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HP 8714 Frac N Module Problems

Question asked by philw on Nov 17, 2013
Latest reply on Dec 12, 2013 by philw
Users of the 8714 network analyser below about 150MHz will have difficulty with instability of frequency and eventual failure of module. This is due to a Tantalum capacitor being used on a VCO control line that operates over about -5.5 to +8V volt range. Above about 150MHz capacitor is always biased correctly, but below this the bias is reverse for the Tantalum which eventually fails. *This is a design fault.*
Solution is to replace capacitor with ceramic type.
I would give circuit and part number references except HP/Agilent don't publish service info to this level. On Analog Bus menu node is A3  Frac-N Assembly node 18 - VCO Voltage Drive. 
If it measures anything negative you will have trouble (eventually).
Fortunately most RF work these days is above 150MHz where the VCO voltage becomes positive.