[time-nuts] Repair of PRS10 Lamp Assembly

Bob Camp lists at rtty.us
Sun Mar 17 20:34:26 EDT 2013


I'd bet that something is keeping the oscillator from putting out enough RF. The circuit is simple enough that the issue is one of a very small number of parts. I'd bet on the FET…..


On Mar 17, 2013, at 8:24 PM, Brucekareen at aol.com wrote:

> 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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