[time-nuts] Repair of PRS10 Lamp Assembly

Brucekareen at aol.com Brucekareen at aol.com
Sun Mar 17 20:24:37 EDT 2013

I am afraid the PRS-10 lamp starting algorithm is a little involved.   I 
believe that once the lamp temperature is in the starting range, the  
microprocessor ramps up the drain and possibly gate voltage of the heating  
oscillator FET until the bulb strikes as evidenced by a DC signal at the  detector.  
If the lamp overheats (as measured by thermistors imbedded in  the back of 
the lamp block), at some point the microprocessor will reduce the  drain 
voltage to a safe steady-state value.  
The starting constants for each individual PRS-10 are factory-set in  the 
unit's programmed software, and as far as I know, access to change these  
settings has not been made available yet to end users.  This makes it  
difficult to swap lamps between units.
I have a PRS-10 with a lamp that is just on the edge of starting  properly. 
 Since I cannot change the factory-programmed values, I have  tried adding 
a shunt resistor to increase the lamp current to the point where it  would 
reliably start.  But, unfortunately, other problems seem to be  keeping the 
unit from working properly.  
The only luck I have had in repairing a non-working PRS-10 with certainty,  
was one in a Symmetricom 2500 Time Source that turned out to have a  failed 
Mini-Circuits VCO on the synthesizer board.  Replacing the VCO  module 
restored proper operation.
Perhaps someday the factory will provide end users with access to what  are 
now "factory-only" settings.
Bruce, KG6OJI  

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