Inspecting, aligning, and testing an HP 5065A Rubidium

[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!
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

[5065AGuru]

Bill,

They are hard to find!

The lamp assy. runs at 92 Degrees C so that does not help.

The film type does not like the expansion/contraction over the years.

Cheers,

Corby

[HighPrecision]

Thanks Corby for the restoration guide !

I've recently got one old unit, no options, 105 OCXO and blue cylinder, this unit is in very good condition, no corrosion visible, most capacitors are previously replaced.

I'm now waiting the 1.3K 2W resistor for lamp oscillator, I don't want to touch anything until the lamp is again operative and see what happens... actually the meter on [PHOTO I] and [2ND HARMONIC] is zero, lamp and cell ovens start at high values (+50) and after one hour both slightly decrease to 40 or less, all outputs are present.

I will start new thread for my unit if I got new questions / problems.

Best,  Stephano

[5065AGuru]

Stephano,

Good luck on the repair!

While the lamp assy. is out you might want to inspect the reflector. Sometimes it gets a bit corroded and not so shiny. I use Mothers aluminum polish and a bit of elbow grease to get it into good shape!

It's a bit tricky to get the perforated diffuser off but once off you can push the circular glass piece out from the rear, I push it out with my finger.

You might want to look over the post by SYAU

https://www.eevblog.com/forum/metrology/restoration-of-a-really-old-hp5065a/

It has a lot of tips!

Check the resistance of the lamp and cell oven windings. They should be around 50 Ohms each and show no continuity to the chassis.



Cheers,

Corby

[HighPrecision]

Corby,
thank you for tips on repair !

After some very hot days I finally repaired the Rb87 lamp assy yesterday, internals of RVFR (lamp side) is very clean, I think the R1 resistor is original 1.33 KOhm, open circuit, replaced with a new 1.3K 2W. 

Assy tested OK with a +20V from bench PS, less than 5 second to lighting, final current from PS is 138 mA, reassembled all.

OVEN windings are both 51.5 Ohms, the isolation from ground was not checked because I've difficult to disconnect the DB-9 from socket, unfortunately my sight is weak...

Now the meter on [PHOTO I] is at +26, I cannot get the [CONTINUOUS OPERATION] lamp ON pressing the reset button, the meter on [2ND HARMONIC] remain at zero, but in [CONTROL] position the situation is completely different, from a over -50 (full scale) indication, now initially indicate about +30 and I can adjust to zero with multiturn potentiometer (NOT the C-Field one)... Anyway the lamp indicator is OK tested 95 Ohm cold filament, the other lamp [INTEGRATOR LIMIT] is open-circuit, strange...

I've seen a couple of brief flash of the [CONTINUOS OPERATION] lamp in OPEN LOOP mode and switching the positions of the meter switch, surely the lamp is efficient !

Best,  Stephano

[HighPrecision]

I've started the procedure of the manual at paragraph [3.9] for cell-flooding, hope this recover the 2nd harmonic that's still absent.

[5065AGuru]

Stephano,

You really need to monitor A7 TP2 with a scope.
It sound like there may be some signal.
When in  open loop and you move the fine Quartz frequency back and forth across 5Mhz (plus minus .5HZ.) You should see a signal that changes to the 2nd harmonic when you are on frequency.
If you see some signal try turning the rearmost pot an A7 CW to see if you can get the 2nd harmonic meter reading up over 10. If so switch to operate and push the logic reset and it will probably lock.
What series number is your A7?

Cheers,

Corby

[HighPrecision]

Sorry Corby, you're right, my initial caution keep me from making unneeded actions touching the actual state of calibration, but I've some doubts that this A12 RVFR module is the original of this 5065A, but apparently it's in good internal conditions...

Some data from this unit:
Instrument S/N:  1320A00472
A7 AC AMPLIFIER:  P/N  05065-6080  - SERIES  960A  (new version ?)
A12  RVFR:  P/N: 05065-6001 - SERIES  956  (blue color,  not  olive-green ?)
A10  OCXO  P/N: 00105-6034 - SERIES  1248  (manual reports  00105-6013 ?)

