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HP 8567A YTO Unlock message!

Discussion in 'Electronic Equipment' started by pdmtr, Jun 6, 2005.

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  1. pdmtr

    pdmtr Guest

    I have a HP 8567A (very similar to HP 8568A/B) spectrum analyzer that
    has an error code flashing every 2 seconds on screen. "YTO unlock"
    message comes on and at the same time the Green SWEEP Led on the 85662
    Display unit flashes instantly. I have figure out that this is related
    to the Yig Tuning Oscillator (Y.T.O.) that is the main sweep osc of the
    spectrum analyzer. Inside the RF/IF section I have seen that the A6 YTO
    PHASE LOCK board has the "YTO UNLOCKED" led permanent on while the "YTO
    LOOP CLOSED" led is mostly lit and goes instantly out (whenever the
    message on screen comes on). The exact same off time of it the "LOW"
    led located on the A22 FREQ CONTROL Board comes on. The analyzer has a
    sweeped display with a VERY low refresh rate (a refresh every time the
    error message comes on) and has a working RF input, checked for level
    with a R&S SMS generator... I can see the R&S output signal but
    spectrum show a difference on carrier frequency of aprox: 13.6MHz lower
    than the actual input freq. The 100MHz are showing (using the MARKER)
    as 86.4MHz!
    Anyone know something about that problem? I cant find even the manual
    of the HP 8567A spectrum analyzer (while i have found where to buy the
    manual for the 8568A/B model), so any help will be greatly appreciated.
    My regards
    Sotris Pdmtr
     
  2. John Miles

    John Miles Guest

    I believe the 8567A was simply a reduced-cost version of the 8568A. It
    is probably OK to use the 8568A's manuals to troubleshoot it.

    I am about to start tracking down the same symptom on an 8566B, so we'll
    have to wish each other luck! (That's a pretty-different machine,
    though.)

    -- john KE5FX
     
  3. pdmtr

    pdmtr Guest

    Hi
    Thanks for the reply.
    Well I have put it on test mode yesterday (pressing "SHIFT" + "w") and
    the analyzer worked with a normal refresh rate!!! Made some test on the
    measurements it makes and ALL of them are inside specs!!! Correct
    frequency accuracy of the applied signal from the R&S generator,
    correct amplitude level, all buttons reacting instantly on pressing
    (before they reacting every YTO message flashing time)!!! Problem is
    that the error messages are still there. And I have more now! Except
    "YTO unlock" I have also "Oven Cold", "EXT REF" (thats the most strange
    as I have put the INTERNAL REF switch right!!!), and 2 more related to
    the YTO unlock message!
    Trying to figure out if thats a hardware failure of the YIG Osc, or a
    failure of the ,master time base TCXO, as it looks like only the
    refresh rate is "blocked" or delayed for some reason. But also it might
    be a mis-programming fault, and not a hardware failure... Also I'm
    thinking that the error it may be located on the 85662 display unit and
    not on the RF/IF base unit, as it looks that the display unit provides
    the RF unit some synchronizing signals thru the Bus cable and also some
    IF signals thru the Multi-Coaxial Interconnection. Not sure where the
    main TCXO is located, but it looks its on the 85662 unit and not on the
    RF/IF base... So if the refresh problem is related to the locking of
    the YTO that will lead to a TCXO REF fault. The EXT REF input is
    located on the 85662 unit and that guide me to think that the INT REF
    TCXO is also based on this unit... as it would be strange to pass a
    10MHz signal input from the upper half to the lower half!
    I've found the User guide of it and maybe I'll buy a HP 8568 service
    manual, and try to see the relationship between failure on these model.
    :(
    If you find something related (anything really) it would be a great
    help, as the basic working philosophy behind the 8568 (8567) & 8566
    Spectrum Analyzers should be the same!!! :))
    My best regards and good luck with yours also...
    Sotiris
    Pdmtr
    @ (Nospam)YAHOO.COM
     
  4. pdmtr

    pdmtr Guest

  5. Here's a link to the manual:

    http://cp.literature.agilent.com/litweb/pdf/08567-90001.pdf

    Don Cleveland
     
  6. Glenn

    Glenn Guest

    I don't remember well and I don't have a set of schematics in front of me,
    but a couple of suggestions:

    It's unlikely that you are running without *any* 10 MHz reference. That
    reference goes into locking everything in the box and even if the
    instrument were in ext ref mode with that loop unlocked, the error is
    probably small enough that the other loops wouldn't have a problem.
    Normally the 10M tuning range is only a few ppm.

