A PA28 PLT RPTS THAT INFLT BOTH VOR RECEIVERS INDICATED 15 DEG ERRORS FROM THE V195 AIRWAY CTRLINE. BELIEVES LCL AVIONICS SHOP IS USING INACCURATE TUNING EQUIP.

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A DUAL VOR RECEIVER CHK WITH A 0 DEG ERROR WAS PERFORMED AND LOGGED FOR AN IFR FLT TO SCK. INFLT BOTH VOR RECEIVERS INDICATED COURSES 15 DEGS DIFFERENT FROM THE V195 AIRWAY CTRLINE. DURING THE IFR DEP NORCAL APCH CALLED TO CONFIRM THAT THE ACFT WAS ESTABLISHED ON V334. AT SJC-R009-10 DME-FIX NORCAL APCH ISSUED A CLRNC 'DIRECT ECA.' VOR RECEIVER #1 SHOWED THE DIRECT COURSE TO BE 060 DEGS; WHICH DID NOT MATCH THE COURSE EXPECTED FOR THAT LOCATION (A COURSE OF 045 DEGS WAS EXPECTED). AT THE SAME TIME THE ATTITUDE INDICATOR FAILED; ATC WAS ADVISED OF THE MALFUNCTION; AND THE IFR FLT PLAN WAS CANCELED. THE TRAINING FLT CONTINUED TO PERFORM A PRACTICE INST APCH AT TCY. IT THEN PROCEEDED TO TRACY INTXN; THEN BACK TO ZZZ VIA V195 UNDER VFR FLT FOLLOWING. WHILE VOR RECEIVER #2 WAS INDICATING ESTABLISHED ON THE V195 CTRLINE AT 3500 FT MSL (2200 FT AGL) THE SAFETY PLT VISUALLY NOTICED A POSSIBLE PENETRATION OF THE N BOUNDARY OF R-2531 SPECIAL USE AIRSPACE. THE SAFETY PLT TOOK CTL OF THE AIRPLANE AND EXECUTED AN IMMEDIATE CORRECTIVE ACTION BY CLBING TO 4000 FT MSL AND CHANGING COURSE TO THE N. THE FLT PLAN THEN CONTINUED TO ZZZ AT 4500 FT MSL WITHOUT FURTHER INCIDENT. THE SAME TYPE OF DUAL VOR RECEIVER ERROR OF 15 DEGS HAPPENED DURING AN IFR FLT ON DEC/SUN/05; IN A DIFFERENT AIRPLANE SVCED BY THE SAME OPERATOR BASED AT ZZZ. 4 SEPARATE VOR RECEIVERS INDICATED AN ERROR OF 15 DEGS WHEN LOCATED WELL INTO SEVERAL NAVAIDS STANDARD SVC VOLUMES. THIS GIVES STRONG EVIDENCE THAT THE AVIONICS SHOP SVCING THE AFFECTED ACFT IS USING TUNING EQUIP WITH INACCURATE CALIBRATION. THE ONLY TYPE OF VOR RECEIVER CHK AVAILABLE AT ZZZ WILL NOT BE ABLE TO DETECT PROBS OF THIS NATURE.