From  A12 label the data for synthesizer setting are:
FRONT DIAL  6.60  -  THUMBWHEEL  9254

Actual setting are:
FRONT DIAL  1.00  -  THUMBWHEEL  9381

Next I'll check the A7  TP-2  as your indications, I live in Italy, got this unit from a guy in Tuscany, not Luciano P. (timeok).

Thank you for your patience.
Best, Stephano

[HighPrecision]

Ok, the signal at A7 TP-2 is present and weak, at 80 mHz offset there is a peak of noisy 70 mV p-p @ 137 Hz, at no-offset only noise (5 mV p-p).

1st problem:
found the coaxial cable from receiver to A7 input has some instability at internal receiver side.

2nd problem:
on A3, TP-2 is only 125 mV DC, clearly the multiplier excitation is insufficient.

Stephano

[5065AGuru]

Stephano,

It's good that you can see at least a weak signal at A7TP2!

Try adjusting the pot on the side of A3 (start max CCW) to peak the signal, there may be more than one point, use the biggest peak.

Also see if the A3 TP2 changes when you adjust that pot.

If no luck you will need to troubleshoot the A3. At A3J8 into a scope (50 ohm) or a power meter, should read between 8.1 and 9.0 VPP or +22 and +23dbm. A3TP2 should be >4.5VDC.

Cheers,

Corby

[HighPrecision]

Corby,

Yes, turning CW the A3R40 increase DC voltage at A3TP2, but the weak signal disappear, after some experiments with R40, C59, C61, L25 and R3 I've increased the signal (at 80 milliHertz offset is maximum) to about 150 mVpp, now I can see the 274 Hz harmonic with NO offset, about 50 mVpp, still low and noisy... the meter is zero in the relative position, now A3TP-2 is about +4.6VDC, i can see over +6VDC tuning the SRD matching circuit.

It is possible a massive flooding of Rb cell ?   Or there are problems at SRD / cavity resonance ?

Thanks,  Stephano

[HighPrecision]

Finally this unit is working, the CO lamp is ON.

After the adjustment of multiplier matching network got a low 2nd Harmonic signal, and the LOOP GAIN adjustment reach maximum 0.28Vdc instead of 0.5Vdc (+- 0.05V), so also the 137Hz signal is low.

I have noticed during the warm-up of RVFR module that the 2nd Harmonic reach about 140 mV and after is 50mV, my suspect is directed to A11 Oven controller for escessive temperature, found this board was not managed for capacitors replacement, this probably confirm a previous exchange of A12 RVFR module with A11 in this unit   

Checked the resistors R5 and R6 on A11 if the ohmic values are consistent with A12 label
[LAMP OVEN - 200 Ohm] and [CELL OVEN - 390 Ohm], found LAMP resistor OK 200 Ohm (measured) and wrong for CELL, 360 Ohm also confirmed by measurement... replaced this resistor with a 390 Ohm.

Now the 2nd Harmonic is about 90 mVpp, and the LOOP GAIN reached 0.45Vdc at CW end of the trimmer A7R17, still low without margin, but the temperature is still excessive, I will try a bit higher value for CELL temp resistor or a trimmer...

Anyway suspect a wrong tuning of the resonance of CELL cavity or the photocell.

[HighPrecision]

Last update,

added 33 Ohm in series with 390 Ohm, this made a BIG change, 2nd Harmonic is 150mVpp and the adjustment of LOOP GAIN has a large margin, A7R17 is at 1/3 of CW rotation.

[5065AGuru]

Stephano,

You mentioned earlier about hooking the TED up.
Did you run it for a week?
If not, try that and monitor the 2nd harmonic level and see if it eventually increases.

I've Never seen a bad photocell!

Did you inspect the reflector for corrosion, if its dull re-polishing it will help increase the level.

If you still are stuck around 150mV I can detail how to tune the cavity to see if that helps.

Cheers,

Corby

[HighPrecision]

Corby

at the moment I had to suspend my work on the 5065A   

TED procedure was executed for only 12 hours, no difference in A7TP2 levels.