    The YTO loop, (the "outer loop") probably needs several other properly
    locked references to operate correctly. In fact, it's "YTO unlock" error
    may only be a logical OR of inner loops. The idea being that if any of
    those other loops are not locked properly, the YTO signal cannot be
    trusted either. Oven Cold and Ext Ref are certainly affiliated with the
    precsion 10MHz reference and probably a detector circuit that looks for
    10MHz external signal.

    Are there rear panel BNCs on that instrument with adjacent "10M out" and
    "10M in" labels that should have a short jumper cable that is missing?
    That might almost explain all of your problems.

    The slow refresh rate is probably related to error reporting of the
    unlocked condition. It will probably go to normal rate once all the loops
    are properly closed. I wouldn't be surprised if test mode ignores the
    warnings so that diagnosis can proceed more or less normally.

    Glenn
     
  7. John Miles

    John Miles Guest

    It sounds like a problem with the diagnostic circuits. Or: "Oven Cold"
    could imply a bad oven-control circuit in a 10811 OCXO module, if it has
    one. That's a pretty common fault, usually caused by a thermal fuse
    that's opened up due to old age.

    I'd be somewhat surprised if the 8567A had a 10811 OCXO, though. If it
    has a TCXO with no oven, then the presence of the "Oven Cold" message
    makes me wonder if there's an interconnect problem between the 8567A and
    85662A. It shouldn't be possible to get that message on an analyzer
    without an oven-controlled timebase, so that'd be an important clue.

    It's almost certainly not the YIG oscillator, if the analyzer works OK
    in all spans in Shift-w mode.
    It'll be in the RF section.

    Hmm, are you sure? That should be on the RF unit, not the display unit,
    if I remember correctly.
    Will do.

    -- jm
     
  8. Sotris

    Sotris Guest

    Hi
    Thank you for your answer.
    To correct my self :)) rear input of external 10MHz ref is on the RF
    unit and not on display... I've done some more tests...
    Switched on external reference and checked with a +3dbm 10MHz sinewave
    .. When i applied the +3dbm signal it works like with the switch on
    internal position. When i removed the signal from input, all system
    freezed... So the internal ref must at least works. Dont know about
    the freq accuracy of it yet (I'll measure it), but its working. So
    thats good.

    Do you have the service manual of the 8567? If you have it, can you
    make some copies of specific parts of it (I'll pay all the cost for
    them of course)?
    Yes thats should be true. However I still cannot locate a 10MHz TCXO
    inside that instrument. Have download some service (verification &
    adjustment) informations for the bigger brother of it (the HP 8568)
    and at the chapter that reffers to the 10MHz time base has a figure
    showing the location of the A27A1 (10MHz TCXO). The strange is that at
    my HP 8567A there is nothing there!!! Only empty space with some nuts
    to install something and a PCB text near it saying A28 BATTERY... I'm
    starting to believe that thats one of the differences between the two
    models... Not sure however, but there isn't a 10MHz time base at the
    suggested location. It might be installed on a different location?
    Oven Cold and all other messages except the "YTO unlock" are only come
    on during the error correction routine. At normal operation mode, only
    "YTO unlock" & VERY slow refresh rate of sweep are the symptoms. Even
    at that refresh rate can make measurements of an input signal... I
    understand the YTO loop as you explain it. The strange thing is that
    during the test mode almost all measurements of inputs signals are
    correct both for frequency and amplitude. I have also checked an AM &
    an FM modulated carrier up to 1250MHz. And I say almost because I've
    noticed that when I set a Span <1.000MHz cannot find the input
    signal. Increasing again the span at >1.001MHz signal appears again...
    There is a BNC for 1st Local out and another one for frequency
    refference IN/OUT depending on an INT/EXT switch aside of it. I've set
    it on Internal position under normal operation.
    Yes thats true as I've read that during error corection routine some
    signal flags are ignored!
    Well I must found a manual now!
    Any other suggestion will be excellent...
    My regards
    Sotiris pdmtr
     
