X axis transistor failed...AGAIN

[Mr. Technical]

Hi all,

I put in the replacement transistors to replace all of the X-axis transistors as a set. After replacing all 4, I was able to jog (or step, can't remember) the X axis! 10 seconds later I tried it again and the axis wouldn't move. So that's what frustration is...

Any help on what's happening here so that I don't keep replacing transistors? The same transistor failed both times. It's the bottom one on the X panel (is that Q4?) I can order a more robust transistor, but that seems to be fixing the symptom and not the disease.

I have the readings for T2 output voltage checks that I did at home, and I think that one or two of the numbers were a little high. I will post the numbers from the different terminals a little later today to see if that helps. What exactly causes the transistor to pop, and how can I track it down? Is it over current or over voltage, or reverse voltage or current spikes? I've got the schematics, but don't know what the common culprits are.

Would now be a good time to remove FU12, stick an ammeter inline there, and adjust the current at P1 of the SMD board to 8.2 amps? I'm a little hesitant to do that, but I may have to. Please let me know what you think. Thanks!!

Ian

[Mr. Technical]

I meant to say: Would now be a good time to remove FU12, stick an ammeter inline there, and adjust the current at P1 of the ZCK board to 8.2 amps?

By the way, this is a Series I Boss 6 with an SMS board. TIA,

Ian

[Mr. Technical]

The other night I performed the electrical checks listed in the maint. manual to diagnose the first time that the transistor blew. On the output of the T2 transformer (terminals 5 and 6), here's what I got:

5A-6A= 58V
5A-6B= 57V
5A-6C= 53V
5B-6A= 53V
5B-6B= 58V
5B-6C= 55V
5C-6A= 68V
5C-6B= 66V
5C-6C= 70V

According to the manual, these numbers should be between 57 and 63V. The numbering convention is due to the fact that each number has 3 terminals to measure from (A,B,C as viewed from left to right).

As you can see, values from the 5C terminal to all of the #6 terminals (listed in red) are outside of the allowable range. Does that point to dirty power from the wall (actually a rotary phase converter), a bad transformer or what?

I'd like to put this thing to bed, so any input, even if it's only part of the solution would be very helpful. Thanks to all!!

Ian

[Mariss Freimanis]

You also may have a motor with an intermittent short in the windings. That will take transistors out.

Mariss

[Mr. Technical]

That's a good possibility Mariss. How do I rule that out?
Ian

[HillBilly]

The motor current is adjusted at the ACC board. You can adjust the current down for trouble shooting.

There are always 2 final transistors on at a time. I had a SMD board with a bad predriver that would make one final transistor try to carry the full 8 amps, blowing it everytime. By reducing the current I was able to trouble shoot it under power without blowing the final.

Darek

[machintek]

The 1st and 4th wire are X axis, The 2nd to 5th is the Y and the 3rd to 6th is the Z. look at it that way. 58 VAC is great!

George

[Mr. Technical]

I swapped fuses F12 with F13. Then I pulled the F12 fuse and put an ammeter across it and got 8.7 amps. I dialed that down with P1 on the ACC board to 8.1 amps. In the meantime, I blew the top transistor on the X board. Once I replaced that, the X axis will step, and hasn't blown the transistor yet! I'm still not sure if I'm entering the step mode correctly though...

Going onward from here, I'd like to do the following:
-Replace the screw in type fuses F12, F13, F14 etc with snap in type holders. Anyone have a good part number for those? I typically shop at Mouser.
-Order replacement fuses for the above holders. Again, any specs or part number available on those?
-Order some more transistors with a higher rating to keep on hand.
-Turn down the rapid speed on the ZCK board using Pot 3, as a preventative measure.
-Install Hillbilly's BOB which just arrived as I was typing this post!!
I am thinking that the BOB will not affect the problems that I've been having with the ACC board, since the BOB is more of a brain transplant and not a muscle transplant, if that metaphor makes sense.

If there are any other good steps to take as preventive measures I'm all ears.

Should I be concerned about the 70 VAC on the Z axis? No problems with that axis yet.
Thanks to everyone for their continued ideas!
Ian

[Mr. Technical]

Well, the X axis transistor keeps popping. I'll do all the things that I stated in my last post, but I need to take a break and think things through.

At this point I'm trying to decide what's the best thing to swap with the Y axis. motor, SMD board... Any more suggestions?

BTW Hillbilly, is your BOB likely to help me rule out any likely culprits on the control side of things if I install it before the X axis is fixed? Would it hurt anything to install it with this problem still in the mix?

Thanks to all for any input.

Ian

[NC Cams]

By replacing transistors, you may be curing symptoms as opposed to cause.

How much current is REALLY being asked for by the device that is connected to the failing transistor?

Is there binding that is causing high current draw? What about motor? Short, siezed bearing, hang up in drive mechanism, whatever?

Could there be a short on the SMS board? Or the downstream wiring?

Just more ideas to look for/at. Sometimes it takes a keen eye for the obvious to figure out what's REALLY wrong.

[Mr. Technical]

At this point, it seems to me that the most likely culprit is the X-axis motor, bridge rectifier 3, or the 'wild leg' of the rotary phase converter. I swapped the X and Z SMD boards, and there were no problems on the Z axis, so that seems to rule out the X SMD board. Also, I plan to flip the ACC board, which should swap the X and Z inputs to rule out the ACC board too. On a side note, when I removed the rubber seal on the X axis motor (around the cable) a tablespoon or so of oil came out. I didn't read any shorts across the wires though. I'll keep pluggin' away!

[signweld]

In my machine, I use the 2 "normal" legs for powering the transformer that runs everything except the 3 ph motor. In my case, I just turn on the disconect on the back of the machine without starting my 3 phaser. You will find that all the controls and table movement work fine and that you really only need the third leg to run the spindle. This wliminates problems related to the wild leg voltage / current.