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XMT 350 Failed Bus Capacitor C13

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  • XMT 350 Failed Bus Capacitor C13

    I am troubleshooting a XMT350, SN LF450328A that stopped working (operator said no display) during very heavy use (hard facing, operator was exceeding the recommended duty cycle). The vent was blown on the upper bus capacitor, C13, with electrolytic showing on the back side of PC2 between C13 and PC2. The welder passed all the pre-power checks except for the bleed resistor R1 was reading 10M ohms instead of 39k ohms (there are no visual indications that this resistor failed). I haven't checked MOD1 and MOD2 IGBTs with an IGBT tester, not sure if it is worth removing PC2 and unloading the pressure on the heatsink compound for the two mods. If the welder passes pre-power checks after replacing R1 and C13, are there other checks I should do on the MODs or PC2 gate drive or PC1 control circuits before powering it up?

    UPDATE: I talked with Miller Tech - will replace both bus capacitors and the bleed resistor. I am planning to replace the bus capacitors and bleed resistors, conduct pre-power checks, then power it up with a scope measurement across the boost igbt and the DC bus voltage to confirm it isn't overcharging and an idea of the IGBT voltage drop vs current. Any thoughts on this plan? Is there any value in testing the IGBTs before powering up the welder?

    Second Update- A few weeks later and all is still going well. Talking with Miller Tech, the original bleeder resistors did not have the lifespan they were expecting, and upgraded later equipment to MILSPEC resistors. The original were the black tubular ceramic, and the upgraded ones are brown-orange. Between the new caps, new bleeder resistors, and preventive maintenance recommendations from Cruizer and ccawgc, I definitely have the peace of mind in using this for a while.

    Thank you,
    Last edited by jjohn76; 11-18-2018, 08:08 AM.