ACS Side-1 Anomaly Review Board Conclusions

1. ACS Side-1Anomaly Review Board Conclusions STScI TIPS Meeting 21-September-2006 Ken Sembach Subset of conclusions as summarized in the final ACS Side-1 ARB Report (some items condensed / omitted for this presentation)

2. Anomaly Review Board Members Consultants

3. Conclusions • The following facts are based on on-orbit data at the time of the anomaly: • All CEB telemetry collected by the Normal Engineering Data (NED) task went from valid data to all ones within one sample period. • No indication of anomalous secondary voltages prior to ACS Suspend • No indication of thermal rise prior to and following the anomaly • Anomaly nearly instantaneous; occurred within 100 ms window • Anomaly caused the +28V Bus current to decrease ~ 0.8 to 1.3 Amps • No out-of-family behavior was observed in the science data prior to the Suspend event • The most likely cause of the ACS Side 1 on-orbit anomaly was a loss of the ACS Side 1 CEB +15V power rail in either an open or shorted condition. This conclusion is substantiated via • engineering evaluations • focused ground tests • historical performance • on-orbit telemetry collected at the time of the anomaly

4. Fault Tree Investigation: LVPS Board #3 • Loss of CEB Power Rail: • Both WFC and HRC CEBs supplied with power from common LVPS +5V, +15V and -15V source. • Loss of the +15V power rail, with or without degradation of the -15V power rail, results in uncharacteristic performance of CEB A/D conversion and associated serial data stream. • Ground testing of Engineering CEB unit resulted in reproduction of on orbit anomaly

5. Most Likely Location of Fault • The ARB identified LVPS Board #3 as most likely location of the cause of the ACS on-orbit anomaly • LVPS #3 contains all of the active components that produce and/or interface with the suspected +15V CEB power rail • However, other possible (less likely) locations exist: • MEB Backplane: +15V trace to ground fault • MEB to CEB harnessing: +15V wire short to ground fault • Short on +15V Input to a CEB prior to power switching relay

6. Possible Fault Locations Most likely location of root cause of anomaly Cannot be rule out the possibility that the source of the anomaly may be located in this area LVPS#3 S/N 001 CEB PWR MEB2 Backplane HRC CEB Short Side Select Relays 2 pins Cable Harness 538548 +15V Rail Short or Open Short or Open 2 pins +35V Rail Same as +15V WFC CEB -15V Rail Side Select Relays Same as +15V Short 6 connector pins per power rail

7. Fault within MFL2815D Fault on LVPS #3 Board, not in MFL2815D Fault mode matches on orbit anomaly signature Anomaly Review Board Investigated Possible Failures

8. Conclusions • A thorough component-level Failure Mode Evaluation and Analysis (FMEA) was conducted by the ARB on the ACS LVPS3 Board circuitry related to the +/-15V CEB power rails. • The FMEA results were compared to a set of voltage rail responses obtained via CEB Engineering Unit lab tests to determine if the postulated failure mode could have resulted in the ACS on-orbit anomaly. • Based on the FMEA results, fourteen ACS LVPS3 board parts (nine distinct part types) and associated failure modes have been identified as possibly resulting in the ACS on-orbit anomaly. • The most likely cause of the ACS failure resides within LVPS Board #3, component U2-7, the MFL2815D Interpoint DC-DC converter. The next most likely cause is a short to ground in the transformer T6 on the same LVPS board.

9. Conclusions • ARB quality record review • Identified an MEB level thermal cycle test failure that matched the on-orbit failure scenario exactly. A short occurred on the +15V rail at the +35V T6 step-up transformer. • The ARB was concerned to find that after the initial replacement of transformer T6, the anomaly again repeated with the new transformer during re-testing with no further part removal and replacement. The ARB has no way to determine quantitatively what potential stress and possible latent damage may have occurred within the T6 transformer over the 3 minutes of heating that it was subjected to during the re-test. • The ARB was concerned to find that the flight LVPS3 board was subjected to uncontrolled heating effects by a heat gun as part of the anomaly investigation. It is conceivable that this anomaly investigation technique produced thermal stresses that resulted in a latent failure on the flight LVPS3 board in question.

10. Conclusions • ARB quality record review (continued) • During ground test inspections, an investigation of the LVPS3 board showed that the board had separated between the Ground/Thermal Plane and the fiberglass board. The ARB cannot help but wonder if such a separation could have occurred that ultimately led to a shorting condition or a stressed component solder joint that resulted in the ACS on-orbit anomaly. Such separation could have been aggravated by thermal transitions. For example, ACS is powered down approximately once every month to perform anneal operations, at which time the LVPS board temperatures transition approximately 30 degrees with the current 6-hour anneals.

11. ARB Recommendations • Modify ACS flight software so the NED history buffer contains 16-bit values exactly as read from the A/D FIFOs. Specifically, do not apply a bit-mask to the data prior to storage in the buffer. Storing the data in an unmodified form would allow better insight into the MEB/CEB interface. Make similar flight software changes to COS and WFC3. (NICMOS and STIS do not have NED history buffers.) • New CARD items should be written to prevent invalid “hybrid” power configurations. These include: • ACS only: prevent commanding of the “off” CEB • ACS and STIS: prevent hot switching the CEB internal relays. • Investigate reducing thermal and power cycling by: • Leaving the HRC CEB powered on while doing SBC operations. • Reducing the number of anneals • Performing anneals in a “hybrid” mode that would leave Side-2 electronics powered while annealing from Side-1 • Investigations have been referred to the Hybrid LEI Task and the STScI ACS Team.