[time-nuts] Repair of PRS10 Lamp Assembly
"Dr. Götz Romahn"
goetz at g-romahn.de
Wed Mar 20 13:02:56 EDT 2013
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
Am 18.03.2013 01:34, :
> 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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