My impression on the lamp reflector was apparently good, but I have problems with my visual capacity, eventually I will polish this reflector.

With last modification of the CELL temperature I have noticed it is still too high, second harmonic level has reached 200 mVpp and then reduced to 150mVpp at final stabilization, considering the big change in calibration points of previous attempt I expect another consistent increase when the LAMP temperature is optimized together with the CELL temp.

What are the typical levels I should expect at A7TP2 ?

I have noticed also that it is necessary to adjust the DC offset with A7R3, but this trimmer is not accessible from top...

Thanks, Stephano

[5065AGuru]

Stephano,

There are a lot of interacting processes that can cause the A7TP2 signal to read higher during warmup.
I'd recommend sticking with the oven resistor values marked on the optical unit.
The only way to be sure is to route a thermocouple or other fine gage sensor in thru the shields to measure the cell temp and or the lamp temp. (the side without the threaded stud is the cell side) after the oven stabilizes the cell temperature should read between 65 and 68 degrees C. Lamp temperature should be between 89 and 92 degrees C.
In either case you need to remove the 3 shields for the side you are working on to route the sensor.
On the Cell side push the sensor against the wall where the locking segment Phillips screw attaches.
On the Lamp side insert the sensor thru the hole in the top of the lamp assembly that goes all the way through to the face of the perforated diffuser, you can attach under one of the 3 Phillips screws that hold the diffuser or just let it dangle flush with the front edge of the diffuser.

A7R3 is accessible  from the rear of the unit, the hole is just above the top of the rear panel.

A7TP2 should be at least 500mVPP to have the unit meet the stability specs.

The fact that you can now see it is good as that allows you to evaluate methods to increase it!

Cheers,

Corby

[HighPrecision]

Thank you, Corby

Hopimg to restart soon to work on this unit.

Ok for the level at A7TP2, 137 Hz is 3-4 times higher, I have mentioned only the 274 Hz level in my posts.

Rb lamp assy has one thermistor installed, but I don't know anything (model / temp table), one attached image show the thermistor at one side, the left side is flat and rectangular, thinking it is a ceramic support.

Other pictures show the conditions and lamp test, one picture show also the new 1.3K resistor (blue body) behind the power input inductance.

Stephano

[HighPrecision]

Hi Corby,

Now my 5065A unit is constantly powered ON from 2 weeks, after various adjustments and temperatures checking the output signal at A7TP2 is increased, @137Hz (offset) is 1 Vpp and @274Hz (centered) is 280 mVpp, all very stable!

The DC offset of A7 amplifier isn't a real problem, I see about +0.5 VDC at A7TP2, but the mesaurement is to be done at input connector (solar cell connection), I have +2.7 mVDC (max +/- 5 mVDC). The hole on the rear for A7R3 adjustment is not present on my unit.

Thank you for the help !

Stephano

[rubidium]

All of this expertise on the 5065A being shared here is a real asset. Now if only I could find a 5065A and be able to put it to practice. 
Just where are people acquiring these things? It took me several years to find a cesium reference worthy of repair, and still I drool over a 5065A!
Jim

[HighPrecision]

Hi Jim,

As Corby said, these units are hard to find, in late June I've casually spotted an offer on a national radio amateur forum... was very hard to resist knowing these particular devices, finally decided to contact the owner, declared as operative (!), he has accepted a bit lower offer by me.

When arrived was clear that the unit isn't operative, the output 5 MHz signal was nice and very stable, but the Rb lamp was inoperative, for the rarity of these devices I decided to repair this unit obviously !

I haven't a primary standard and don't want to go for the hard "time-nuts" way, too expensive, at the moment I'm using another Rubidium unit (EFRATOM FRK-H) got some years ago in NOS conditions for only 30 euros, I do not trust the frequency calibration of these units, I will try to compare with a GPSDO (OSA Star-4) in next months.

GOOD Luck to find one !

Stephano

[notfaded1]

Look at these old prices!
http://www.barrytech.com/hewlett-packard/standards/hp5065a.html

Nice old gear site where he listed many old auction prices on multiple units.  I'd love to get some of the 2002-2004 prices he listed today! ;^}

Bill