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Repairing a BWD 521 cathode ray Oscilloscope - no traces, hissing noise

Discussion in 'Repairing Electronics' started by joshAU, Mar 26, 2016.

  1. Les Jones

    Les Jones Well-Known Member

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    Hi Josh, (I assume the AU means you live in Australia.)
    Most of your post makes sense. Starting with the top row (1 to 5) From what you say it looks like 4 & 5 are the secondary that generates 80 volts (Lower right winding on the schematic) I can't see any winding wire going to pin 5. (There may be one that I can't see or you may have made a mistake.) terminals 1,2,3 are probably more of the same winding and that part is not used.
    On the left hand side (6,7,8) from what you say terminals 6 & 7 seem to be the feedback winding (Bottom left on the schematic.) 6 does not seem to have any winding wire going to it and terminal 8 does but you do not say if anything is connected to terminal 8
    Along the bottom (9,10,11,12) I think this is the primary winding. (Top left on the schematic.) I think 10 & 11 are taps in the winding that are not used. You say pin 9 connects to C74 and pin 12 connects to C73 This is the same point. You do not show any connection from the transformer to the collector of the 2N3055.

    Les.
     
  2. joshAU

    joshAU Member

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    Sorry all for the delay - I had family over from interstate, so haven't had time to get back to it.
    Thanks Les - and yes, the AU does mean I live in Australia. :)

    I'm at work now, so I'll try to check the connections and fill in the blanks tonight.

    Pin 5 doesn't seem to be connected to a winding, or to the pcb either, nor does pin 6, but I'll check.
    Pins 7 & 8 (L H side, bottom two) do have windings of the same diameter wire going to them, and both connect to the pcb, so I'm guessing they would be the feedback winding.
    And yes, the bottom 4 - (9,10,11,12) looks like its the primary winding.
    Pins 9 & 12 connect to the mainboard, 10 & 11 just seem to connect back to 9 & 12 - ie no resistance between them, from memory.
    "pin 9 connects to C74 and pin 12 connects to C73" - I'll have to check again.
    I'll also check which pin connects to the collector of the 2N3055.

    I've just bought a Tektronix 2235A oscilloscope. :)
    I should get it in a week or two, so it should help the diagnosis.

    In the meantime, I guess I'll check all the details above, and then start testing the diodes and other two transistors using the information provided earlier.
    Thanks again all, I appreciate it.
     
    Last edited: Apr 12, 2016
  3. spec

    spec Well-Known Member Most Helpful Member

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    The TEK 2235A is a nice job- I have one (the user and maintenance manuals are available free on the net) :

    [​IMG]
     
    Last edited: Apr 12, 2016
  4. dave

    Dave New Member

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  5. joshAU

    joshAU Member

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    Hi again.
    Sorry for the delay, I've put it on the back burner while awaiting the Tektronix scope to arrive.

    Which finally arrived yesterday!
    Yah!
    Except..... damnit, lol, used it for about half an hour, checking it out, calibrating it, etc. when I noticed a burning smell. :(

    The trace was still active, and I immediately turned it off.
    I disconnected the power cable, and as I did I noticed a black goo coming out from under the connector.

    I pulled it apart and it appears the power input connector also incorporates a EMI filter.
    This seems to have failed.
    The rest of the components all appear visually fine, no bulging caps, etc.
    I do like the design and layout of this scope, its certainly is a generation or more after the BWD521.

    The EMI filter is a Schattner FN323-1/01, which no longer appears to be available, but there are several other similar ones available.

    A couple of questions.

    Is the FN9222R-1-06 a suitable replacement?
    The datasheet is here if needed:
    http://docs-asia.electrocomponents.com/webdocs/121b/0900766b8121ba74.pdf

    Is it likely that the failure was caused by some other component, or is it more likely that it just gave up the ghost of its own accord?

    I'm not having much luck with secondhand scopes, am I? :)

    I've ordered a FN92222R-1-06 in the meantime, and I'm pretty sure its the correct one, but thought I'd check before I install it.
    Josh
     
  6. spec

    spec Well-Known Member Most Helpful Member

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    Hy Josh,

    Nice to see you back.

    You are having some luck with scopes.:wideyed:

    Odd fault- never seen a Shaffner input filter fail before.

    Yes the FN92222R-1-06 mains input filter will be a good replacement and has a higher specification than the original.

    The three things that I can think of that could destroy a mains input filter would be excessive current or volts or heat. Unless the mains fuse has been shorted out, over current would not be possible because the fuse would blow and protect the filter. The scope circuitry would not be able to generate a voltage high enough to stress the filter. A badly fitting IEC socket can overheat a filter housing and destroy it and there are some very poor IEC connectors around.
    That leaves three plausible causes for the filter failure. Alternatively the internal contacts to the filter may have gone high resistance, but that is unlikely.

    (1) Over voltage on the mains input line.
    (2) Heating
    (3) Failure of the filter

    To me (3) seems the most likely, but surprising.

    As far as I know, the only common failure mode on the 2235 scope is that the power supply electrolytic capacitors fail due to aging. If the scope has lived in a 110V country all it's life, applying 240V mains could be a shock to the capacitors- as a precaution in that situation I gradually wind the input volts up from 110V to 240V with a Variac to ease the capacitors in.

    spec
     
    Last edited: Apr 26, 2016
  7. joshAU

    joshAU Member

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    Hi spec.
    You're up late. :)

    Yes, amazing luck I'm having, or lack thereof. :)

    Thanks for your information and advice.
    I'm glad you agree that is a good replacement, having already ordered it..

