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HP 141T power supply question

Discussion in 'Electronic Design' started by Glenn, Apr 27, 2013.

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  1. Glenn

    Glenn Guest

    I have looked at the power supply schematic for the HP 141T with the
    intention of improving it, and found something that is very odd (also
    found others - maybe another time):

    Old one - "neon tube" stabilized (pdf-page 35):
    141T Display.pdf

    New one - zenerdiode stabilized (pdf-page 10..12):
    141T_changes.pdf

    (both downloaded from here: http://bama.edebris.com/manuals/hp/141t/ )

    The three lower driver and power series regulator transistor base
    resistors - to high voltages - bothers me. I would prefer e.g. 100 ohm
    to the emitter.

    If the output transistors turn off, just for millisecond, the base is
    pulled "downward" to the high voltages. The (-)Vbe should never be
    greater than 5 volt for most transistors - even for the high power.
    Almost every datasheet tells you so.

    -

    What is your opinion about this? What was the PS designers thinking? I
    know that the instrument was the state of the art. Please help me to
    understand this PS design.

    Glenn

    (the question is also asked in news://sci.electronics.design )
     
  2. Guest

    It looks to me like the Q7-8 diff amp could put -20V Vbe across
    Darlington pair Q6-3 under fault conditions (R37 CW leg open). Pretty
    unlikely failure mode.

    Whether or not they could take it depends on which'un trannysauruses
    dems was (is). CR13,14, and 18 suggest the answer was "no."

    Cheers,
    James Arthur
     
  3. miso

    miso Guest

    Either way, that is an instrument that stood the test of time. I was
    given one a few years ago. One of two things I use with the old style
    IEC plug.
     
  4. tm

    tm Guest

    Hi Jeff,

    The old version supply uses a neon bulb as the reference for the 100 volt
    supply. This regulator drifts over time as the bulb ages. It gets to a point
    that the supply cannot be adjusted to spec and the special bulbs are hard to
    find. The problem gets worse as the 100 volt supply is used as a reference
    for the other supplies.

    The new design changed to a 9 volt low TC zener that makes things much
    better.

    HTH,
    tm
     
  5. Glenn

    Glenn Guest

    On 28/04/13 17.51, Jeff Liebermann wrote:
    ....
    ....

    Was done at the same time, but I have followed this group some time and
    read many posts from many electronics expierienced people. So I thought
    this group was as good as the HP scope forum.
    I also just found a page that made me suspect the problem might be "not
    enough radioactivity" in the tube stabilizers, but I was not sure. The
    radioactivity ionizes the gas, which is then much easier to start.

    Now - I think that the problem of the sometimes blown fuses are because
    of the tube stabilizer can not start, and therefore the voltage
    regulator send max. voltage out which trips the overvoltage SCRs in the
    plug-ins at -12.6V. SCR trip at approx. 14.4V and the unregulated
    "-12.6V" is approx. -18V.
    I would like to make a new PCB with the zenerdiode circuit, but it
    should not look like a rat nest...

    The "zener" would be chosen to be a TL431-like IC.
    As others has written (might be in the HP forum) these zeners is not
    temperature stabilized.

    That 69.4v zener is already occupied, it is used to save the transistors
    from voltages higher than 70V at start - and at regulator output short
    circuit. Then the fuses blows som hectomilliseconds later.

    Glenn
     
  6. Glenn

    Glenn Guest

    ....

    I have four 141T :)

    The reason was to use component from one or two to repair the others.

    But I have repair and calibrated all power supplies.

    Two of the 8555A "wheels" has been disassemblied, drilled, glued so they
    work again. The plastic that holds the contact-metalblade arm had broken.

    I have these problem now:

    * one scope can not get high enough intensity and can not get in focus,
    but it is useable. Tripler blown? I presently do not own a multimeter or
    a probe that can measure 6.6 KiloVolts.
    * two blows fuses occasionally
    * two 8555A has blown diode first mixers (one blown by me - the antenna
    cable was not earthed before measurement) - but I have two other 8555A
    that now works fine. One has a missing "pointer" belt though. Then it
    should be easy to take a module from the other busted ones - NOT! they
    use a different socket plug - how can this happen for the same product ;-)
    * one power supply uses zener - and the other three use tube
    stabilizers. The difference came as a surprise :) But later the newer
    schematic was found.
    * one chassis is missing many knobs.

    I presume that the storage do not work - or I have not read enough about it.

    -

    The reason I bought them is that they for the most part use common
    components, and I would like to know how they work. The schematics are
    fascinating to study.

    Two years ago I would never expect to own a single multi GHz spectrum
    analyzer. Now I have approx. 3.5.

    I also use the preselector in front of 8555A, so signals do not get
    aliased into the band of interest.

    Glenn
     
  7. Glenn

    Glenn Guest

    On 28/04/13 17.51, Jeff Liebermann wrote:
    ....
    ....

    Hi Jess

    Actually my question was about electronics (re)design :)

    Glenn
     
  8. Tim Williams

    Tim Williams Guest

    Ya know, I wonder if you could take the bulbs down to the local physics
    department's "neutron howitzer" and get some neutron activation going.
    It's not going to replace Kr85, but there's probably a few elements
    present which will absorb with a reasonable cross-section (i.e., it won't
    take weeks of exposure to have a useful effect) and half-life (most
    neutron activation experiments generate half-lives in the minutes range,
    which is great for detecting them, but not so useful for ionization and
    stability). It would need to be aged to allow those short-lived isotopes
    to decay, of course.

    Tim
     
  9. ....

    That range looks almost right for a SG202B (??202?) type gas stabilizer,
    its range is nominally 81-86V. These things were made in large enough
    quantities in the Soviet Union, back when that existed, so that they're
    still easy to get and cheap. Fleabay has some for a buck plus shipping.
    Also Pollin (an electronics discount shop from Germany) sells it for 2?.
    They sell mostly to Germany and Austria, but can ship internationally.
    link: http://www.pollin.de/shop/suchergebnis.html?S_TEXT=SG202B

    There is no guarantee that it starts (this lamp has a maximum specified
    strike voltage of 135V and most are about 30 years old now), but that
    spec is pretty loose to allow for a lot of age-related wear and tear.
    Maybe try 2 or 3 in parallel and use the one that turns on as it will
    be the lowest-voltage one. If that works, it may replace the original
    more or less "as is" without significant modification.

    If you decide to put one to use, note the polarity (center pin = anode),
    otherwise it will drift out of spec before its rated lifetime.

    Dimitrij
     
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