KTR-953 Service Bulletin 4 addressed the replacement of the original crystal over assembly in the unit with a more accurate, higher reliability Temperature Compensated Crystal Oscillator (TCXO) assembly. The TCXO provides instant-on operation eliminating the need for the 2-3 minute warm up time as with the original crystal oven assemblies. The effectivity for this SB is as follows:

• KTR-953 P/N 064-1015-00 units with serial number 3196 and above have SB 4 incorporated during original manufacturing.
  
  
• KTR-953 P/N 064-1015-00 units with serial numbers between 2940-3195 have SB 4 incorporated during original manufacturing but were not stamped.
  
  
• KTR-953 P/N 064-1015-01 units with serial number 25644 and above have SB 4 incorporated during original manufacturing.
  
  
• KTR-953 P/N 064-1015-01 units with serial numbers between 25350-25643 have SB 4 incorporated during original manufacturing but were not stamped.

The cost to incorporate SB 4 is approximately $2000 (2010 pricing).

While KTR-953 SB 4 is not specifically a required mod, it is unfeasible to offer warranty on a unit without the SB incorporated. Therefore, purchasing and exchange criteria must be accounted for when accepting a unit without SB 4.