Tektronix 465M HV Power Supply Issue

Discussion in 'General Electronics Chat' started by litmister, May 12, 2016.

  1. litmister

    Thread Starter New Member

    May 12, 2016
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    My main oscilloscope is a Tek 465M. I recently noticed an issue with it where the trace did not extend to the screen's edges properly. I determined the issue was the 95 volt rail, and found some suspect capacitors. To access and replace them, I had to remove the vertical module (which pulls out all in one piece in this scope). Upon powering up, the trace was way off the screen's top, and could just barely be brought into view by turning the position control all the way to the end. It was also extremely fuzzy. I figured I must have not properly connected something on the vertical module, probably the connection to the y-deflection plates. I removed the module, reconnected all external wires, and reseated the main connector. Upon applying power, the HV supply makes a loud, high buzzing sound. I'm not sure why this is the case, or if it is dangerous to leave the scope running in this state. Any ideas?
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    The deflection coils are difficult to align without the added problem of figuring out what you did wrong from a less than 100 word description of your hours of troubleshooting.
    Pictures maybe?
     
  3. litmister

    Thread Starter New Member

    May 12, 2016
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    Well, I wasn't intending to affect anything to do with vertical deflection. Here's a better description... The original fault (a low 95 volt rail) was unrelated to that, and did seem to be fixed after I switched out a couple of filter capacitors. To access the caps, the vertical control module had to be removed; this simply entails disconnecting the vertical deflection plates, a ribbon connecting to the horizontal control module, a piece of coax which carries the channel 2 signal to the horizontal module (presumably for x-y mode), and the entire module itself, which connects to the mainboard using a simple edge connector. This procedure is one shown in the service manual, and (according to the manual) should not affect vertical alignment at all. When the caps were replaced and the vertical module reinstalled, voltage rails read the proper values and operation was mostly normal, ignoring the fact that vertical deflection was way out of whack. I assumed that I must have just made some error in reinstalling the vertical module, so I pulled it back out and reseated all associated connectors (vert. deflection plates, coax, ribbon, and the edge connector) to see if that fixed the issue. When the scope was powered on, a horrible whining buzz sound came from the HV supply area. I have tried a few preliminary things to solve this issue, but none have worked. I am nervous to leave the scope on in this state, as I am unsure of the cause and thus unsure of potential damage that could be caused from leaving the unit powered on. Thus, I came to this forum to see if anybody has encountered something like this before, and possibly shed some light on a more precise possible cause of the buzz / if it's safe to leave the unit running.

    I do not understand how disconnecting and reconnecting the vertical module could cause such a fault, but evidently it's possible. I've scoured the internet but have found no information on such HV supply faults.
     
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Remove it again and clean the edge connector and the socket for it as best you can. When you reassemble be sure that any standoff posts are fully contacted. Be sure a wire isn't pinched and that no traces near the mounts are shorting.
    If you followed a published procedure then execution is the likely culprit. Old components also have a bad habit of going out during a repair on something unrelated. Let's hope it isn't that.
     
  5. litmister

    Thread Starter New Member

    May 12, 2016
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    No beans... Just tried that and did a close visual inspection for any visible damage/loose connections/screws fallen into places they shouldn't be/ etc... Can't find anything and it still makes that horrible noise. I'm really not sure where to go from here
     
  6. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    My guess the main filter caps are shot .. Check esr..
     
  7. litmister

    Thread Starter New Member

    May 12, 2016
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    And the timing of failure is just coincidental?
     
  8. litmister

    Thread Starter New Member

    May 12, 2016
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    Did a little more testing today. Don't know if the information is useful... Buzz is definitely coming from the HV section, like I thought before. I would guess it's at about 600 hz. Doesn't seem to produce a trace, and if the scope is left on for about 3 seconds the pitch seems to shift a little lower (~50 hertz less?). I've checked for visible arcing in case that was the issue, but couldn't notice anything. I'm quite baffled by this one.
     
  9. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    I would suggest checking out Yahoo Group Tektronics and they will be able to give better support..
     
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