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Need help with Tektronix 2710 spectrum analyzer

Discussion in 'Electronic Equipment' started by Esa Heikkinen, Nov 4, 2008.

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  1. Hi

    I need help with Tektronix 2710 spectrum analyzer...

    It's having some frequency domain fault above 1 GHz. Infact there are no
    exact point where the fault begins: sometimes it will work even to 1.3
    GHz but there's a days when even 800 MHz is too much for it... However
    with lower frequencies (below 700 MHz) it always works.

    Firmware version is 01.15.90 and installed options are 300 Hz RBW
    filter, phase lock, tracking generator, centronics and frequency counter.

    When the fault kicks in there's something going wrong with center
    frequency control. Frequency domain of the spectrum is shifted about 26
    MHz lower, for example is center frequency of 1.2 GHz is set and that
    signal is fed to the analyzer the peak will be found at 1174 MHz. Some
    times it may "jump" between correct and wrong frequencies when the
    center frequency knob is turned. Amplitude is measured correctly.

    When changing to narrow spans (20 kHz/div or less) and the phase lock
    option is turned on it will sometimes even lock to the wrong frequency!
    When this happen the frequency counter option (which can be used to
    count any displayed peak's exact frequency) will also show about 26 MHz
    wrong.

    However it's quite common situation that when the frequency domain fails
    the phase lock and frequency counter options won't work anymore. Error
    message "FREQ NORM SUGGESTED (1ST LO)" or "FREQ NORM SUGGESTED (FIND
    SIDE)" is displayed instead. Of course running that normalization
    process does not help and the normalization process will be passed
    without any errors.

    This unit has YIG local oscillator. I have service manual and I'm
    already done the reference normalizations, span attenuator and center
    frequency accuracy adjustments, LFVCO and HFVCO adjustments and counter
    level adjustments to fix the fault - no help. Service manual won't help
    with those error messages for example what's the "FIND SIDE" is meaning?

    The center frequency control system is quite complex (it's 80's
    technology) and infact there's 26 MHz VCO on center frequency board,
    which controls the YIG's coil currents/center frequency. This could
    somehow explain the 26 MHz frequency error but how - and why this
    problem only happens on higher frequencies?
     
  2. Joerg

    Joerg Guest

    You could check the power supply and bias amp for the YIG. But if the
    YIG itself is toast I think most are sealed assemblies that can't really
    be field-repaired.
     
  3. Max65

    Max65 Guest

    Hi Esa.
    First of all, let me tell you that I never seen the 2710 schematics,
    the things I write below are just suggestions based on intuition
    (maybe you had already do that).

    The 26Mhz VCO you find on the center frequecy board, should enter into
    a mixer. Look for the LO intput level of the mixer, maybe it is too
    low or near the level at which the mixer starts to work (a passive
    mixer usually need a LO signal of 4-10dbm to work).
    Of course, maybe the 26MHz VCO frequency or the other frequency
    incoming into the mixer the LO signal, so check both the signals for
    the right level.
    Maybe it's just a "cold" solder joint.

    Hope the above will help.

    Good luck.

    Have a great day.

    Massimo
     
  4. Hi Max.
    I did some scans from service manual schematic and circuit description
    pages. Maybe these will help to get further:

    http://www.amigazone.fi/files/tek2710/schema1.png
    http://www.amigazone.fi/files/tek2710/schema2.png
    http://www.amigazone.fi/files/tek2710/schema3.png
    http://www.amigazone.fi/files/tek2710/schema4.png
    http://www.amigazone.fi/files/tek2710/schema5.png
    http://www.amigazone.fi/files/tek2710/description1.png
    http://www.amigazone.fi/files/tek2710/description2.png
    http://www.amigazone.fi/files/tek2710/description3.png
    http://www.amigazone.fi/files/tek2710/description4.png
    http://www.amigazone.fi/files/tek2710/description5.png
    http://www.amigazone.fi/files/tek2710/description6.png
    http://www.amigazone.fi/files/tek2710/figure1.png
    http://www.amigazone.fi/files/tek2710/figure2.png
    http://www.amigazone.fi/files/tek2710/figure3.png

    I do how the frequency synthesizers work but still can't get the figure
    of this! For example there are no N divider at all and the description
    claims that the harmonics of signal called "strobe" is used instead? And
    this "strobe" signal is infact that ~26 MHz signal mentioned before! But
    there are 11 MHz VCO also... And sometimes it counts "beat" and
    sometimes phase offset like normal PLL. I don't really know why this is
    so complex - may be there wasn't dividers fast enough to count 2.11 -
    3.91 GHz in the end of 1980's?

