up to eleven. The photos remain as site records, but the conflicting total is not retained as a technical conclusion. A required equipment inventory should be checked against each nameplate.
The confirmed fault and the unconfirmed cause
FANUC 31i-MODEL A mainboardFurther inspection found a short circuit in the mainboard power section. The original author suspected instability of the external 24V switching power supply. However, the record contains only a normal voltage check, without waveforms, a change trace or data demonstrating causation. The board short circuit is therefore an inspection finding; external supply instability remains a hypothesis requiring verification.
The post-repair record reports normal test operation, followed by parameter backup and testing on the machine. The author also recommended that the customer use a reliable-quality 24V supply. No specific damaged component, repair duration or long-term stability result has been added here.
Fuse replacement is not fault elimination
The fuse chapter of the 30i/31i/32i-A maintenance manual B-63945EN/02 requires identifying and removing the cause of a blown fuse before replacement. Confirm the actual specification from the control unit and applicable documentation, not experience from another machine, the old part’s marking or the trial value in this case. The original statement about “usual mainboard current” had no configuration-specific basis and is no longer retained as a selection criterion.
What to preserve when handing over this kind of fault
Provide the complete controller specification, original fuse marking, external wiring state, measurement conditions and symptoms before and after repair. Qualified maintenance personnel must follow equipment procedures when disconnecting power or the mainboard. “The display lights after fuse replacement” is insufficient proof that the short-circuit cause has been removed, and repeated fuse replacement must not be used to verify the fault. See the system identity recording method (screen example limited to 0i-F), and other cases in the FANUC section.
References and applicability
- FANUC Series 30i/31i/32i-MODEL A Maintenance Manual B-63945EN/02 (public mirror of the manufacturer’s manual): §2.7, Replacing Fuse on Control Unit, printed page 178.