[time-nuts] SR620 fails in Self Calibration routine 07

[Bruce Griffiths]

Jim Robbins wrote:
> My SR620 fails in its Self Calibration routine at "Cal Error 07".  The Users Manual describes this error as a problem with the "Start TAC" (Time to Amplitude Converters) and indicate that this flags a Hardware problem in the circuits which perform the analog interpolation between the 90 MHz clock ticks.  Anyone have any experience with this?  Is this a repair shop problem or user correctable?  I am unable to find a "Service Manual" for this unit also.  Many thanks for any input.  Jim Robbins
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Jim

The TAC is a fairly straight forward circuit comprising a capacitor that 
is pre charged to a reference voltage and discharged at a constant 
current during the short time interval to be measured.
At the end of this time the voltage across the capacitor is measured by 
an ADC which also measures the STOP TAC output among other things.
Since it uses discrete components and the manual describes how it works 
in some detail it should be fairly easy to troubleshoot the circuit even 
without a schematic.

Bruce