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optical knob encoder: HP/Agilent 33120A freq generator, tuning-knob failure

Discussion in 'Electronic Repair' started by Winfield Hill, Aug 8, 2011.

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  1. We have about 10 Agilent 33120A frequency-generator / synthesizers
    in our lab, and a few other similar synthesizers. They feature a
    convenient digital frequency and amplitude programming knob that's
    based on optical encoders. A pair of left-right pushbuttons sets
    which decade you're adjusting with the knob, and over or underflows
    operate on the next higher decade. There's a little indent near
    the edge of the knob so you can spin it with your fingertip. You
    can also use up-down pushbuttons or directly punch in numbers, but
    the spinning knob is very convenient. Typically such encoders have
    16 or 32 ticks per revolution. A very nice design feature.

    The optical shaft encoders were originally made by HP, but now of
    course they're made by Avago. Sometimes they call these a digital
    potentiometer or a "Panel Mount Optical Rotary Encoder". For
    example, the HRPG-AD16 product line, about $34 each at Mouser.

    Since they use an optical 2-bit quadrature grey-code (as opposed
    to using a pair of switch contacts), they're supposed to be
    extremely reliable.

    But recently I've encountered two instruments with identical
    tuning-knob failures, one in an elegant 33250A 80MHz model
    that's on my bench, and the other in one of our standard
    33120A 15MHz instruments.

    In both cases the knob seems 'stuck" and not to work as you
    turn it, except every now and then you'll get one or two
    increments or decrements. Totally useless.

    I'm wondering if anyone else has encountered this failure?
  2. N_Cook

    N_Cook Guest

    Its probably patentable so is there a patent outline for them somewhere, to
    show what the innards are?
  3. JW

    JW Guest

    I have not seen a failure on the 33XXX series of generators, but the part
    # is 0960-0892. Looking at the picture on the find-a-part website shows it
    as a mechanical encoder.

    Which is obsolete and replaced by 0960-2545, also a mechanical one. It
    looks like its the same as used on the 65XX and 66XX power supplies. I do
    know that the older 65XX and 66XX do use the (obsolete) optical ones. It
    is possible to use the mechanical ones to fix the obsolete optical ones
    with a bit of hacking, but the mechanical ones need more turns to change
    the output of the supplies for a given amount.

    Are you sure it's an optical encoder you're looking for?
  4. JW wrote...
    Thanks, great info!
    No, and as you say, the service manual identifies the mechanical
    one as the part, now that I bothered to look. Given the failures,
    perhaps being mechanical makes sense. I had just assumed that
    because HP long-ago developed and manufactured a fine optical part,
    they'd use it in their expensive premium instruments. Heck, I've
    long used their expensive optical encoder in my own designs!

    The mechanical part looks much smaller than the optical one, which
    might not fit as an upgrade replacement. Hmm, their mechanical part
    looks a lot like the ones Spehro was selling as excess inventory.

    BTW, as you can see at my Mouser link, the HRPG optical encoder
    is not obsolete (yet). Hah, it costs $34 instead of $4.23.
    Do we get what we pay for?
  5. Olaf Kaluza

    Olaf Kaluza Guest

    I bought a power supply E3647A and the encoder feels like very bad
    quality. My boss bought the same power supply six month ago and the
    encoder feels much better.

    So I thought the quality of HP goes south and I bought the freq
    generator from Rigol. When HP is selling the quality of
    Rigol now, I dont like to pay the price for HP anymore!

  6. shrtrnd


    Jan 15, 2010
    It's the 5V 'rice' bulb mounted in the encoder housing.
    The bulb needs to be replaced, but you have to be careful.
    There are two bulb lengths easily available, with 3 different current ratings.
    You have to center the filament exactly in the encoder, or you get intermittent
    reading problems.
    (I always make sure I've got the same bulb length, and the bulb is pushed down into
    it's holder at exactly the same depth as the original).
    Good luck
  7. JW

