I have seen several instruments obtainined a HED error/fault during sequencing or during the tuning process. These instruments have been in various cuontries with varying electrical output, but all protected by high spec. UPS systems.
Occasionally the HED fault disappears for a while and functions normally then reappears agian maybe a month or two down the line. The HED volts are fine and below 1500 V (with a limit of 2500 V).
What causes these faults to occur?
I have also seen HED faults occur and the only way to fix them is to replace the HED power supply located in the MS circuit board case. We have tried changing the horns first but the issue was still there until the power block was replaced.
Again what are the causes for this issue? As we don't know why this is happening so we don't know how to prevent it.