The cause of H# value failure on the Beckman LS6500 may be due to a problem with the front end electronics, including the PMTs, pre-amp housing assembly, or high voltage power supply. However, it could also be due to a problem with the Cs137 source drive itself. To isolate the issue and confirm if the front end electronics is functioning correctly, perform a simple CPM count to check for valid sample counts. If it fails, the likely cause is a faulty pre-amp board in the pre-amp housing, specifically the board on the left-hand side near the cooling fan.

If the front end electronics are confirmed to be working correctly and the problem persists, check the mechanical movement of the source spring responsible for moving the radioactive source from the pea to the pig to ensure it is moving smoothly and not slipping. If the problem continues, it is recommended to seek the assistance of a trained expert for further investigation.

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