    JW Guest

    You're welcome.
    Could be, but I've yet to see a mechanical one go south unless there was
    some abuse. Even then they are sometimes repairable. The bend over tabs
    that hold the encoder together come apart a bit when the shaft is hit at a
    right angle, which seems to happen all the time on the power supplies I
    mentioned. I've lost count on the number of times I've repaired them.
    After repair I solder two 24ga pieces of bus wire across the top from tab
    to tab which helps sturdy them up a bit.
    If you *do* have an optical one, it may depend on whether it solders
    directly to a PCB, or is panel mounted. If the latter, it probably has a
    cable which attaches to a PCB, and you should be able to adapt the
    mechanical one. If you go that route, I've found that it is important to
    ground the case of the encoder. You'll probably have to fiddle around with
    the wires to get the order right. Trial and error...
  8. JW

    JW Guest

    And what a pain in the butt to get to on a 6034A! I wish I could find a
    "side firing" LED that would not only fit in that little slot, but also be
    the right wavelength for it to work correctly. I've experimented a bit
    with this, but never been successful.

  9. I see mechanical ones go bad ALL THE TIME. I even keep a few of the more
    common ones in stock. I was going to mention this sooner, but it was said
    that there was a pushbutton function on the OP's unit which the encoders I
    see don't have, but one of the pics previously posted look like some I
    stock (and pay about 1.00 each for). They go bad from tarnished contacts, in
    my opinion, but my friend Arfa opines that it is the migration of silicon
    lubricant that is the root of the problem.

    Mark Z.
  10. N_Cook

    N_Cook Guest

    I don't know if its relevant with these encoders but a few times I've found
    with preset size pots that appear in audio mixers. The grease hardens and
    then lifts the very feeble wipers of the pots. No track wear at all as the
    wiper action is so slight , so why the grease ?
  11. I think the silicon grease is there to provide the "feel" when you turn the

    Mark Z.
  12. N_Cook

    N_Cook Guest

    I'd not thought of that as a reason but it makes some sort of sense. I
    suppose the first pot one ,I cleared the grease out with meths and
    reassembled dry, refitted, and returned to owner must be about 5 years ago.
    That one and subsequent ones, have not bounced back with worn tracks , so
    superfluous as far as electrical function is concerned , IWS. I suppose
    someone will now tell me I should have replaced like for like, not going
    against the "designer's" wishes.
  13. N_Cook

    N_Cook Guest

    Of course being a seller of lubricants there is no tabling of the long-term
    performance as far as immunity to hardening. That "velvet feel" becoming
    gumming up the works . And pot track lubrication being insufficient to cause
    "aquaplaning" and then with aging/ chemical reaction, hardening and forming
    a wedging under the wiper and so loosing contact for all or most of the
    track run.

    A job for tomorrow is to take some measurements of one of those tiny preset
    size pots.
    The wiper arcs are something like 3 off 10mm long , and section .2x.4mm IIRC
    but I will try and measure the contact force , I'm guessing at this stage of
    order 10 gm. Once one wiper lifts I imagine its not long before all 3 lift
    at some part of their travel.
  14. N_Cook

    N_Cook Guest

    Alpha type 4D3-11 pot , footprint 9.6x 11.2mm. 2 wiper arcs about 7 and 9 mm
    long. Wiper metal 0.1mm thick x 0.3mm.
    Could not measure contact force in-situ. With wiper removed from recess, 10
    gm would visibly move one wiper relative to the other. No visible track
    scoring under a x30 magnification.
    Of course , those old enough, would have come across the effect of hardening
    grease in cassette tape mechanisms then VCR. Where an arm pivots on an axle
    and return action is just a light torsion spring.
  15. N_Cook

    N_Cook Guest

    Easily dissimilar metals , Al and brass and sliding fit , then the naval
    Cold enough to freeze the (iron canon) balls off a brass monkey (trivet)
  16. Hey, Win:-

    We had a mechanical encoder failure on a 33220A arb function
    generator. It is a known issue with these encoders and Agilent sent a
    repair kit gratis (or we could have sent it in). It's an easy fix, so
    I elected to do it myself.

    Best regards,
    Spehro Pefhany
  17. JW

    JW Guest

    Here's the service note:
    The expiration for free service or parts is February 1, 2012.
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