Monthly Archives: March 2015

Troubleshooting Fanuc Servo Devices

Recently we had a customer that we helped with troubleshooting a Fanuc servo A20B-1003-0090 board that he was installing into a A06B-6058-H005 drive. Fanuc lists some troubleshooting options in the manual that we wanted to repost to help anyone who is having similar issues.

Troubleshooting for the DCAI alarm:

[check items]

  1. Setting S2 for the S series
  2. Machine load
  3. Check connection of separate discharge unit

[Adjustment procedure]

A. Check amplifier setting S2. If the setting is incorrect, go to Cause 1. If the setting is correct, go to A-0.

A-0: Check whether a separate discharge unit is being used. If it is being used, go to A-1. If not being used, go to A-2.

A-1: Check the connection of the separate discharge unit. If the connection is incorrect, go to Cause 2. If connection is correct, go to A-2.

A-2: Check the acceleration/deceleration frequency. If the frequency is too high, go to Cause 3. If the frequency is low enough, go to A-3.

A-3: Replace the servo amplifier. If a DCAL alarm no longer occurs, go to Cause 4. If a DCAL alarm still occurs, go to Cause 3.


1). If the setting S2 of the S series servo amplifier is incorrect, a DC alarm is caused.

2). If the separate discharge unit is connected incorrectly, a DC alarm occurs.

3). Compared to the regenerative power of the amplifier, the regenerative energy of the motor is too large. (The inertia is too large or the acceleration/deceleration frequency is too high.) In this case, try to decrease the acceleration/deceleration frequency or install a separate discharge unit.

4). The discharge transistor (Q1) in the servo amplifier is defective.

Powering Up a Siemens PCU 50

Recently we had a customer who we helped to walk through with booting up a 6FC5210-0DF22-0AA0 Siemens PCU 50 unit that they were replacing. Their original unit was starting up and then freezing after several minutes so they decided to go ahead and replace the unit, as their original was diagnosed as having a blown main board which was unrepairable. There were some issue with getting the replacement up and running, so we wanted to give a simple explanation for powering up a 6FC5210-0DF22-0AA0 unit, along with an accompanying video showing the final result.

To start, the PCU 50 need to be hooked up to 24VDC power using the accompanying green plug. On the PCU 50 itself there is a wiring diagram showing all of the connections for the ground and the +/-. Next the display monitor is hooked up to the VGA port, and the keyboard/mouse are hooked into either the USB connections or the designated connections on the side of the 6FC5210-0DF22-0AA0 unit. Before powering on the unit, you need to turn the black knob on the CPU card from the “non-operating” position to the “operating” position. If it is not changed the unit will still power on, but you will be unable to access the CPU. Once everything is set you can turn on the DC power and the 6FC5210-0DF22-0AA0 will power up successfully, bringing you to the login screen.

Here is a link to a video showing the connections and the booting process.

Setting Up a Fanuc Test System

Here an example of a Fanuc test system made up of the power supply, spindle amplifier, and servo amplifiers. On the far left of the system is the A06B-6120-H030 Power Supply hooked up to the incoming power source. Next on the rack is the A06B-6122-H030#H570 Spindle Amp module which is the control for the spindle in the system. Following that we have four A06B-6127-H202 Servo Amplifier modules which control the four axis directions in the setup. All the units are powered up and currently set to the ready state.

Replacing a 140CPU43412 With a 140CPU43412A CPU

One of the more common Modicon Quantum CPU modules is the 140CPU43412A CPU module. This unit replaced the 140CPU43412 unit which has since become obsolete. Currently, many systems are being updated so we consistently get questions about 140CPU43412A CPU replacement and what the differences between the two units are, and if they are plug-and-play replacements. To help with this transition, we put together a few notes to help anyone who has questions.

The redesigned unit designated with the “A” at the end is functionally identical to the original non-“A” unit, with a few slight considerations. If you are using this CPU in a hot standby system, you must use matching units, so either two 140CPU43412 units or two 140CPU43412A units. Additionally, the 140CPU43412A version has a unique flash executive, and the two unit’s flash executives are not interchangeable. For the software, all the standard systems (Concept, ProWORX, and Modsoft) support the 140CPU43412A unit, and any 140CPU43412 program will load directly on to the new 140CPU43412A without needing to be changed.

Rebuild a Fanuc Motor – Model A06B-0131-B075 Process

Gallery of pictures showing the process that it takes to rebuild a Fanuc motor, part number A06B-0128-B675#7008.

Original pictures of the motor. Fan is dirty, all sides show significant use.

fanuc motor repair
fanuc motor repair

Upon  dissasembling the motor,  residue was found in the stator prompting the removal of the motor shaft. This would have caused a short in the stator once running.


The finished product, after the motor is rebuilt and fully tested.

fanuc servo motor repair
fanuc servo motor repair
Unidrive Classic

Unidrive Classic HF Trip Codes: HF82, HF83, HF84, HF88

HF82 – Large option module missing:
The trip would be expected if one of the UD7x large option modules is removed while
the Drive is powered up. If this trip occurs at any other time than the case above, then
there is a problem with either the large option module or the UD90A control PCB.

HF83 – Power Board Code Failure:
This trip means that the UD90A control PCB could not recognize the power rating of
the power PCB it is connected to.
On Unidrive Sizes 1 to 4, the trip is probably due to the power PCB in the Drive but a
problem with the UD90A control PCB could also cause this trip.
On a Unidrive Size 5 the trip is cause by either UD99 PCB or the UD90A PCB. The
interconnects between the PCBs could also cause this trip and should be checked.

HF84 -Current Offset Trim Failure:
This trip means there is a problem with the current feedback on the Drive.
On Unidrive Sizes 1 to 4, the trip is probably due to the power PCB in the Drive but a
problem with the UD90A control PCB could also cause this trip.
On a Unidrive Size 5 the trip is cause by either UD99 PCB or the UD90A PCB. The
interconnects between the PCBs could also cause this trip and should be checked.

HF88 – Watchdog Failure:
This trip can result from a faulty UD7x Co-Processor.
With power off, remove Co-Processor and re-apply
Hardware Faults are typically fatal. If powering down and letting the drive sit for 5
minutes before re-application of Power does not clear the HF Fault it would require
Drive repair.

MRO Electric stocks new and refurbished Classic Unidrives. We also provide pre-priced Control Techniques repairs. For more information or to request a quote, please call 800-691-8511 or email

Fanuc CHECKSUM Error

We recently shipped out two Fanuc A16B-3200-0040 boards to a customer and upon installation they received a Fanuc CHECKSUM error on their control. Here are the notes that we use to troubleshoot this Fanuc error which can be used to try and solve the problem.

There are two common causes for this problem.  The first and most likely is TP communications.  If you see 3-5 “_” appear under the checksum number, and no red LED’s on the CPU, then this is the problem.  This means that 1/2 the communications signal is missing.  Swap the Pendant and cable (Cable is probably crushed or cut), and everything should hopefully be great.  There is a possibility of damage on the CPU side however.

The second is a corruption of software on the CPU.  This will result on the middle red LED (going from memory, so it might be a different LED) on the CPU turning on.  The only way to recover from this is to dump in an image, or reload core & put in your most recent backup.  Corruptions can occur from the backup battery being dead, noisy power being supplied, or any major power glitch while the system is on (power loss, lightning strike, etc.)