  9. Glenn

    Glenn Guest

    Correct, sounds like it is using either the internal or external 10 MHz
    reference.
    Sorry, I don't. I'm doing all this from very old memory!
    Given the symptoms, I wouldn't worry about that at the moment. Since it
    works in "Int 10 MHz" position, it's finding something.
    I think that may be a clue. Narrow spans probably use somewhat
    different phaselock circuits, in order to improve phasenoise
    characteristics close to the carrier. The 8568 was very different from
    the 8566, as someone (John Miles? Hi John de n6gn) has already noted.

    The 8568 used a "pilot channel" which was sort of a twin of the main
    signal path but there only to allow improving the phasenoise of the LO.
    I wouldn't be at all surprised if things are switched differently for
    narrow spans where narrow RBW filters mean that the improved phase noise
    was important.
    However, the fact that it sweeps slowly and issues an unlocked error on
    wider sweeps leads me to suspicion that there is still something more
    general wrong with the YO loop.
    Judging from what you've written, I'm shifting focus away from
    suspecting the 10M reference.

    I agree with you that you really need at least an accurate functional
    block diagram so you can unravel things.

    Sorry I don't have any documentation and that I can't remember more detail.

    Glenn
     
  10. Sotris

    Sotris Guest

    Hi John
    Thank you for the follow up message. Please read also my other answer
    to Glenn.

    About the Oven Cold message I reffer at it on reply to Glenn. It only
    displayed running the "Error correction routine" mode.
    Well I've found that at narrow span modes (<1MHz) there is a
    problem...
    Yes it should be there!!! Only thing is I cannot find it! :))
    Nope, you're right, I've corrected that already... :))
    My major problem is that here in Greece we don't have a lot of HP
    support and to find something (used or not) we must take our chances
    with the "SLR" (shipping lose risk) parameter as I call it :( Have
    allready lost 3 manuals for test equipments ordered from US and
    another one delivered to me after 5 months from shipment!!! And mostly
    here, there is a total lack of (second hand) spare parts sources for
    things like test instruments...
    I have seen in the past, your tracking generator for the 8566. Nice
    mod you've done there on the 86222A plug-in. Will something similar
    work with 8567A/8568?
    My best regards to all that helping at anyway...
    Sotiris Pdmtr

    PS. any follow up message is welcome ;)
     
  11. John Miles

    John Miles Guest

    Sounds like there's a TCXO module buried somewhere, instead of the
    standard 10811-60111 oven. I wouldn't worry about it any further.

    In the case of my broken 8566B, the problem turned out to be A20C7, a 10
    uF electrolytic that was open. The YTO main-coil driver circuit was
    oscillating at about 1 MHz, which kept the outer YTO control loop from
    locking up.

    I started troubleshooting by verifying that the M/N and 20/30 MHz
    synthesizers were both working properly, using a frequency counter.
    That meant the problem had to be the YIG or its driver circuitry,
    including the pretuning DAC. I went through the YTO loop adjustments
    section in the tests/adjustments manual (which should be freely
    downloadable from Agilent), and found that while the pretune DAC
    adjustments were OK, I couldn't bring the actual YTO output frequencies
    into range. There was a 1 MHz oscillation in the voltage-to-current
    driver circuit that clearly didn't belong there, so I poked around with
    an ESR meter and found an open electrolytic (A20C7). Replacing it and
    recalibrating A20 took care of the issue.
    Thanks! Spare parts are a problem everywhere, unfortunately. I was
    glad that I didn't actually have a bad YTO, or I would have been in
    trouble. The 1.5-GHz analyzers are pretty different with regard to
    their tuning/conversion schemes, so no, the 86222A mod won't work with
    the 8567/8 directly. Check out the article linked from the DKD
    Instruments page at http://www.dkdinst.com/products/TGdescrip.html for a
    good discussion of what it takes to track an 8567/8.

    -- jm
     
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