I recently took one of my 5334A’s off of the house standard to lend to a friend. I thought it a good idea to check the onboard oscillator before lending it out. Both of my 5334A’s lack Opt 10 oscillators. This particular unit was almost 40Hz high in frequency. I tried to set it to frequency and had trouble getting it closer than 2Hz. The set-ability was very poor. I checked the service manual and found that HP only specified a final frequency within 8Hz. I can now see why. I then turned the instrument off for 12 hours. The next day I powered the unit back up. The oscillator was 20Hz higher than where I had measured it at shutdown the night before. It took well over an hour for it to re-stabilize and overshot the previous set point by 2Hz. At this point I decided to lend a Heathkit IM-2420 to the friend and continued experiments with the 5334A. I now took the second 5334A down and ran the same tests on it. This instrument was somewhat better but still shared many of the problems the first unit. 1) 60+ minutes to stabilize after power-up. Oscillators would start out 9 – 19Hz high and settle in at +- 1Hz. 2) Poor retrace at turn on. Final settling frequency can vary by 1Hz. 3) Poor set-ability due to the ceramic trimmer. 4) Oscillator pulling of 2 – 3Hz when I connected another counter to the rear panel oscillator jack. The oscillator buffer is on the same ECL chip as the oscillator. After looking at the circuit I decided that a new TCXO would be a better solution than trying to modify the existing circuit for better performance. Since these units are used at room temp most of the time, I need not worry about a wide temp range specification. I set the following goals for the replacement oscillator. 1) Improvement of stability by an order of magnitude and set-ability of two orders of magnitude. 2) Replacement to be built on a circuit board that would mount in the oven oscillator connector on the main circuit board. 3) Use of the full time 24vdc at this connector to power the oscillator at all times. Use of an on-card battery supply for oscillator backup. 4) Cheap 5) Use of readily available TCXO oscillator module. After checking Mouser and Digi-Key I settled on a Mouser supplied FOX801BE 10mHz unit @ $13.46. to start my experiments. This oscillator draws only 2ma. and frequency is set by an external trim pot. I would be interested in the experiences of others with respect to the performance of their non-oven 5334’s (or 5328’s) and ideas that support the design goals. Thanks Colin
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