[jhoanrojas]

Good day everybody.

I have an Okuma CNC lathe with Fanuc 6T control And the velocity control unit is A20B-0009-0320.

Last days there's an alarm (420: z servo) and the tool post moves abnormally when it moves or when it's stopped. I checked DGN 801 grows up instantly (more tan 2000 or 3000), what produces the alarm.

Fortunately I borrowed a similar PCB and the machine is runnig percfectly. Now i Know that the parameters are well configured.

Well, I have to repair my PCB but I dont know where or what component controlls the velocity feedback signal. I 'm going to try to make a pcb map but it's gonna be hard if there is an hybrid component.

Waiting for some Help.

Many thanks.

[guhl]

check transistors, which drive power transistor
also isolated amplifiers (small yellow box) break, try to swap with one from another drive

[jhoanrojas]

Thanks for your answer guhl.

I was wrong. Now there are other alarms (401). The error stills there. I disconnected the servomotor cable and the tool post was quiet for some minutes but then, the alarm 401 appears and the HCAL or HVAL is ON sometimes:

Guhl, Ill check the transistors and the transistor modules. I'm not sure, but I'm going to check the servomotor brushes.

Regards

[guhl]

HCAL or HVAL?
HCAL
Attachment 207776
HVAL

[jhoanrojas]

Today the machine was working fine first 10 minutes. I checked the transistor modules, drive transistors; they are ok. I checked the encoder signal too, it's ok. Then, the led HCAL turned ON with 401 alarm. I remember that the TGLS led was ON sometimes (last friday). . After a few minutes the machine was Ok again. Looking all the causes for this alarms... It could be the motor brushes).

I'll chek it and post.

Thanks!!!

[guhl]

also a yellow box on the PCB (isolated amplifier) causes these symptoms

[tc429]

sounds silly, but unplug the motor connectors and see if they stink...that plastic they use has a real pungent odor if any arcing is going on inside...the blue/brown plastic inserts are 2 piece(both inside the motor, and inside the cable end), often even a little moisture will start carbon tracking between the layers on the motor plug, or greenish corrosion in the pulsecoder plug- but they really stink if theyve been arcing at all.

if you can turn the motor by hand, see if its moving freely- a dry gib, bad screw or thrust bearing will overload- unfortunately fanucs 401 is a catch-all alarm, often misleading...tg can be from open armature circuit, or simply a current spike, although hcal is supposed to show, it dont always...

in looking for odd mechanical catches, ive used a cordless drill battery directly to the motor leads(disconnected from the drive/control power off, of course) with a dc ammeter to look for current spikes as it travels- careful, watch the slide dont hit anything... if the motor drives ok, its gotta be a drive issue- one note-
*some* M series servos have a physical tach sandwiched under the encoder...only ever seen a couple, always on older, high-end machines...otherwise the 'tach' is generated in the control or on the drive via a freq/voltage converter. might pop the yellow cap and make certain you dont have a tach/brushes too...

[guhl]

guys, we forget the first post where was mentioned that with another PCB drive's behavior is fine, hence the problem in the PCB
I really faced with same problem three times in my shop and the reason was in yellow box
I suppose that characteristics of yellow box with time get down
BTW we have managed to make the copy of it using modern components

[jhoanrojas]

Thanks guys.

I found an isolated amplifier from a Fanuc 10T spindle drive, but I haven't changed it yet.

Yesterday I cleaned brushes and the machine worked fine for five hours.

Now I'm going to change the new pcb, it should work fine too. But if the trouble continues, i'm going to change the isolated amplifier.

I'll take out the servo motor for maintenance first, and i'll check the connectors.

Thanks again.