[time-nuts] Repair of PRS10 Lamp Assembly

Herbert Poetzl herbert at 13thfloor.at
Mon Mar 18 00:14:26 EDT 2013

On Sun, Mar 17, 2013 at 08:24:37PM -0400, 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. 

Assuming that this is the correct image for an PRS-10
(http://goo.gl/Q4ztp), the CPU used looks like an
MC68HC711E20CFN3 (http://goo.gl/Zhxzv) which would mean
that it should be simple to reverse engineer the program
and data stored there ....

> 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.

If there is really an interest in this, just send me
one or maybe two units and I'll give it a try, no promises
though, but I've done a lot of work with motorola CPUs
in the past ...


> Bruce, KG6OJI  
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