Electronics > Metrology

Inspecting, aligning, and testing an HP 5065A Rubidium

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5065AGuru:
Hi, Thought I'd start an ongoing post about how to test and align a "fresh picked" HP 5065A Rubidium standard.
The one I just got in has the 10811 oscillator mod installed.
First thing is to remove the top and bottom covers and set the unit vertically on its left side.
Then remove the following plug in cards one at a time and inspect the 100uF axial
Tantalum caps for any corrosion or leakage around either lead. Replace any
suspect ones and repair any traces that might have been damaged by corrosion. Replace
the card in its place.

A15 3 caps (See PIX for an example of the caps))
A8  3 caps
A6  1 cap
A4  1 cap

Now remove the A7 metal module and remove its cover to inspect its capacitor.
Then remove A3 metal module and remove its cover (it's a job!)and inspect its
capacitor.
While the A3 is open you should locate the two tuning caps for the matching
network and make sure the rotors are not stuck, too much force when stuck will break the rotor to tuning post connection! (These caps are the two closest to the 3 SMC jacks).
I usually put pressure with a plastic rod or side of a tuning tool sideways against the rotor until I hear it snap free. Then it can be tuned easily.

Replace the covers and reinstall A7 And A3.
Pull the A11 card and replace the two non-tantalum axial caps. I use 22uf 63V caps. Reinstall A11.
If it has a battery installed remove the battery fuse (on top of the battery assy.)

OK, now we are ready to power it up! Place the mode switch to open loop.
Place the meter switch to supply and plug the unit in. The supply reading should jump up to around 40. Connect a DVM to the chassis and to the + and - 20V supplies. The probe can be placed into the
back of the Red +20 and Violet -20 push-ons on the A7 module. They should be within .1V for now. Note the time.
If they are OK switch the meter and observe the cell and lamp oven readings.
They should both be quite high or even pegged. They should (one at a time) drop down before you reach 45 minutes, if not power down and troubleshoot!
While waiting for the ovens you should see the photo-I reading come up and the 5Mhz output should settle close to 5Mhz (within a Hz or so).
One other check while waiting is to scope the input and output pins of the chassis mounted DC choke. The input side should be an unfiltered full-wave rectified waveform amd the output side pretty musch a flat DC.

More to follow!

Cheers,

Corby

5065AGuru:
OK, Now to start checking the alignment. But first we need to check the coarse tuning of the 10811 oscillator. HP had several versions of a mechanical kludge and I always just remove them as they are unneeded! This unit would not tune and as I suspected the tuning shaft had disintegrated and clogged the tuning capacitor. See PIX!  Remove the front tuning shaft bezel by fitting an allen wrench in the front and removing the nut on the back. make sure you catch the toothed washer! Then pull the shaft out of the oscillator slot. As is often the case you will need top (with power off!) remove the 4 screws holding the oscillator assy to the bottom chassis. then rotate the oscillator out so that you can remove the two screws holding the 10811 onto the board (thru the two holes in the rear). Then pry the 10811 from its socket. I spend some time with a sharp pointed tool picking degraded fiber from the tuning capacitor threads and blowing hard into the hole to clear particles out. Once the capacitor threads look OK carefully turn the shaft CW a bit. This usually works as you are going down into threads not gummed up. Then see if you can make a couple CCW turns working back and forth if it sticks. If OK at this point then reinstall the 10811 and the assy. Since the tuning hole lines up with the hole in the front panel all you need is a long tuning tool and can throw the mechanical kludge away! :phew:
Now with the unit warmed up and the Quartz fine frequency pot to 250 (midrange) adjust the coarse for 5Mhz plus minus .1Hz.

Set the meter switch to error and connect a scope to A7TP2 and hopefully as you adjust the fine Quartz pot back and forth and you should see a 2nd harmonic noisy sine signal as you cross resonance and the meter move plus or minus as you move the pot.

Set the pot for zero error and observe the 2nd harmonic amplitude this unit reads 200mV P-P on TP2 which is not too good, however an Allan deviation plot of the unit locked shows it just making spec.
Now adjust the two pots on the top of A3 max CW or until the signal at TP2 peaks. This unit now gives 3.4V P-P at TP2!

Switch the meter to the 2nd harmonic position and adjust the rear pot on A7 for a reading of 30 to 45.
Place the unit in operate and push the logic reset button. You should get a green operate light.