    I guess that it locks to wrong harmonic of the strobe, because the
    frequency is always ~26 MHz off when the fault kicks in. But why this is
    happening only with higher frequencies - just no idea. Also, the service
    manual claims that with wide spans the strobe signal is not even turned
    on and the YIG main coil is driven with suitable current controlled by
    CPU. So how the frequency can be same amount off without "strobe"? Only
    explanation that I could guess is that the fault was on when frequency
    normalization was run and because of that there are wrong DAC values in
    some kind of "main coil current table" in the CPU.

    Clearly I'm too novice with those 1980's RF circuits...
     
  5. Max65

    Max65 Guest

    If I were you, I checked the incoming signals at the mixer HY450. If
    I'm right, its output (BEAT NOTE) should be either the PLL feedback
    signal when used in small span scans and the center frequency tuning
    signal in the other cases.
    Does Q460 work correctly?

    Massimo
     
  6. If I were you, I checked the incoming signals at the mixer HY450. If
    Yes I already did some measurements from it's output from (from J430)
    with oscilloscope. Signal at this point is weak, infact it can't be
    clearly measured with digital oscilloscope at all, execpt when it's
    frequency is low (couple tens of kHz) when it's amplitude raises. The
    frequency here changes when spectrum center frequency is tuned. The
    strobe signal in J460 is strong.

    Infact this strobe signal is not always turned on when instument is in
    normal mode. It turns only momentary on when frequency correction is
    done between sweeps, occassionally. I guess the reason for that is
    keeping noise floor good (-143 dBm in this unit). But of course it's
    possible to turn the strobe signal continuously on with instument
    diagnostics menu.

    When J430 is disconnedted there will be an error message "CANNOT COUNT
    BEAT FREQUENCY" in the screen and sweeping also stops, instrument goes
    useless. There are also frequency counter tests in diagnostic menus.
    With this function it's possible to monitor the VCO and beat frequencies
    real time. If J430 is not in place / no signal these counter tests will
    also fail. So there must be some signal on J430 because there are no
    error messages and instrument works perfectly below 700-1 GHz.
     
  7. Max65

    Max65 Guest

    It should be continuously on when you use narrow spans too (below
    10kHz),
    the reason it's not continuous on moderate spans is because it is
    used
    as reference for a while, then the correction error is sampled and
    hold
    to keep the YIG sawtooth signal centered on the desired frequency.
    Do you know what's the frequency test?
    I mean, does the test run below or above 700MHz?
    I don't agree your statement above, maybe that (in some
    circumstances)
    the mixer is underdriven by Q460 or the YIG signal is too low, in
    those
    situations the mixer may have some conversion losses above 700MHz.
    The fact that the defect is frequency dependant, leads me think that
    the
    problem should be into the YIG interface (limited to the HY450 mixer
    circuitry) or the YIG oscillator.
    Check the YIG level on J100 too. The YIG oscillator is ovenized.
    For that reason its circuitry is thermal stressed every instrument's
    power on/off cycle, and it's not rare to have cold solder for that.

    Have a nice day.

    Massimo
     
  8. Max65 kirjoitti:
    That could be bad. I'm quite sure that there are no spare parts
    available for those. Expecially HY460 which internal circuit is unknown.
    Need another spectrum analyzer for that because I don't have any power
    meter for those frequencies... It seems that there are no adjustments
    for YIG level. The RF level adjustment in the local buffer seems to
    control only the 1st mixer injection but not HY460 input. The YIG has
    some internal trimmers but those are glued. I have no idea what those do
    and there's nothing about those in service manual. Service manual says
    that if YIG is defective it will be changed as a compete installation.
    Is it possible to fix soldering INSIDE YIG? There is a PCB on top of YIG
    can, it's the 1st lo interface. I have already check solderings in it.
    But the YIG can itself is closed with screws. I didn't open it because
    I'm unsure if it will damaged if opened?? However schematics show that
    there is circuit inside YIG also and cold solder could be there.
     
  9. Max65

    Max65 Guest

    Hi Esa,
    I don't want let you down for this, my own was just an hypothesis.
    The HY460 should be just a mixer and I don't believe it could be
    defective
    because it's placed in a safe area (I mean, it's not like the input
    mixer that
    could be damaged by the unknown incoming signal of the analyzer test
    port).
    I can't figure out what trimmer you see inside the YIG, since the
    schematic
    doesn't show any there.
    Anyways here in Italy we say: "don't cry over spilt milk, go ahead".
    If it is really defective, you could try to open it and see what to
    do.
    But before it, you should establish whether it is faulty or not.

