[TFGC]

Hello-

After several months of inactivity on my 1981 Mycenter 2, I attempted to power up and exercise the machine. Machine powered up, but Fanuc 6m-b control went into an immediate 426 Y axis alarm (Y axis Fanuc pulse coder position feedback is abnormal - Disconnection alarm). Immediately contacted my friend/mentor in the CNC world and he told me to check all associated contacts which I proceeded to do. Here is an interesting observation that perhaps someone can weigh in on: With power off, I have found that I can get the machine to respond normally after I press the small black contactor button in multiple times on the Y axis drive unit (Fanuc A06B-6045-H001/H002) and then power the machine on. This work-around only lasts for a short period of time (from 15 seconds to many minutes) and there appears to be a correlation between how many times I exercise the contactor button and how long normal operation lasts. I have carefully accessed the Y axis servo and reseated both canon plugs as well as checked the brushes (which look great).

With that being said, I replaced the existing Y axis drive with another used drive and received the same alarms (401, 420, 422 & 426 which ALL go away after exercising the contactor button), so I don't believe the drive is the issue. I think that the exercised contactor button is having an electrical effect in some other part of the control or power supply that is causing the machine to function normally for a short period of time. Any thoughts and experiences are welcomed to root out the cause! Thank you.

[STLMachinist]

The drive only handles the command cable from the main board. (Out of CAV on main to CN1 on drive PCB) The feedback goes back to the resolver/inductosyn board, or the main board. Try swapping the feedback cable to another axis and see if the 426 alarm swaps to the other axis.

[TFGC]

Thank you for the advice. To date, here is what I've done to try and find the root issue:

1. Swapped the Y axis servo motor with another exact match from a partial donor machine I have: 426 alarm still active.
2. Swapped the encoder cable connectors between X and Y on the main PCB: Alarm stayed on the Y axis.

With the alarm staying on the Y axis after the encoder cable connector swap, my thought is that I have an issue with the circuitry specific to the Y axis encoder feedback residing on the main PCB. Does this sound logical? If the alarm went away after the motor swap, then more than likely there was an issue with the encoder. If the alarm followed the encoder cable to the X axis when swapped, then the encoder cable itself was faulty. Since the alarm has not followed the encoder cable, the problem must be on the main PCB. If anyone has any information contrary or in addition to this line of thinking it is certainly welcomed!

Can anyone point me in a direction as to where and what to look for on the main PCB?

With this being said, I have access to a replacement main PCB, but have been told there will be some chip swapping required from the original MoBo to the new MoBo; is this correct? If so, which chips need to be swapped? Can I assume the chips needing the swap will not be associated with the encoder circuitry for the axes? MoBo is a20b-0008-0410. Any experience or insight into this procedure is also welcomed!

[STLMachinist]

On the Main PCB, there should three (3) LSI chips. The big purple ones in the center. Swap the leftmost one with any of the other 2. Swapping these will not cause any sort of memory loss. If the alarm switches axis(or goes away), obviously you have a bad chip.

[TFGC]

Thanks for the advice. I actually did that today with no change to the alarm. This weekend, I am going to check all the parameters dealing with encoder feedback and see if any magically changed while the machine was out of use. Might explain why I have been unable to locate the problem via the usual suspects! Will report back after I have checked.

[TFGC]

ProbleFixed: Turned out the main PCB did in fact have an issue specific to the Y axis encoder feedback (possibly a hybrid chip, but do not know for sure). After an issue with my first used PCB purchase, I found another with the same board number as the original, put it all back together and BINGO back to normal operation without any hitches to speak of! Hope this helps someone in the future. I would like to thank: DB (my friend and mentor in the machine world), Dan Fritz, Warren Goyer and STLMachinist for all of their time and effort in helping me with this issue...THANKS!