Help with Oscilloscope

Discussion in 'General Electronics Chat' started by ryugreen, Feb 2, 2014.

  1. ryugreen

    Thread Starter New Member

    Feb 2, 2014
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    Hi there,

    New member here. I picked up a Bk Precision 1476A at a HAM radio swap here in Richmond last week for pretty cheap but wasn't able to test it before buying it.

    I am new to scopes so I don't know exactly what to look for but something seem s a bit off on this one.

    The problems i'm noticing is when I try to adjust the astigmatism control, and set it to xy mode and ground, the manual says I should get a small circle/dot. I get instead a line about 2-3mm's long. No amount of turning the adjuster turns it into a dot. I have attached a picture to show this.

    ALso when I run a wire to the probe adjustment square wave jack on the front, I get what looks like a good square wave, but a second identical wave seems to be behind kind of bouncing. I can never get the square wave stable as it just bounces.

    This also occurs when I have nothing hooked in at all. It looks like the line is out of focus but when i focus it up and zoom in I notice that in fact the flat line is rising and falling every so slightly. This seems to happen on both channels. So there is some kind of noise or something.

    Could this be indicative of a bad unit? Really disappointed if so. Seems like I should be getting a flat line at the very least. You just can't trust people at meetups any more.

    Either way, thanks so much for you help,

    Ryan G.
     
  2. Mike33

    AAC Fanatic!

    Feb 4, 2005
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    Hi,

    While I'm no pro, I do have an analog scope & know how to use it pretty well.

    1) Did yours come with probes? Could be that if you're 'running a wire', you're introducing a bunch of noise into the signal that is causing those weird artifacts. There's no shielding if you're trying it out that way. No compensation, either...results would have to be VERY poor. You can get basic probes for $20, new, on Ebay! Not the best, but useable. Esp. for the freq range your scope can handle.

    2) Do you have it set to "both"? Meaning, both channels. If you can't get to EITHER channel 1 OR channel 2, you may get that ghosting you showed.

    3) It could need to be calibrated (?) For hobby purposes, no clue how critical that is (I haven't had mine calibrated, and it's fine - a 1982 Tektronix 2213).


    4) I'd download the manual for it, and go thru the 'startup' section. They generally will tell you how to get it up and running.

    After that, it's repair time. Mine came cheap, and channel 1 was not working. It was a diode on the input board that I fixed myself for a 10 cent low capacitance diode...a BJT junction! :D

    Hey - at least you get SOMETHING, so you know the CRT works! You're 1/2 way there.

    Hope this helped...
     
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  3. ryugreen

    Thread Starter New Member

    Feb 2, 2014
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    Mike,

    Thanks for the quick reply. I did not at the time of these pictures have the wire plugged in so that was just plain no input flat line. UNless you need a probe to get a flat line, I don't think the wire was the problem.

    I did find a service manual and will crack that open but am wondering if there isn't an easier fix out there.

    Maybe somebody has seen a similar problem and can point me in the right direction I guess?

    Always worth a shot!

    Thanks again.
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    It is not a good idea to have a single well focused spot on the screen for too long. You can burn a spot on the phosphor screen.

    I think your intensity is set too high.

    Let's take it one step at a time.
    Set both channel inputs to GND.
    Set SWEEP TIME/DIV to 5ms

    Do you get a horizontal line across the screen?

    When you buy equipment at a Ham flea market you have to remember you are making your purchase AS IS. You may very well still have a great bargain. Don't despair.
     
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  5. #12

    Expert

    Nov 30, 2010
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    1) That's not a flat line. It's a diagonal line. Try one channel at a time and see what happens.
    2) the jitter is a synchronization problem cause by lack of amplitude. You must have at least one section of graticule of amplitude (probably 2 sections) for the sync to lock in.
    3) the rising and falling is either from picking up power line hum with the "probes" or a filter capacitor in the power supply.

    Edit: MrChips was typing at the same time as I was. Please forgive any duplications.
     
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  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Could be PSU noise. Often you need to replace a few caps on old scopes to get them working tip-top. :)
     
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  7. ryugreen

    Thread Starter New Member

    Feb 2, 2014
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    Thanks. I didn't leave the high intensity dot there but for a few seconds to snap the pic. I was just trying to display that the dot was in fact a bit of a diagonal line of sorts.

    I get a horizontal line when set to ground but it seems to have noise (rises up and down slightly).

    Yeah it was AS-IS for sure, I just should have plugged her in and gone through the motions but it was the end of the day and he was leaving. Might be a good learning experience either way.
     
  8. ryugreen

    Thread Starter New Member

    Feb 2, 2014
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    Hmmm... but shouldn't I get a straight line with no voltage at all? There still seems to be a bit of jitter. Maybe I should go ahead and order a probe and see if that fixes the problem.

    It is also worth noting that when i hooked up to the probe adjustment square wave I got a decent looking square wave that kind of bounced almost like there was a ghost of another square wave just behind it.

    It sounds like the filter capacitors would be a good place to start if I open it up. I just want to make sure I'm aware of where the real high voltage is before I do any of that. Need to spend a bit more time with the diagram I guess.
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    I am inclined to agree with #12 and RB. Looks like some capacitors in the power supply need replacing.
     
