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Lecroy 9424 calibration faillure

Discussion in 'Troubleshooting and Repair' started by nvs, May 11, 2013.

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  1. nvs

    nvs

    2
    0
    May 11, 2013
    I recently bought a Lecroy 9424 oscilloscope in an auction: a 4 channel digital (aged) one...

    While testing, I found out no valid channel/traces are shown on screen: a weird ugly 8khz repetitive non-sinewave signal is shown on all channels, regardless of any setting chosen...

    Started debugging the fault(s)... looking for a service manual...searched the internet, but did not find a manual for free... but found a service doc for the 9450 where the frontend looks very similar... hence started debugging using this one

    So far, I found 2 rootcauses for defects:
    1. a 74HCT4094 lacking output signals causing some leds not been displaying some settings on the front.
    2. the supply of the front-end has a huge 4V ripple on any supply: replacing the 6V supply by a standalone labsupply solves the ripple (6V @ 280mA - no clue if this within spec) --> suspect a LM78GCP regulator being defective.

    Right now, there is one problem I could not solve yet: the 1MOhm input (OAC and DC) of channel 1 is failing the built-in calibration. Anythin else seems working...

    Does anyone know how this 1MOhm AC&DC input/calibration is working? And/Or a hint to the rootcause to solve this issue? And/or owns the 9424 service manual?
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,390
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    Jan 21, 2010
    Have you fixed the 2 faults you have found?

    I'd certainly be looking at the root cause of the power supply problem first. Is there huge ripple on the power supply for the 74HC device? That would probably explain its failure.

    p.s. Guess what's on TV here :)
     
  3. nvs

    nvs

    2
    0
    May 11, 2013
    Rootcause of the power supply problem is a misbehaving LM78 when loaded: disconnecting the voltage regulator and replacing the output by a seperate 6V lab supply solves the supply ripple completely. (Input of the regulator does not show a (huge) ripple.)
    Expect replacing the regulator will solve the issue.

    The problem of the 74HCT device looks to be one single broken ic:
    board contains multiple of these chips connected in series... only one is having high impedance outputs while the input and control signals are correct and are the same for the remaining good working ones. Replacing chip will solve this problem.

    The remaining problem I'm looking at is the offset regulation circuit used for the 1Mohm input of channel 1 only. or the bandwidth limiting circuit.... (or something else...?)
     
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