    You should look the Beat Note at J430 with your oscilloscope when the
    analyzer shows the wrong behaviour and see if the signal is similar to
    the
    one you see at lower frequencies where it works right. Since the
    signal is the
    result of the beat of the YIG and the VCO signals, you should change
    the
    center frequency slowly and see what happen for at least 26-30MHz and
    compare the two behaviours.
    If you see differences, the faulty component/solder should be before
    J430,
    otherwise it should be after J430. This could be a good start point by
    my
    point of view, don't you?
    I remember that a good friend of mine who worked for Rohde & Schwarz
    for
    a long time repaired one of that one day, I see it. The real problem
    was to
    solder the components on the alumina boards which are highly thermal
    dissipative, so you need an high power soldering iron and limits the
    time.
    (one other issue was don't lose the small yttrium iron garnet, I
    remember
    it was not greater than 3mm in diameter).
    Yes, unluckily often the cold solders are there because of the
    thermal
    stresses I told you before.

    Check for the behaviour on J430 and let me know, nothing is lose for
    the
    moment.

    Massimo
     
  10. Max65

    Max65 Guest

    Hoops I feell an idiot, in the previous message I wrote: "I can't
    figure out what trimmer you see inside the YIG, since the schematic
    doesn't show any there." Better looking to the schematic I realized
    that you refer to R330, R340 and R130 on the YIG bias board called
    A19A1 1st LO INTERFACE.

    More, did you adjust the RF OUT LEVEL (R288 on the 1st LO BUFFER
    board?
    If you had to do it because the signal on J380 was to low it clould be
    a symptom of bad behaviour of the YIG.
    Anyways you could measure the voltage level on the emitter of Q190.
    The higher is the voltage the lower is the YIG signal incoming on the
    board. You should measure it below 700MHz and above it and loot for
    any difference.
    Massimo
     
  11. Hi Max,
    Here's a picture of HY460:
    http://www.amigazone.fi/files/tek2710/HY460.jpg

    I checked that with microscope and no any visual defects. But of course
    that doesn't prove anything about it's condition...
    That's a good idea and I planned to do that now in weekend. But like I
    told before the signal here is so weak that it can't be seen with my
    digital oscilloscope and my analog oscilloscope is too slow. So I plan
    to build some quick-and-easy preamp for oscilloscope - with some OP AMP!
    However after digging all my "treasure shelfs" it seems that I haven't
    got any opamps with enough bandwith. Maybe I will try one with 5 MHz
    bandwith (-3 dB point) it's possible that it will give some gain also
    with higher frequencies - or not... :-/ However the signal on the
    digital oscilloscope is so weak that i would need a gain of 10 or 100 or
    more. Maybe some video amplifier taken from scrap VGA monitor could do it...
    Last time I tried to do that the signal was visible only to few hundred
    kHz and then the amplitude decreases rapidly. But maybe with preamp...
    Yes I'll back when there's someting new.. Have to find some solution for
    the oscilloscope preamp..
     
  12. More, did you adjust the RF OUT LEVEL (R288 on the 1st LO BUFFER
    For now I haven't touch it at all. If I understood correctly it will
    only adjust the 1st mixer injection and has no effect on HY460 input?

    Also I'm not sure if I lose the amplitude measurement calibration if I
    adjust that? Of course it's possible to recalibrate the analyzer after
    that, but may be it's wise to do some reference measurements before
    adjustment...
    Have to try that also when 1st lo buffer is open - it's almost hour of
    disassembling, tens of screws etc. and some semi rigid cables have to
    replaced with coax until it's possible to test run 1st lo buffer on the
    table... :) I hope that these kind of set-up or running it without it's
    shielding will not change it's performance too much. You can see the
    part of 1st lo buffer assembly on HY460 picture.
     
  13. Max65

    Max65 Guest

    Here's a picture of HY460:
    Uhmm... It looks like a signle balanced mixer, but the outcoming
    signal from T450 should have a great shape, it must have harmonics
    over the GHz limit to work with that couplers.
    Do you have tried to see the signal on R450 with the scope?
    Never use x10 probes (if you did) since they highly reduce the
    bandwidth of the scope. Anyways the T450 transformer output seems to
    be insulated from ground and have very low impedance, so your x1 probe
    should be used without any problem (doens't matter where you put the
    ground terminal).
    If your scope has at least 40MHz bandwidth you should see a discrete
    square wave there (I guess).
    I wonder reading that you read a so low signal at J430 because looking
    to the schematic of the following stage, you can see just two opamp
    having low gains (about 10) connected there.
    Yes, it is. I asked just to know if the YIG has fall down of some dB
    there.
    Do not adjust any trimmer if you don't know the procedure for do it, I
    asked just to know if the service manual said something about it.
    Have a nice weekend.

    Massimo
     
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