  10. ryugreen

    Thread Starter New Member

    Feb 2, 2014
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    Sounds like that's the best place to start. Can anybody give me the low down on the best way to find which caps need replacing? OR should I just do em all? I have a multimeter and a fair amount of soldering experience. No oscilloscope though obviously =). I know I need to discharge the caps regardless but would the PS circuit be at tube level voltage (as in one hand in my back pocket the whole time)?
     
  11. MrChips

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    Oct 2, 2009
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    If I had the service manual in front of me I would be able to help. Photographs of the power supply section would also help.

    I would begin with the power supply section. There should be one or more physically large capacitors. Look for bulging or leaking capacitors. Replace any that are suspect.
    Once you have identified the major power supply caps I would consider replacing those.

    I will see if I have the service manual.
     
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  12. PackratKing

    Well-Known Member

    Jul 13, 2008
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    FWIW>>> I have a B&K 1541A 40 Mhz dual, which was discontinued in 1990, and was wondering where to take it to have calibration checked...

    I went google-fishing for B&K customer service, and asked about the issue. I was answered by Marian Stauffer who was supremely helpful... she wrote...

    "I am sorry but we no longer service the 1541A that was discontinued in 1990. I will attach the service information below that will come with calibration instructions. "

    I was flabbergasted by the detail of the information she sent... It included PCB trace patterns, component layout, component list, and an EXHAUSTIVE calibration procedure...
    I have never in my life been treated so well by any company in regard to outdated equipment... B&K has a customer for life !!!
     
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  13. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    I picked up a BK persion 2120 20mhz and love it ..But I would deffently post up pics to better help with the repair .. Also the cal procedure might be long and a pain but is worth it in the end...
    Good luck with it and please keep us updated




    Thanks
    Jason Sr
     
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  14. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Photos of the PSU will help a lot.

    If you don't have an ESR meter to test the caps, you can look for bulged caps (or shrunken outer jacket) or just replace any cap that is near a heatsink or near a component that gets hot. :)
     
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  15. ryugreen

    Thread Starter New Member

    Feb 2, 2014
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    Just for further specificity, when I set sweep to 10ms and ground, i still get a bounce in the line. Seems like some kind of power supply cap issue is the front runner here.

    I found a service manual online actually. I'll attach it. I took the case off today and snapped a few pics of the power supply section. I see some very very old caps so it seems like pulling the board and replacing those would be a good place to start. I haven't really done much with high voltage so I want to review how to discharge these caps... and build me a capacitor discharge tool. Looks like it'll be tough to get at the leads since they are so well inserted into the PCB. Do you sometimes need to wait until after pulling the board to discharge them? Seems risky. Does look like the board is easy to pull though.

    Anyway, here is a close up pic of the caps immediately following the AC in. Nothing looks busted, but they all are in rough shape. The largest one almost looks to have a dent/line in the top but can't tell if that's normal. I also took a further out pic showing the general layout around the PS. Any tips much appreciated.

    Thanks for lending a hand to a newbie guys.
     
  16. #12

    Expert

    Nov 30, 2010
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    Just use a 1K resistor to discharge the caps. 2 or 3 seconds each should do most of the job. There is no law against leaving the resistor connected for 10 to 20 seconds.

    and, yes. They are supposed to have those dented looking lines. They are intentional weak points so the cap can find a way to leak without exploding.

    Replace a dozen caps if you can. They still won't cost more than the price of shipping.
     
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  17. Mike33

    AAC Fanatic!

    Feb 4, 2005
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    +1 High likelihood they are the cause, or at least a big part of the cause, of your symptoms. Either this unit was in high humidity, or something leaked/spilled...so clean up in there, too (contact cleaner).

    I work on tube amps (high voltages) a lot. I connect a jumper to ground, and put a resistor (1K as said) on the other end. I use an insulated tool to touch the + cap terminals for a little bit each, to make sure they're discharged. You can read them with a DMM if you're not sure. Leaving it unplugged overnight will help this process if you're concerned. After you do this a few times, you'll see that it's little bother - but glad to see the RESPECT you have for charged caps, which can really ruin your day!

    Changing them all is worthwhile - all you're losing is your time and really low parts cost. If you can solve the issue, you'll have a great scope!
     
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  18. MrChips

    Moderator

    Oct 2, 2009
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    The service manual you posted is for model 1476. Your model is 1476A.
    If the power supply circuit is the same or similar we should be able to work with this.
     
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  19. ryugreen

    Thread Starter New Member

    Feb 2, 2014
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    Correct you are. I had assumed this was meant to cover the 1476A but upon further examination the 1476 looks to be an older version of which there is very little to be found on the internet (other than this excellent service manual).

    Its unfortunate but I haven't been able to find a similar manual for the 1476A. I will contact B&K and see if they can provide some info as the guy a few posts ago did with his.

    My plan of action for the moment is to go ahead and order replacement caps for the entire power supply and depending on how successful that goes, perhaps for the other sections as well.

    I will certainly keep you guys posted on the work. May be a week or so before I have time to tear into it.
     
  20. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    How buried are those caps? Even though it was working great, I sold my old Tek for fear the caps would go bad. Those things were really buried. It would have been very difficult to replace.
     
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