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Thread: TB 251 no output.

  1. #1
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    TB 251 no output.

    I have a Trailblazer 251 that welded fine a couple of weeks ago, but today there is no output on a/c d/c or aux power. I checked the CBs on the front, but there is no power to them. I also checked the fuse under the hood and it has continuity across it. The brushes are making good contact, and the slip rings look good. I also have continuity from the sliprings across the brushes to their respective wires. It is a TB 251, Ser no.kg219496. Also, i did not jump start it.
    Any help would be appreciated.

  2. #2
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    Re: TB 251 no output.

    The main common denominator between weld and auxiliary output is the center (+) rotor slip ring that receives 12VDC battery voltage upon start up for initial flashing.

    Without it - no output. So check for 12VDC+ at the center brush lead while cranking and following start up. If no voltage there, the first thing to check is flashing diode D2.
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  3. #3
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    Re: TB 251 no output.

    Duane, I grabbed the manual and went out to the truck and this is what I found; Sliprings and brushes are good, rectifiers SR2 and SR3 are good, disconected leads 23 and 24 from brushes and there is continuity, Capacitor C1 is not open, diode D2 is good. I began the troubleshooting the pcb and got as far as checking voltage between pin 1 and 8 on RC 3. I get no voltage here. This is how far I got before the rain came.
    Any ideas?

  4. #4
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    Re: TB 251 no output.

    From your Miller forum thread:

    Voltage may or may not be present at RC3 pins 1 & 8 depending on the position of the CONTACTOR switch and/or if you have a remote connected or not.

    However, you're getting ahead of yourself. Did you check for battery (+) voltage between the center brush and ground with the unit running? If no voltage there you have to find the open in the flashing circuit. For instance, check the connection at PLG5/RC5 pin 3.
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  5. #5
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    Re: TB 251 no output.

    Yes, there is 13 volts present there. Interestingly I was probing at the plug that goes down to the brushes (wires 21/24) and one of the connector wires (21) came out in my hand. I reattached it, but still no output.

  6. #6
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    Re: TB 251 no output.

    Continuity between wires #23 and #24 is only part of the picture. More importantly, is there continuity between the center (+) slip ring and #23 and #24.

    I don't have the specific values for the TB 251 but resistance between the center ring and ring for #23 should be around 3.5ohms and between the center and #24 ring around 30-31ohms if my guess is correct.

    If the above continuity and resistance checks are OK, check for AC voltage between SR2 and SR3 wires #43/44 and #22/29 respectively with a focus on SR3 power input on #22/29. Again, no specifics on the TB 251 but I'm guessing +/-30VAC at weld rpm.

    Make those checks and we can go from there.

    FYI, if you contact Miller Customer Service with your unit SN, they will likely email you the Technical Manual for it if you ask.
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  7. #7
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    Re: TB 251 no output.

    The list of checks I did was out of the tech manual for it. I will check those items tomorrow.

    Thanks

  8. #8
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    Re: TB 251 no output.

    If you have the Technical Manual it will have the specific resistance and voltage values for the checks I listed.

    Sent you a PM.
    Last edited by duaneb55; 08-23-2012 at 08:39 AM.
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  9. #9
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    Re: TB 251 no output.

    I went out this morning to finish trouble shooting my TB251. Yesterday I had voltage at lead 21 going to the center slipring, however the molex connector, through which I was probing to get the reading came apart while I was doing so (unit is running), so I grabbed a jumper wire with alligator clips and reconnected it (unit still running) and still had no power. This morning, I checked for continuity from the center slipring to the other, which I had, and then repaired the broken connection at the lead 21 connector. I fired it up and now have voltage everywhere! Hurray!
    This repair, with a bullet connector is temporary, as I dislike having cobbled connections like that, and I need to find molex pins to fix it with. It is running right now to warm up, and I will check my run speed (via hertz on the recepticle). Is there anything else I can do to protect my $652 circuit board, aside from not jump starting and checking run speed from time to time?
    Also, thanks a bunch for the help, it saved me a lot of time trying to figure it out alone, and likely a lot of money buying a new board.
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