Oscilloscope repair

Discussion in 'The Projects Forum' started by bance, Nov 28, 2013.

  1. bance

    Thread Starter Member

    Aug 11, 2012
    315
    34
    Hi guys,

    I've just started a new project and thought I'd post it here. I bought an old Kikusui oscilloscope, "spares or repair" from e-bay. Actually I wasn't after a scope, I was after a couple of beefy linear power supplies, from the same seller (also "spares or repair",) but they went for far too much money!!!!! (This was a collection only item, so I got a chance to speak to the seller. He was buying and selling audio gear, and told me that the scope was part of job lot he'd bought when a repair shop had closed down.)

    As a consolation prize I won this:-

    [​IMG]

    Uh-Ohh, Two missing knobs.......... Two missing buttons.......... And three missing lever knob ends!!!!!! Yikes!
    Anyway it came with a power lead, so I powered it up and waited for the magic smoke......

    Nothing, nada, nicks..... No trace, no lights, no smoke, nothing.

    Ahh Hah, must be a fuse.... No, the fuses are OK.

    OK, let's open this sucker up. Nine screws later and Tah-dah....

    [​IMG]

    UH OHHH What's this:-

    [​IMG]

    Pretty catastrophic failure, I wonder what caused it? Anyway I've had the scope for a few weeks, and had already tracked down a service manual.Also tucked away inside, under the CRT was some hidden treasure.... most of the missing hardware from the front panel and the burned out components.Looks like some heart bypass surgery is needed!!!!!!!!!

    The top board had already been removed at some point, you can probably see the board to board connectors are lying all over the place in the case open picture. All I had to do was unscrew the shielding can. pop the plastic rivets that held the board to the chassis, and lift it out.

    [​IMG]

    Now we get a better idea of what needs to be done!

    Ok, so I've seen this kind of repair before (on one of those Pace soldering videos on 'you-tube'.)

    I need to cut back the damaged PCB until I find sound fibre-glass, make a new section to match and then patch it in with epoxy resin, and solder bridge the tracks.
     
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    5,003
    745
    well if you have the manual , then identify the components from their pcb positions, at a guess i would say the large burned hole was a power transistor , or regulator on a heatsink that got fried.
     
    Last edited: Nov 28, 2013
  3. MrChips

    Moderator

    Oct 2, 2009
    12,452
    3,371
    I wouldn't bother to cut back the burnt PCB. It's not like you're trying to patch a hole in drywall that someone put a fist through.

    Send us a link to the schematics and we'll take it from there.
     
  4. #12

    Expert

    Nov 30, 2010
    16,354
    6,852
    Physical layout in a high frequency circuit can be very important. I would find out what part of the circuit is burned away and consider whether to do it like a drywall patch. It is possible to do that if it's only a one sided board. Please discover what part of the circuit has been burned up.
     
  5. bance

    Thread Starter Member

    Aug 11, 2012
    315
    34
    Spot on, Dodgydave, it's a power transistor OK, but it also took a couple of resistors and a cap.

    A section of the schematic:-


    [​IMG]

    A section of the layout:-

    [​IMG]

    MrChips, you're right, I don't have to make a complete repair to get the machine up and running, but I'm a bit old fashioned like that (do it once and do it right!) So I think I will make a new section of board and patch it in, eventually. But,I don't know what actually failed first or why, so I'll just solder some components in "point to point" to begin with. Luckily I have everything I need, except the power transistor. However I looked it up on alltransistors.com
    and it appears that a tip 122 will work, and I've got some of those as well.

    Going to get the soldering iron out....

    I'll include the complete schematic/layout for this particular board just for completeness.

    I'll keep you guy's posted!

    Steve.
     
  6. John P

    AAC Fanatic!

    Oct 14, 2008
    1,634
    224
    I see Q651 on the layout diagram, but on the circuit diagram I see no identifying information at all. Very likely a TIP122 would work.

    An easy thing to do would be to check the transformer T651 (do those part numbers show an error, with two components numbered 651?) and make sure that it has no shorts or opens. Maybe the transistor failed because the transformer did. Or even something on the output side of the transformer, overloading the transistor that drives the primary.
     
