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tek 465 alike osciloscope help

Discussion in 'Electronic Repair' started by Steve Sousa, Jul 24, 2007.

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  1. Steve Sousa

    Steve Sousa Guest

    Hello:

    I've been trying to repair my osciloscope, it's not a well known brand
    but it's fully discrete and the circuit is similar to a tek 465.
    The problem is that i can't see square signals, they are very distorted
    on both channels. I tried to follow the signal with another scope but
    didn't get far, because my readings didn't show a similar signal to what
    was displayed and maybe because i don't know how to properly measure on
    a diferential amp.
    I tried the trick of shorting the transistors shown on the tek guide on
    how to repair scopes and it didn't help me, and i tried replacing the
    diode switches with new ones and isolating each chanel but could never
    reach a conclusion.

    I'm lost.

    I made a test circuit to get some reference signals: a 20MHz dip
    oscilator module (8 pin metalic can) feeds two 51 ohms resistors, that
    go through 20cms of rg174 coax into a bnc male that goes into a bnc tee
    were the other ends go to the osciloscope input and to a 51ohm resistor.
    This circuit displays the"20Mhz" pics.

    I also probed with a X10 osciloscope probe a board that has an 74hc74
    flip-flop clocked at 16MHz and got the "16MHz" and "8MHz" pics, these
    last one didn't come out the same because i had to capture the event but
    it shows the ringing at the rising and falling edges. For the
    osciloscope probe i used a tiny (1cm) piece of wire from the tip ground
    to the ic ground pin, not the usual ground clip.

    The pics are at: <http://sweet.ua.pt/~etsteve/osciloscope.htm>

    Osciloscope history of repairs that i made was a bad bridge on the power
    supply (intermitent open diode) and a logic chip that togles the chanels
    in ALt mode.

    Any help/pointer are apreciated.

    Thank you

    Best Regards

    Steve Sousa
     
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