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Tek 465 5v Rail Issue

Discussion in 'Troubleshooting and Repair' started by RossH, Feb 9, 2021.

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  1. RossH

    RossH

    5
    1
    Feb 9, 2021
    Newbie-to-Tek-scope-repair here. I was given a 465 for free. I would like to repair it.
    Initial symptoms as reported by previous owner: no activity on panel upon startup, fan only.
    Initial power-off inspection and investigation: nothing showed on visual (no dry soldering, no burnt or otherwise iffy-looking components). Didn't smell anything that was suspect. Continuity all good with fuses, plug to switch activation, etc.
    Power-on investigation: all rails showed good except the 5v, it tested -0.45. Re-examination of 5v area using a 30x loupe showed a cracked rectifier (no other abnormalty, such as burning, etc). Pushing on one side with a dental tool caused the halves to shift slightly.
    I replaced the rectifier, double-checked solder joints, and powered up. The rectifier unsoldered itself within 3 sec. I removed the rectifier and re-checked the other rails, all good.
    So, what likely happened?
     
  2. bertus

    bertus Moderator

    1,612
    638
    Nov 8, 2019
    Hello,

    When the rectifier gets so hot that it unsolders, there must be a short somewhere.
    Does the PCB have tantalium capacitors mounted?
    I have many times seen them create a short.

    Bertus
     
  3. RossH

    RossH

    5
    1
    Feb 9, 2021
    Yes, tantalums a-go-go. I haven't checked those yet and should have first. However, I know they're a potential problem and will likely replace them as a prophylactic measure since I have to unsolder one side to test anyway.
    I figured it's a short somewhere, but I wanted some input before proceeding further. This time I'll be a good boy and follow the isolation procedure for that rail.
     
  4. davenn

    davenn Moderator

    13,785
    1,936
    Sep 5, 2009

    tant's often fail short cct
    Takes me back 12 years to when I got my 2 x Tek 465B 's up and running. Still have one on the shelf behind me.

    I will do some looking back to see what I discovered
     
  5. davenn

    davenn Moderator

    13,785
    1,936
    Sep 5, 2009
    @RossH

    Here's the 5V rail starting point and a couple of the other rails

    Tek 465B part PSU.jpg


    NOTE: if the +55V rail is missing, then so will the 5V rail and several of the other rails


    Dave
     
  6. RossH

    RossH

    5
    1
    Feb 9, 2021
    Thanks, Dave. Ya, I've been going over the schematic and wondering about the cans. I may have to just dismantle this further than I wanted to. All other rails are good.
     
  7. RossH

    RossH

    5
    1
    Feb 9, 2021
    Thank you, that's a nice offer!
     
    davenn likes this.
  8. 73's de Edd

    73's de Edd

    3,064
    1,299
    Aug 21, 2015
    C1559 ?
     
  9. RossH

    RossH

    5
    1
    Feb 9, 2021
    That is the tant I thought would be a good candidate. C1552 also caught my eye, but wasn't sure if it was as good a candidate for the BIG melt. I'm new at oscilloscope and PCB work and I'm drawing on my tube radio knowledge to attempt this. I must admit I'm loving the learning experience and all the help I'm getting from you all is much appreciated.
     
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