Since the HGAG and WSA pots had no bearings, each one was placed into a functional test fixture and 5  VDC was applied across its input pigtails to energize the resistance element. While its rotor was manually  rotated, the wiper or output voltage of each pot was plotted on a strip chart recorder. All pots were then  placed in a vacuum chamber. Once evacuated the pots were subjected to three thermal cycles of -120” C  to +110” C. Finally, each flight pot was given a second room temperature functional test.  During the two functional tests, problems were noted with the following flight pots: MIL-PRF-39023-Style  S/N’s 1035 and 1041, HGAG Pot S/N 1017 and WSA Pot S/N 1039 (Note: all S/N’s were randomly  assigned by JPL). JPL Problem / Failure Reports (PFR) were initiated to describe the test failures. Shortly  after the PFR’s were initiated the cognizant engineer for the MER pots left JPL and I assumed her role.  Though I was experienced in the construction, cleaning and use of non-flight pots, I had no knowledge of  the MER pot effort prior to my taking it over. The remaining three sections of this paper discuss each of  the failure investigations and present lessons learned.  MI L-PR F-39023-Style Pots  The Initial Problem  Functional testing of the MIL-PRF-39023-style pots before and after the three thermal cycles revealed:  a) S/N 1035’s total resistance varied from approximately 5130 to 5720 Ohms as the shaft was  turned, when nominally the value should have remained constant. In addition, there was an open  circuit through the wiper terminal for all shaft positions, and  b) S/N 1041 showed an open circuit through the wiper terminal for shaft positions that should have  corresponded to voltage ratios between 0.0 and 0.1.  After duplicating the original functional test setup and now taking the proper precautions (see Lessons  1 - 6), the problems were verified, the out-of-spec electrical performance was characterized and a  “scratchy, detent” feel was noted as the shafts were rotated.  Figure 2. X-ray images of the SIN 1041 pot at increasing magnification show wiper damage  FeinFocus real-time X-ray images of the pots revealed wiper damage (Figure 2). Subsequent disassembly  of S/N’s 1035 and 1041 revealed that the resistance element wiper, slip-ring wiper and the conductive  plastic disk were all damaged due to high current and/or arcing (Figure 3).  The wipers showed arc scarring: pitting coupled with discoloration and warping due to overheating (Figure  3). The end of one S/N 1035 wiper finger was burned off and embedded in its conductive plastic disk. The  disks were damaged in two places: a) heavy arcing damage to the triangular shaped junction between the  “CCW” end of the resistance element and the mound of conductive epoxy connecting the “CCW”  terminal’s flat ribbon wire, and b) light arcing damage to the slip-ring portion of the disk where one of the  two slip-ring wipers was located. The carbonized shell of a “bubble” was observed across the wiper tracks  on S/N 1035’s conductive plastic disk where vaporized disk substrate “puffed up” the overheated top  layers. There was a trough across the wiper tracks in S/N 1041’s conductive plastic disk where the  substrate was vaporized away.  305