Electronics > Metrology

Inspecting, aligning, and testing an HP 5065A Rubidium

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--- Quote from: notfaded1 on September 18, 2020, 05:26:44 pm ---Look at these old prices!

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! ;^}


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Ah ... looking in the rear-view mirror can indeed be depressing.  :palm: At those prices any one of us would buy them all today.

--- Quote from: HighPrecision on September 18, 2020, 04:18:38 pm ---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 !


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I've learned to be a (mostly) well-disciplined and patient guy when it comes to this stuff - largely as a result of a few failures in the past. About 5 years ago I gambled on a CS4000 that turned out to be little more than a "tire chock" in that it had an exhausted CBT. Of course it was advertised with one of several luring statements like "Working", or "Pulled from a working environment", etc.  Fortunately, I was able to get a refund from the seller. Recently I lucked out on a PRS-50 at low cost after convincing myself that it probably had a bad PSU board and perhaps held the promise that the CBT still had some decent life once the PSU was repaired. Quite honestly, some of the more favorable gambles I've made on surplus high-end gear involved bad power supplies - in particular, failed electrolytics.


--- Quote from: HighPrecision on September 17, 2020, 04:39:08 pm ---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!

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New update:

Found another problem, in the C-Field coil there is no current flowing despite the presence of a voltage on A15 R10 and R11 current sense resistors, about 2V DC variable with the C-Field control.

The RVFR "C" coil is about 12 Ohms, found the dual transistor A15 Q6 (1854-0221) broken, either B-E junctions are shorted; ordered a NOS original replacement and changed also the C6 tantalum capacitor slightly leaking (47µF).

Now the C-Field adjustemnt acts on output frequency for about 12 milliHertz. The major surprise is the 2nd Harmonic level on the A7TP2 test point is increased to 1.32Vpp @ 274 Hz (centered) and over 4Vpp @ 137 Hz (offset) !

The 2nd Harmonic signal now is clean, noise is greatly reduced.



--- Quote from: HighPrecision on October 10, 2020, 06:34:36 pm ---The major surprise is the 2nd Harmonic level on the A7TP2 test point is increased to 1.32Vpp @ 274 Hz (centered) and over 4Vpp @ 137 Hz (offset) !

The 2nd Harmonic signal now is clean, noise is greatly reduced.


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Sounds like transistor fix made a nice difference.  Now I'm wondering if I should buy some 1854-0221 while they can be had still.  WTH I just bought a couple new ones.



With no C-field current you were probably locking on the wrong line.

One of the lines a Rubidium can lock on at low C-field currents is HIGHLY sensitive to magnetic fields.

The HP 5065A when working properly is designed to provide enough current even at the minimum pot setting to avoid this line.

Great that you are making progress!



thank you for the explanation on C-Field !

Actually I'm trying to calibrate the 5065A output frequency with the only method available for me, using the Oscilloquartz GPSDO Star-4, TC setted to 3600 sec. and using a FA-2 counter statistics to average 10MHz frequency variations, approx. 260 microHertz P-P, after some weeks and adjustment of C-Field, the averaged difference is stable around -4.4 microHertz @ 5 MHz.

I think this small difference is too difficult to compensate with C-Field control.

I have found a replacement connector for mains input, same MIL serie and pin number (3), but different rotation and reversed socket/plug sides, not very safe to have live mains on plug side, but I will avoid to modify the rear panel of the 5065A  :)

The RVFR signal at A7TP2 is very strong, the A7R29 trimmer for the meter indication is somewhat difficult to set, from full CCW position the meter needle jumps immediately over full scale with a very small rotation of this trimmer in the '2nd Harmonic' position of meter switch.

That is a very interesting experience with this unit !

Best, Stephano


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