Connect with us

Tek 465B

Discussion in 'Electronic Repair' started by Jim Yanik, Nov 7, 2007.

Scroll to continue with content
  1. Jim Yanik

    Jim Yanik Guest

    They would have to be matched,else you get an offset.

    check this out;
    http://www.sphere.bc.ca/test/tek-parts/tekparts2.html
    it appears they have some 151-1090's for sale.
    they also cross ref to a "2N5911 family".
    the 155-0078-xx is used in a lot of different scopes,several 7000 series
    plug-ins,too.
     
  2. bg

    bg Guest

    Both channels on the verticle front end are damaged. One dual FET follower
    is a definite (Q1297). It is tek part number 151-1090-00. I haven't seen one
    yet using google. Is there a replacement or two discretes out there that
    will work?
    I noticed that the pinouts for the chip following the dual FET (U1290), are
    incorrect. Doing a diode test on the chip, there are several pins
    mislabeled. The external circuitry seems to go to the pins as shown on the
    schematic, but the internal circuitry ohms out to different pins. This would
    only be an issue if you ohm out the chip like I did to see if the chip was
    bad. If anyone is interested, I can post what seems to be the correct
    internal circuitry.
    bg
     
  3. Jim Yanik

    Jim Yanik Guest

    I've seen the input BNC connector cause DC offsets!
    seems it absorbs contaminants and becomes a weak "battery".
     
  4. James Sweet

    James Sweet Guest


    Now that's certainly not something I'd have expected. I'll keep it in mind
    should my 465 ever misbehave in that way.
     
  5. Jim Yanik

    Jim Yanik Guest

    are there input protection diodes on that FET input? did you disconnect the
    atten PCB and still have the DC?
    (and then GND the gate and the offset goes away?)
     
  6. Jim Yanik

    Jim Yanik Guest

    definitely a weird one. only had a couple of those,as I recall.
     
  7. bg

    bg Guest

    Jim Yanik wrote in message ...
    Thanks Jim. I had already been to that site and didn't see it the first
    time, even with my glasses on!
    Offset is the issue I am having.
    bg
     
  8. bg

    bg Guest

    Jim Yanik wrote in message ...
    I replaced the channel 1 connector, but plugged in one of the power
    connectors backwards. So I blew one of the dual fets and the other one seems
    to have a leakage problem. There is DC on the gate lead right after the
    attenuator. The 1 meg resistor in the attenuator output isn't low enough to
    pull the gate to 0 volts. Out of the circuit I can't measure anything
    abnormal with the FET but in circuit the DC is there.
    BTW , you are a great weath of info for these tek products.
    bg
     
  9. bg

    bg Guest

    Jim Yanik wrote in message ...
    I replaced the ch1 connector, but connected a power connector backwards. It
    blew one of the dual FETS and the other seems to have a leakage problem.
    There is DC on the gate where the attenuator output connects. The one meg
    resistor in the attenuator output is not low enough to pull the gate to
    zero. Out of the circuit, I don't see anything wrong with the fet.
    BTW , you are a great resource for these tek products, thanks.
     
  10. Jim Yanik

    Jim Yanik Guest

    No,the atten grounds it almost directly.Maybe thru a ~47 ohm R.
    I forget the exact circuit values.
    Check your schematic.
    1 megR is the basic input Z,it's no "ground".
    You HAVE to see signals across that input Z.
    Have you done your DC bal adjustments?
     
  11. bg

    bg Guest

    Jim Yanik wrote in message ...
    I've disconnected the gate completely from all of the external circuitry and
    then connected it to ground thru a 1.5 meg resistor , which I assume is what
    it sees from the attenuator. Seeing as the gate draws no current , there
    should be no voltage right, or perhaps just microvolts. I get way too much.
    If I short the gate to ground or use low resistance like about 50 ohms, I
    can drive it with a generator and all is fine. But with High Z to ground,
    the offset is too high for the verticle position control to bring the trace
    back on screen and none of the balance or position pots have enough range.
    bg
     
  12. bg

    bg Guest

    Jim Yanik wrote in message ...
    The output of the attenuator has a 1Meg resisitor shunted across it. The
    gate connects to this resistor with a 470K. I think the short only exists
    when the front pannel AC/OFF/DC switch is in the off position.
    It's supposed to short the output of the attenuator. If I put that switch in
    the off, center position, the trace does come back to center screen. Balance
    adjustment works, gain adjustment works. The attenuator measures 1 meg in on
    all ranges. I've ruled out DC in the attenuator by disconnecting the FET
    from it.
    I think I just need to get a new fet because everything points to the gate
    having leakage of some sort.
    bg
     
  13. Jim Yanik

    Jim Yanik Guest

    DC-GND-AC.
    You just said that shorting the FET gate thru the 470K R removes your
    offset;that tells me the offset is coming from the atten/BNC.
    Not relevant. You're dealing with mV offsets.
    And grounding the gate,you still get an offset?
    The FET gate has an extremely high input Z(100's of Megs),the 1 MegR -sets-
    the scope Z to the desired input Z.
     
  14. bg

    bg Guest

    Jim Yanik wrote in message ...
    With the gate totally disconnected from the rest of the scope, I connected
    it to ground thru a 470k resistor. There is about 1.5 volts DC on the gate.
    The spec sheet says this gate should deliver about a 100 pico amp reverse
    biased with 15 volts. That comes out to 47 microvolts of offset on the gate
    lead. So this FET has some pretty severe leakage at the gate.
    Sorry for the confusion.
    bg
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-