LOW IFR TIME PA28 PLT DSNDS FROM ASSIGNED ALT WHEN ON APCH TO ABI DUE TO LOSS OF SITUATIONAL AWARENESS AND LESS SEVERE EQUIP PROBS.

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I AM A NEWLY IFR RATED PVT PLT WITH 4.5 HRS OF ACTUAL IMC; 49.3 HRS SIMULATED INST; 180.5 HRS PIC TIME. I WAS PIC OF A PA28-140 AT ELMDALE ARPT; (6F4) ABILENE; TX. WE DEPARTED ELMDALE AND I RPTED AIRBORNE AS I TURNED TO 120 DEG HDG AND CONTINUED CLBING. WE WERE QUICKLY IN THICK IMC. APCH VECTORED ME AROUND TO THE N AND WE WERE ON TOP OF THE OVCST AT ABOUT 3800 FT AND I CONTINUED TO CLB AND LEVEL OUT AT 4000 FT. APCH VECTORED ME ONTO THE 020 DEG RADIAL OFF OF TUSCOLA; TQA; VOR. I WAS CLRED TO DSND TO 3100 FT AND WAS ADVISED I WAS 7 MI FROM FAF; BOWIE INTXN. AS I DSNDED INTO THE CLOUDS I WAS BUSY CTLING THE AIRPLANE AND KEEPING UP MY SCAN. APCH ADVISED ME TO EXPECT CLRNC TO RWY 17R. THE AIRPLANE DSNDED BELOW THE 3100 FT I WAS CLRED FOR. I FIRST NOTICED THAT I WAS AT 2600 FT BECAUSE I HAD DSNDED BELOW THE CLOUD. I WENT FULL PWR AND BEGAN CLBING; BUT WAS AT INDICATED 2500 FT BEFORE I COULD REVERSE MY DSCNT. I NOTICED THE PRISON WATER TWR AND KNEW I WAS WELL ABOVE ANY OBSTRUCTIONS SO OUR SAFETY WAS NOT SO MUCH A CONCERN TO ME AT THAT POINT AS THE FACT THAT I KNEW I HAD INADVERTENTLY BUSTED A MINIMUM ALT. I SET MYSELF UP FOR TROUBLE WHEN I CHANGED THE WAY I HAD BEEN TRAINED TO TRIM BACK TO 80 MPH IAS AT 2200 RPM WHILE MANEUVERING FOR AN APCH. APCH ADVISED ME WITH A LOW ALT ALERT AND I ADVISED I WAS ALREADY CORRECTING. APCH TOLD ME TO TURN R TO 270 DEG HDG AND CLB AND MAINTAIN 4000 FT AND ADVISED THEY COULD NOT CONTINUE THE APCH SINCE MY XPONDER HAD STOPPED WORKING. I TURNED TO 270 AND CLIMBED TO 4000 FEET. I ADVISED APPROACH WE WERE TRYING TO FIX THE TRANSPONDER PROBLEM AND ALSO MENTIONED I HAD JUST HAD IT WORKED ON. I NOW HAD BROKEN SKY BELOW AND I ADVISED APCH I WANTED TO GET BACK ON THE GND TO TROUBLESHOOT THE XPONDER PROB AS IT WAS NOT A BLOWN FUSE. I ADVISED I WOULD LIKE TO CANCEL IFR CLRNC AND GET A SPECIAL VFR CLRNC FOR DIRECT FLT BACK TO ELMDALE BELOW THE OVCST. APCH ASKED FOR A POS RPT AS WE MANEUVERED FOR A L DOWNWIND. WE LANDED AND RPTED THAT TO APCH. THE NEXT DAY I PERFORMED SOME TROUBLESHOOTING AS I HOLD AIRFRAME AND POWER PLANT MECH LICENSES. I FOUND THE POSITIVE LEAD TO THE KT-76 XPONDER ALSO FED THE LORAN AND HAD A COLD SOLDER JOINT TO THE FUSE HOLDER AT THE BUSBAR. THIS COLD SOLDER JOINT WAS DETERMINED TO BE THE CAUSE OF THE INTERMITTENT OP OF THE XPONDER. I MADE APPROPRIATE REPAIRS. IN ORDER TO PREVENT GOING BELOW ASSIGNED ALT ON AN APCH I AM GOING TO IMPROVE MY SCAN WITH MORE PRACTICE. I AM ALSO GOING TO SLOW DOWN TO THE LOW CRUISE (80 MPH AT 2200 RPM) BEFORE REACHING THE INBOUND RADIAL OR LOC. THIS SLOWS THINGS DOWN FOR ME AS A BEGINNER AND MAKES IT SIMPLER FOR ME.