[time-nuts] anybody have hp-5061 stories

[tom jones]

I won a one day auction for hp 5061 with high preformance cesium beam tube.
  This was 15 months ago. The unit pictured on ebay showed lots of stickers I was hopping for a unit with recent maintaince.
   
  I received a 5061a with standard cesium beam tube no stickers and had complained to the seller.  He told me to get my story straight.
   
  I didn't mind the sellers comment and was happy anyway. I didn't expect the unit to work and it did not work. It did show promise though.
   
  The cesium tube wouldn't pump down and of course the cesium oven, electron high voltage supply was locked from operating due to the units inability to pump down.
   
  After 1.5 months trying to ion pump down cesium beam tube I decided to defeat cesium oven and electron high voltage lockout circuitry.
   
  Sure enoubh cesium oven began heating, high voltage supply fired up after everything looked stable I switched to the closed loop mode and it appeared to operate of about 5 seconds then went into alarm mode, loosing 2nd harmonic reading.
   
  I trouble shot the unit to AC amplifer card where I suspect many other users of 5061's are porbably having problems maintaining continous lock and operation
   
  After repairing the AC amplifer card I only lost lock once in 13 months where I just turned up ac gain slightly..
   
  Any to address what I found with the AC card that migh help others with old cesium beam tubes and loss of lock problems;
   
  The amplifer card uses three seperate amplifer stages each composed of 3 to 4 dc coupled transistors  This direct coupling (collector to base) of transistors can be a nightmere for maintaining propper biasing at the respective transistors.
   
  But after about a week of trying to get the right voltages (and biasing) at each transistor, I found only the very last output transistor was causing all previous stages to have the wrong voltages and biasing..
   
  Thus removing all my weeks work and focusing on the last transistor in the string of amplifers I simply rebiased the final output transistor by changing the resistor at the base to ground to allow the final transistor to have 9 volts on its collector instead of 2 volts.
   
  Voltage to the whole amplifer card is supplied through a 100 ohm resistor. The final output transistor is  the first component connected to the 100 ohm supply resistor and this final transistor was pulling all the amplifer stages supply voltages down to nothing. Rebiasing any transistor effected the final output transistors biasing and changed voltages for all transistors and amplifer stages.
   
  Anyway if your having continous lock problems I would go right to the 100 ohm resistor where 19.4 volts is applied to the card. The other side of the resistor connects to the colllector of the last transistor in the string of transistors and this collector is supposed to be 9 volts I found operation is fine down to 5 volts but any lower expect problems getting enough gain to hold continous lamp light lit.
   
  Yes you can turn the modulation level up to hold lock but the modulation distorts very easily because of all the frequency multiplication and this distortion causes the microwave output to change frequency.
   
  I found that higher modulation level was pulling the microwave output off frequency so I keep the modulation level as low as possible to achieve a 2nd harmonic reading of 10
  My unit goes into alarm around  6 or so.
   
  I also had to demagnetize the beam tube to get the unit running good.
  I run cesium oven on hi I almost certain that low cs oven will not provide lock.
   
  The daily drift is usually arrond 70-100 nanoseconds which I believe is temperature dependent or sun and moon gravity dependent (high tide low tide drift) ?  
  Weekly and monthly drift doesn't exist.
   
  Anybody else running 20 + year old 5061 cesium tube. Tell me about it !
   
   
   

 		
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