  7. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    That looks like a flyback transistor used to generate the high voltage to run the CRT. Look at the 3kV rated components on the other side of the transformer secondary. The massive smoked crater you see could be from a number of reasons, such as HV arcing across the transformer or a failure in the CRT itself. Flyback circuits that run a CRT typically cause more failures than all the rest of the circuits.
     
  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    A flyback transformer can not be accurately tested that way. You need a HI-pot tester to catch high voltage arc over and usually it gets worse when the transformer heats up. Those transformers are notorious for failing and probably put more TV repairman's kids through school than any other source of income.
     
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    I agree totally to check the transformer. It generates a high voltage rail (uses 3kV cap on the secondary). Check for any conduction between primary and secondary, or between the primary and the feedback winding.

    My guess is that someone tried to repair it, realised the transformer was dead too and gave up. :(
     
  10. bance

    Thread Starter Member

    Aug 11, 2012
    315
    34
    OK,

    Thanks for the advice, so I tested the transformer for continuity, here's the results:-

    [​IMG]

    P/N Continuity/ohms
    1-2 21
    1-6 38
    1-7 596
    1-8 596
    2-6 18
    2-7 576
    2-8 596
    6-7 558
    6-8 558
    7-8 good
    10-11 good
    10-12 good
    11-12 good

    All other combinations showed open circuit.

    The pins were numbered arbitrarily since I don't have a data sheet for the transformer,
    reverse engineering the traces lead me to believe:-

    Pins 1,2 & 6 are the centre tapped secondary.

    Pins 7 & 8 are the feedback winding.

    Pins 10,11 & 12 are the primary.

    So there doesn't appear to be any conduction between primary and secondary, or primary and feedback winding. Does this mean the transformer is OK?:D

    As you can see from the picture above I went ahead and soldered point to point the components that were burned out. However when I plugged the scope in and switched it on it tripped my RCD and blew the 300mA main fuse.:(

    I didn't have any of the correctly rated/sized fuses to replace the one that blew, so I've ordered some, should be with me on Monday.

    I did have some 1A fuses of the correct size, and thought I would disconnect the secondary of the main input transformer and check that. Wouldn't you know it, when I switched it on, the same thing happened.

    Compared to my Tek scopes this is a very simple machine, There are two main boards and three subsidiuary boards and of course the CRT.

    When I switched it on for the second time there were only two subsidiuary boards and the main input transformer connected, of the two smaller (subsidiuary) boards, one simply "breaks out" the two primary windings of the transformer for 120V or 240V operation. It contains no components so there is nothing to go wrong.

    The other board that comes from the front panel is connected directly via B2B connection to the power switch, again nothing to fail.

    I disconnected the primary coils of the transformer from the "break out board" and tested them. And guess what, they failed short circuit!!!!
     
  11. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
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    Repeating my earlier post:

    It may be OK or it may fail when you try to power it up. You'll find out when you do.
     
  12. bance

    Thread Starter Member

    Aug 11, 2012
    315
    34
    What is a HI-pot tester?
    Just looked it up, you mean a Megger?
     
  13. #12

    Expert

    Nov 30, 2010
    16,354
    6,852
    They are similar enough that you might get useful information.

    A megger sends a pulse. A hi-pot has steady DC.
     
    bance likes this.
  14. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
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  15. Experimentonomen

    Member

    Feb 16, 2011
    331
    46
    It feels to me like this scope might be beyond economical repair, ie it will cost more to repair it than to just get another scope that is in working condition.

    Judging from the totally burned out board, someone either tried repairing it by just keep changing the transistor to the point where the board got charred to the current state, or a small torch was used as an attemt to desolder it.

    With this damage to the board, chanses are the transformer might be totally melted down inside as well.
     
  16. bance

    Thread Starter Member

    Aug 11, 2012
    315
    34
    Experimentonomen, You could very well be correct in thinking this scope is uneconomical to repair, however, I shall not be throwing money at it, but will try to fix it anyway ( as an exercise.)

    I don't need it as a scope, I have Two good scopes, and one that needs some TLC, but I do need some experience in tracking down faults and fixing them.

    I'm currently working on the mains input transformer, will make another post when there is something to report!!!

    Steve.
     
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