[time-nuts] Repair of PRS10 Lamp Assembly

["Dr. Götz Romahn"]

Thanks for all your comments so far. I did sniffing with a cable loop 
located in the lamp cell (thank you Bob for this advice), and surprise 
(!) RF power does not degrade with time but does stay rock solid after 
ignition. So, sorry Bob, you did not win the bet this time. Next step 
should be to check, what is going on with heating. Not so easy since all 
the many diagnostics are within range. As a provisory measure I decided 
to cheat the lamp heat regulator somehow by paralleling a 39 KOhm 
resistor to R226 an thereby shifting the operating point of the 
thermistors in lamp assembly. PPR10, when started properly, now seems to 
work as expected.
One comment to the lamp starting algorithm: oscillator FET drain and 
gate voltage are controlled via factory set values but not on an 
individual basis. All lamps are started with FET voltage set to maximum 
available via SD2=255  and after inginition to some reduced value also 
via SD2=55. So there is no need to change set values for different lamp 
assemblies.
Goetz


Am 18.03.2013 01:34, :
> Hi
>
> 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…..
>
> Bob
>
> 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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