[time-nuts] Are these PRS10's worth it ?

brucekareen at aol.com brucekareen at aol.com
Tue Dec 30 13:00:50 EST 2014

My experience has been similar to Vasco's.  Of three defective PRS-10s acquired for about $100 or less each, one had a bad MiniCircuits 400 MHz VCO module and was an easy fix.  Another had a rubidium bulb issue that I traced to the driving FET.  I managed to replace the FET, but the replacement's bias characteristics were somewhat different than its predecessor's, keeping the sensing circuits in an alarm mode.  At the factory, bias adjustments are made in the software; but SRI does not provide access to this feature for customers.  I tried removing and replacing the FET with still another, but messed-up the circuit board in the process.  I wish now I had tried to alter the FET bias by adding external series or shunt resistors. 

The third unit has an intermittent condition that causes it to be slow to lock at times and fine at others.  I have spent hours trying to trace this, but so far without success.

If you decide to take a chance on these units, I would not be inclined to offer more than $100 as their repair is a crap-shoot.

I suspect many of the units offered for sale on eBay have been removed from Symmetricom TS-2500 and TS-2700 telephone industry timing sources.  PRS-10s from these seem to have Customer Number 123-44101-4 on the label.  I have also encountered PRS-10s with the Customer Numbers 123-44101-08 and -10 on the label and assume these were from other Symmetricom instruments.

By the way, The TS-2500 is a GPS-referenced source; however, the unit seems to compare its internal crystal oscillator with GPS, the PRS-10, and other 10 MHz sources (that can be connected for monitoring) and keeps track of their behaviors with a microprocessor.  It appears the PRS-10 is not locked to GPS, but is simply monitored and kept in reserve as a replacement frequency and timing source should GPS service fail.  If the PRS-10 frequency wanders out of the acceptable range, the TS-2500 shows a fault signal, but does not attempt to adjust the PRS-10.  Apparently the PRS-10's frequency is set at the Symmetricom factory and the unit is on its own after that.  It is included in the TS-2500 box as baggage to be used only if the GPS timing fails. As it is kind of expensive to burn an SRI PRS-10 simply as a standby, although this provision is understandably important to telephone companies, an unmodified TS-2500 is of questionable utility to hobbyists.

Bruce, KG6OJI 

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