Electronics > Metrology

HP5065A Odd Fault

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My nice HP8065A is playing up.

I confess that it does not get run full time, but I run it up for a few days every few months and it has been fine for many years.  It is stored reasonably warm and dry.  It has an RVFR dated 1991, although a number of the circuit boards have caps with 1968 datecodes.  No serial number (scraped off), but the lid shows Serial No 916-00184 and RVFR Serial A07436 which matches the RVFR.

A few days ago I wanted to do the long-awaited caps change so I warmed it up for an hour and it started first time.  I noticed that it had developed an odd fault.  If I tapped it, even gently, the 2nd Harmonic leapt then returned to normal.  Anyway, I got it stable and left it on for a day.  I then powered down again to change all the caps that 5065AGuru (Corby) recommends, and heatsink that poor little mushroom rectifier on A2. I haven't done the 1.33K resistor in the RVFR yet.

Once done, another 1 hour warmup and it fired up fine so I left it on soak test overnight.  Come daylight the Continuous Operation (CO) lamp had gone out.  Damn!
But if I tapped the case a few times I could get the 2nd Harmonic to jump and if I pressed logic reset the CO lamp lit but extinguished after a few seconds.

It feels like I have something like a cracked resistor - does that make sense?  When it was working, everything was looking fine on the meter and on the various outputs.  I waggled boards gently to prove that the fault was not just bad pcb seating.

Are there certain resistors prone to cracking (maybe due to heat)?   I'll be doing a thorough check of the various circuit boards to see if anything measures oddly, but any hints would help.

Any help or advice welcome!


Update: some carefully calibrated tapping all over the instrument seems to locate the intermittent effect to the RVFR lump itself.  I'll remove it and check and if necessary replace those often-toasted resistors inside.

The 1.3K resistor may be toasty but it's unlikely to be the source of the problem at hand.  You can rule it out by looking at PHOTO I instead of the 2nd harmonic.  If the photocurrent remains steady while tapping on the unit, then the problem is unlikely to be associated with the lamp.

Once the covers come off, the first order of business is to check the various SMC coaxial connectors on the bottom of the chassis to make sure they are tight.

It's a closed-loop system, so a good second troubleshooting step is to open the loop, and there's a switch on the control panel for just that purpose.  After you familiarize yourself with the block diagram, check out some of the documents published by Corby Dawson and others for some good test points to monitor with a scope. 

Another suggestion: switch back to closed-loop mode and see if you can tell whether the problem is in the OCXO versus the rest of the loop.  (In an updated unit at least, the OCXO is right next to the RVFR; you can't really bang on one without affecting the other.)  Compare the 5 MHz output to a known-good source with a scope, either in X/Y mode or simply by examining the two traces directly.  When the glitch occurs, does it ring down or recover immediately? 

If you can't tell anything with the scope, tune an HF SSB or CW receiver to 5 MHz and listen to the tone you get from the output jack when you tap on the unit.  It should remain rock-steady (at least from the perspective of your ears.) 

In either case, if the signal breaks up with a burst of fast static or jumps rapidly on the scope, the problem is likely in the OCXO or one of its power supplies.  If it comes across as a slow, subtle tone shift or artifact, the problem is more likely ahead of the integrator.  You could confirm this by pulling the integrator board.

Also, wiggle all of those SMC cables while monitoring the scope or receiver.  They can and do become flaky. 

On my newest 5065A the 10811 coarse tuning piston cap was flakey. Sometimes it would lock, other times not. Tapping to 10811 or turning the coarse adjucstment screw isolated the problem tp the the variable cap. When I get a little free time I will open the 10811 osc again and clean the piston cap better.

Thanks for all the suggestions.

An evening spent checking everything has thrown up a few faults.

- Dry (or suspect) joints under the 1.5ohm resistors on A11.  Cleaned up and remade all nearby joints.
- Loose SMC connector under A7 module.  This connector is carefully isolated from the chassis so a bad connection leaves a flaky earth return. - tightened up.
- All SMB/SMC connectors checked.
- Also opened up the 3 lids of the optical unit and checked the 1.33K resistor inside.  It looks really good, no signs of more than moderate heating, all bands readable and measures 1.33K cold so I am happy to leave that there for now.  No fault on the 12ohm resistor soldered to the two tags on the other side of the board.  Carefully re-asembled (no sign of an "spacers" as detailed by Corby).
- Checked the 10811 OCXO trimmer shaft.  Perfect, moving freely and allowed easy setting of 5MHz.

After re-assembly, warmed up for 45 minutes (it's already in a nice 21 degree ambient) and hit go.  Green light. Currently checking the output on an Agilent 53220A graphical display, locked to an SRI rubidium.  No glitches on the 2nd Harmonic unless I really thump the case.

I'll leave it on for a few days and see if it stays happy.  I'll report back.


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