AN MLG PLT RPTS THAT A WRITE-UP IN HIS ACFT LOGBOOK WAS SO POOR THAT THE WRONG PART WAS CHANGED LEAVING THE ORIGINAL PROBLEM UNSOLVED. PIC WAS 'LED DOWN THE PRIMROSE PATH.'

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ON ARR AT ACFT FOR PREFLT; I FOUND MECHS ON BOARD. THEY WERE LEAVING WITH THE ACFT LOGBOOK. THEY TOLD ME THAT THEY HAD TO REPLACE THE STANDBY ALTIMETER AND AIRSPD INDICATOR. THEY TOOK THE LOGBOOK INTO THE OPS BUILDING; LATER RETURNED TO THE ACFT; REPLACED THE STANDBY ALTIMETER AND AIRSPD INDICATOR. I LOOKED AT THE LOGBOOK; VERIFIED THAT ALL THE APPROPRIATE BOXES WERE CHKED AND SIGNED OFF. WE DEPARTED AND ALL WAS NORMAL UNTIL DURING DSCNT WE WERE CLRED TO 4000 FT AND ATTEMPTED TO SET THE ALT WINDOW IN THE MODE CTL PANEL TO 4000 FT. IT STUCK AT 6400 FT. ONLY THEN DID I REALIZE THAT MAINT HAD PERFORMED AN INCORRECT SIGNOFF PROC. THE ORIGINAL WRITE-UP IN THE LOG WAS WRITTEN IN AN ABBREVIATED FORM -- 'ALT ALERTER STICKS AT 6400 FT.' MAINT MISREAD THE WRITE-UP AND THUS CHANGED THE WRONG PART. I WROTE UP THE ALT ALERTER UPON LNDG AT THE NEXT DEST. THE ABBREVIATED WRITE-UP LED THE MECH TO DO THE WRONG THING. I READ THE WRITE-UP TOO; BUT AFTER HAVING ALREADY BEEN TOLD WHAT THE PROBLEM WAS. I MISUNDERSTOOD THE PROBLEM; AND SINCE THE WRITE-UP WAS SIGNED OFF IN THE ACFT LOGBOOK; I ASSUMED ALL TO BE IN ORDER.