Mori SL-1 spindle drive varispeed 505mt needs exorcism

Hi everyone, my first post, but I've spent hours and hours reading these forums, fixing my machines using the info I obtain for free. Thanks to all who work together here to merge knowledge bases, and past experiences into usable data for all of us.

This post is in regard to Mori seiki SL-1 No.499
Yasnac 2000b(single line) control
yaskawa drives

The issue I'm having goes something like this:

Power machine up
Power control up
power servos up
Machine is in ready state.
Wait.
Wait s'more
Wait s'more
(About 2-10 minutes)
Spindle system begins to buzz on and off(just a little juice actually flowing to the motor)
This symptom begins to get worse the more time elapse
The buzz and movement of the spindle becomes longer in duration(instead of a dot it becomes a dash)
Eventually the spindle begins continous rpm and gradually rises to approx 1100rpm.
At this point the spindle may continue to run at 1100rpmfor an hr or sometimes just a few seconds
The spindle will then slow to 0rpm briefly and then back up to 1100rpm



Sorry for the poor use of space there, I wanted the symptoms to read, just like they happen in hopes somebody familiar with this drive or someone who experienced this same problem in the past could shed some light on my demon drive.

Voltages all check out in terms of power into the drive and exciter.
Tach gen functions properly.
All relays seem to function correctly.
N.C. signal is always 0v during these outbursts.

If I immediatly fire up the spindle with the jog dial when I power the machine up the spindle works perfectly. No funny noises, no erratic rpm. It's rock solid......for about the same 2-10min before it becomes uncontrollable and acts exactly the same as previously described.

So please explain to me how a drive that is seeing no signal to the drive, drives anyways? Leaky scr's?

I guess I should also state that the machine never errors out. It will run on forever in the manner described without setting any codes, nc or spindle drive.

Well, after untold hours spent in the drive cabinet, I finally grew the balls to put the drive board from my Sl2 in place of the current board. This was only after I was absolutely sure the machine wasn't going to damage the other board. Good news is that ot fixed the problem completely. Bad news is now I have to go through both boards side by side woth a multimeter and see if I can figure out whats toasted.

Good luck and let us know what you find out, glad it worked out that you had something else to try it with.

Charles

Well, I've got a huge cup of coffee, a multimeter, and 2 giant boards. Preliminary tests reveal two possible problems. There are 4 small 24v matsushita relays(tiny smt type), one of them seems to have continuity on both contacts, and one transistor seems to conduct oddly compared to others(in testing).

Lawrenceville eh? You know buddy at amts?

Whale. I've spent about 2hrs here amd there when I've had time testing back and for on each board to compare readings in different circuits and not much progress.
Heres what a I think I understand:
There are about 4 parts of this circuit board.
The Igbt and pulse coil output section to the thyristors
The tach gen comparison section
The current amplifier comparison section
The "logic" input output section

Due to the actual fault symptoms I've come to the conclusiin that the board is failing to keep it's "gate blocking" circuit functioning properly.
The board runs the spindle smoothly, identically to the way the good board does.
The motor runs at what I now know is it's base speed, meaning that there is no input from the field weakening system.
This is fact. You run the stated tag voltage of a motor to it, it will run at base speed, in my case 1150rpm. Thats what the motor tag says, and thats what the tachometer says the motor is spinning.
This tells me that the board functions properly, in terms of sending proper phase and voltage control to the motor.
The problem lies in it's ability to arm/disarm the thyristor firing circuit.
So, I'd bet money that the problem has to do with a simple switching action of one of the small relays or transistors controlling this circuit.
I can't find schematics anywhere, and the yaskawa site had 1document, a 34 page user manual that while being a nice thing to have, doesn't help me one bit. I mean, theres check pins right on the board, and I assume a very simple test procedure that if you had access to it would take 20 minutes to give the entire board a diagnostic. Please excuse any grammatical errors. I typed this on a damn smart phone.

Oh god....noooooooo........oooooooooo
Not the curse of the unknown repair!! Anything but that!

So.... I fiddled and tapped and rapped, I probed and checked and compared, I finally gave up. I contacted a couple of repair firms in an attempt to get the a quote on repair.
Nobody called me back.
I put the board back in the machine, hooked everything up, and just for kicks I powered it up.
The fu+%&#ng thing works perfect now.
I have no idea what I did. The only thing I can come up with is that the evil spirit that was inhabiting my board viewed my probing as some sort of incantation or voodoo and decided it was time to vacate.
It's either that, something onto thea board was loose or a tiny piece of swarf was lodged in the board somewhere and with all of the flipping over, standing up, tapping and probing I must have knocked it loose.

For those of you who aren't aware what the curse of the unknown repair is, it goes something like this:

You tinker with something that has malfunctioned or is giving you trouble, you attempt to repair said item. When you try to start(or power on) said device it works.
Heres the curse part: Now that you've fixed it, you put it back to work and everything is wonderful. Then the problem rears it's ugly head a few months later. Well, that shouldn't be a problem right, you already fixed it once. Ahh but now, try as you might you can't seem to work your magic this time. So you keep going, keep trying, because you know that you already made it work just by dicking with it for awhile. If the curse is strong, you will manage to fix it again, looping you back through to the begining of the hell your now trapped in because as we all know, there is no way in hell that you're going to pay someone a giant pile of money to fix it when you know that it just needs a simple fix.