Old Millermatic 200 question

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My buddy has two older [1980's?] Millermatic 200s. One will weld fine but when the trigger is let go the MIG wire stays charged with low voltage and sparks against a ground. He thought the trigger on the gun was faulty. But when he puts that gun on the other machine, it works fine.  I said that if the wire feed and gas valve stay off, but the wire stays hot, there is an internal problem. I see on the parts sheet a relay inside. Is that for the arc current? Can I assume that when the trigger is pulled that enables the internal relay to power the arc current? And that there is probably an internal short that is letting low voltage get to the MIG wire?
Reply:Does it spark once, or repeatedly? If only once, it may just be residual charge on the capacitor bank. If it keeps being re-charged ("but the wire stays hot"), something is letting current through the charging system. To know what schematic applies to that machine, we either would need the serial number or a good copy from the supplied manual which should be appropriate to that version.
Reply:My MM200 does that, one fairly small spark if I touch the wire to the grounded work right after finishing a weld. I always assumed it was a small residual charge in the caps as Oldiron said.