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Thermo Fisher Scientific

Mono instability in Y direction


Mono instability in Y direction.


Titan 80-300


These are possible resolutions for the issue:

  • Replaced 3 boards of GMS, DPA and SDCA.


These are possible causes for the issue:

  • Boards defective.


Emitter was replaced 2 months ago, at that time FEG can’t be started, it had communication problem, replacing GMS, SES, CPB boards fixed it.
Now FEG emission current = 589 uA, before it was about 400 uA.

To determine if the instability source is HT or FEG or charging related: 

  1. HT emission current = 0 uA.
  2. Back focal plane check per WI-300259 at 300 and 80 kV. 
  3. HT AC stability test per WI-103190 in service and normal mode by HCA X‐100 jumper, normal mode OK (< 500 mVpp). 
  4. HFP test per WI-105031, Ibleeder and Imeasuring-resistor and Cascade OK. 
  5. FREF test from TAD, OK. 
  6. Follow WI-106115 “Mono instability TFS”, and goes to HT conditioning, not favorable. 
  7. Source of instability confirmed to be in FEG (WI-306046, WI-106115).





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