LESSON 16. When a design uses one feature for multiple purposes (like the circumferential groove  in the lid which was used both to retain the lid and to provide a labyrinthine path for venting), make  sure all the effects of even the smallest design change are well understood before approving it.  Turnina Gold Into Base Metals  Prior to delivering the pots to the end users I noticed that the original cognizant engineer ordered the MIL-  PRF-39023 pots manufactured to a drawing which specifies that its solder terminals are made from brass  which has first been silver plated, and then very thinly gold plated.  JPL’s internal requirements state that gold shall not be used as a surface finish for soldering, so I  instructed the end users of the MIL-PRF-39023-style pots to “degold the terminals before soldering.  LESSON 17. Soldering to gold can form brittle intermetallic compounds that can crack under thermal  cycling. Gold plated conductors must have their gold plating removed by immersion in a designated  solder pot, then tinned using the same processes in a different solder pot. Alternatively, gold plated  leads must be tinned twice with solder wire, wicking off the solder in the first tinning to remove the  gold, then tinning again.  Trust Nobodv With Your Fliaht Hardware  About this time MER’S Deputy Mission Assurance Manager requested a private meeting to better  understand the pot details described in the PFR’s. Afterward this manager asked to borrow several of the  pots that had been written up, and I foolishly obliged. When I asked for my pots back, S/N 1036 could not  be found: Mission Assurance had lost my flight hardware!  LESSON 18. When cognizant of flight hardware, especially small hardware, never let it out of your  sight, and never, ever lend it to management.  HGAG Pots  Functional testing of the HGAG pots before and after the three thermal cycles revealed that on S/N 1017  the total resistance varied from approximately 5440 to 6190 Ohm as the rotor was rotated. The total  resistances of the other HGAG pots tested did not vary with rotation angle. Nominally the total resistance  should remain constant.  After duplicating the original functional test setup and now taking the proper precautions (see Lessons  1 - 6), the problem was verified.  Next the build documentation was examined to determine if it held any clues to this anomaly. The pot  manufacturer’s ATP describes the resistance check, but does not mention rotating the rotor, nor does it  specify that the total resistance must not vary with rotor angle. The spec only called for a total resistance  of 6250 Ohms * 20%. The in-process data sheet filled out by the pot manufacturer listed the total  resistance as 6046 Ohms. After environmental testing, the in-process data sheet listed the total resistance  as 6121 Ohms. 6121 Ohms represented a 75-Ohm increase which was not in keeping with the rest of the  pots in that lot which showed total resistance changes of no more than 5 Ohms. Random rotor angles  during total resistance measurement explained the 75 Ohm change, and indicated that this problem may  have been inherent at assembly.  S/N 1017 was still a flight pot, so I was in no rush to tear it apart. Besides, FeinFocus X-ray images of SIN  101 7 revealed no internal damage.  Instead I elected to do more testing. The pot was powered with 1.0 VDC across the input wires and the  wiper voltage was measured and recorded as the rotor was rotated through 360 degrees in 5 degree  increments. The wiper voltage was indeed the required linear function of rotor angle. Next the total  resistance was measured and recorded as the rotor was rotated through 360 degrees in 5 dagree  increments. The total resistance was a smoothly decreasing function of rotor angle (Figure 9).  31 3