Now log the 2nd harmonic meter reading and the P-P value of TP2. Leave the unit as is now and log the 2 readings a couple times a day for a few days. They might slowly increase and then stabilize. This unit took 5 days to stabilize. This is causes by the Rubidium level in the cell equalizing.

Once its stable for a day we can then continue with the alignments!

More to come!

Cheers,

Corby

5065AGuru:
One more thing to do before alignments. You need to remove the optical unit from the chassis, then remove the nut and washer on the stud on the lamp end, remove the 1st shield.
Remove the internal nut on the stud and remove the 2nd shield.
Remove the 3rd shield. Remove the 3 tiny phillips around the lamp assy. circumference and pull the lamp assy out.
Locate and inspect the 1.33K resistor. If it is original it will be a film resistor and will need to be replaced. (even if it looks good replace it!) They degrade and crack over time! I replace them with a 1.3K 2% MOX resistor. See PIX.

 If careful you can do it without further disassembly but you can remove the 3 big phillips on the same side as the 3 tiny ones and it will come apart. Be careful not to bump the lamp or lose the 3 spacers that fall out.
This unit also had a cracked 12 Ohm that I replaced.
Reassemble everything making sure that you space the inside nut so that the outer shield will be snugly clamped between it and the outer washer and nut!
Reinstall the optical unit into the chassis.



Now for some alignments! Warmed up and in open loop for now.
First a match of the 137HZ filter frequency of the A7 module is made.
I use a short wide bladed tuning tool cut down to 5/8" with heat shrink on it to give better purchase. This allows tuning the card in place without an extender.
Clip an oscilloscope to Pin 13 on the A8 edge connector. You will see a Yellow wire connected to this pin.
Adjust the Quartz fine frequency to give you an observable signal on the scope.
Now adjust the R8 pot on the A8 module for a peak amplitude.
You have just adjusted the A8 output frequency to match the "as built" center frequency of the A7 fundamental filter. This will make the phasing adjustment
we will make later much easier!

Move the scope to A7TP2 and adjust the fine Quartz pot for zero on the error
meter.
On A3 Set the core of L22 or L25 to a little over 1/8" from the top of the form and then leave it
there.
Observing the TP2 signal adjust for a peak on A3 the matching network C61, C55 or C59 and bias pot R40 or R53. Depending on which A3 version you have.
Check TP2 on A3 to make sure it reads >4.0VDC. If not try a different position of the R40 or R53 pot.

Installment 4 to follow!

Cheers,

Corby

5065AGuru:
Now to adjust the phasing.
Connect the scope to the blue wire on the A8 connector pin 14 and move the Quartz fine frequency pot back and forth across resonance.
On one side the waveform will go positive and on the other negative.
Position the pot to give a positive waveform and adjust the phasing pot R43 on A8 to eliminate the "step". See the PIX which shows good (left) and bad (right).


Correct phasing occurs when there is no "step" at the base of the peaks.
As you reduce the amplitude with the Quartz fine adjust there can be some residual "step" when you get close to the null. That is usually OK. If you can't seem to phase it then try reducing (CCW) the level of the modulation at A3 R11/20 a bit at a time and rephase. At this point the A7TP2 reading at resonance should be greater than 800mV P-P

The loop gain adjustment is next and the easiest method is to (in open loop) adjust the
fine Quartz pot to zero the meter (in the error position.)
Note the reading on the counting dial, add 50 to it and set the dial to that number.
Say the reading at zero was 235, add 50 to get 285 and set the dial to 285, now adjust the
front pot on A7 for a reading between 35 and 40. Return the dial back to the 235 reading.
Now switch the meter to the 2nd harmonic position and if it's not between 35 and 40
adjust the rear pot on A7 to make it so. If you have a good 2nd harmonic level you can end up right at the bottom of the pots range making it tricky to set the level! Place the unit in operate and push the logic
reset button and the green light should come on.
If you have a standard to adjust the frequency against, use the C-field pot to match
frequencies and your 5065A is done. :-+
 

notfaded1:
This is great guide Corby!  I just wanna know where you find "freshly picked" 5065's these days!  :-+  I recently just acquired a 5061B but haven't seen many HP Rb's around.  The heat must get to those resistors and that can't be good.

Bill

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