    I've tested the power outlet at the wall and I get 242-246 volts so I think that's ruled out.
    The unit didn't seem noticeably hot when I unplugged it, although I may have not noticed it.
    Looking at the components near the filter, there seems to be no discolouration due to heat or the like.

    The unit appeared to work well, I had just followed the manual for the initial setup and was calibrating it using the 5v calibration point.
    I left it running on the bench, got sidetracked with a few other repairs, phone, etc, then noticed the burning smell.
    When I returned to the scope, the 5 volt trace was still active.

    I then turned it off, turned it around, then noticed as I removed the plug the black/brown goo leaking out of the filter.
    None went onto any components inside the case fortunately, it just ran down the back of the case over the serial number sticker.
    So I'm guessing, and hoping, that it is option 3, failure of the filter.

    Re going from 110 to 240 volts, I hadn't really thought that it could be problematic.
    The caps in the power supply section all look pristine - in fact the whole unit is pristine, not even any dust inside.
    I realise caps can fail without any visible signs, but the electrolytic caps all look dead flat on top and are flush with the board.
    The larger 120v radial caps look equally as good.

    I don't however have a Variac, so I guess I'll have to hope they cope with it.

    So, fingers crossed, and toes as well, that replacing the filter is all that is needed.
    I guess we'll see, tomorrow hopefully, if they do indeed ship it overnight.

    Thanks again spec, and speak soon.
     
  8. spec

    spec Well-Known Member Most Helpful Member

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    Hy Josh,

    If I don't do anything physical during the day I tend to wake up in the night and then read or converse with antipodeans on ETO. :) Then I go back to bed.

    From what you say, it sounds more likely that the filter has failed in its own right- fingers crossed as you say.

    The overvoltage I mentioned would be a surge on the mains supply, which could have many causes. The overvoltage may have occurred before you got the scope and may have weakened the insulation on the components in the mains filter inlet.

    The Variac thing is only me being ultra cautious- I don't trust electrolytics.

    spec
     
  9. joshAU

    joshAU Member

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    Hi again spec.
    Got sidetracked at work finishing jobs and missed your post.

    Fair enough re getting up in the night and conversing with antipodeans.
    Probably not many other people awake on your side of the world at that time...

    I spent years staying awake to 3, sometimes 4 in the morning, working on various computer related hobbies, and then have to drag myself out of bed the next day, sleepy, cursing myself for the self imposed lack of sleep.
    Then I'd of course do the same the next night.

    I think I've finally gotten over it though.. as I'm usually in bed by 1 ish lately.
    I do find I'm waking up 3 or 4 times a night though, but I usually just go back to sleep pretty quickly. :)

    Glad to hear you also suspect the filter has failed.
    I really hope so.
    I just got email confirmation that the item has been shipped, so I should get it tomorrow.
    rs components are quite amazing.. the total cost was about $10 AUD, with free shipping.
    It'd cost me that much just to post it via snail mail, let alone the part.

    Re the overvoltage, do you mean that the internal power supply could be causing the issue, or that a previous external power surge could have weakened the insulation?

    Fair enough re the electrolytic caps, given the plague of bad caps throughout my industry, I'm pretty suspicious of any of them.
    It has provided me a fair bit of work replacing caps in motherboards, video cards and the like, so I guess I shouldn't complain. :)

    I'll let you know how the new filter goes.
    josh
     
  10. spec

    spec Well-Known Member Most Helpful Member

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    Hy Josh,

    I went back to bed around 5:30 am and slept like a log until 9am.

    I am like you; if I have a project on will often work on it until around four am and then will be like a bear with a sore arse all the next day.

    About the over voltage. No, the internal power supply would not cause the overvoltage. It would be caused by transients on the input mains supply line, but the filter probably failed either due to a manufacturing fault or a deterioration of the insulation with age.

    Yes, let us know how the repair goes. You will find the TEK scope a bit different to your original scope. :happy:

    spec
     
  11. joshAU

    joshAU Member

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    Hi spec.
    I think the most productive times for me to focus on stuff is usually from about 12 to 3 AM.
    But perhaps that's why I'm not so productive the next day. :)

    Its probably the complete lack of distractions at that time that helps productivity.

    OK, thanks for the info re overvoltage.
    I hope it is as you said, that it failed without some other part being the culprit.

    Re the differences between the scopes... yes, in the admittedly very brief time I've had a chance to use them, the 2235a looks promising.
    The internal boards and overall design seems more modern, and more what I'm familiar with through work.
    Nice, neat, compact... a good design all round I think.

    I'll keep you posted, the replacement filter should be here tomorrow.
    josh
     
  12. joshAU

    joshAU Member

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    Hi again spec.
    OK, the part arrived overnight...
    Soldered in the new filter, held my breath, and turned it on...
    And its working beautifully!
    Nice to come back with some good news finally, lol.

    I had it on the bench at work for two or three hours, calibrated it, and its still going.
    I've decided the cheap 1/10x probes from O/S are probably crap... but should do so long as I'm aware of their limitations.
    $10 bucks not so well spent I guess.

    A Big sigh of relief though. :)
    Thanks so much spec... its nice to have some moral support when struggling through things.
    Its easy to blow things way out of proportion when you're stuck trying to diagnose something yourself, especially with little knowledge or experience in the item.

    So, thanks again, and thanks to everyone else who's generously donated their time and thoughts.

    I WILL get back to the BWD 521 at some stage... and no doubt I will seek advice again from you knowledgable folks... but for now I'm going to have a play around with the new scope and see how it works, and hopefully put it to use with the detector kit I'm working on - the original reason for buying the first scope.
    I do love the design, and the tube of the BWD, and, during its brief operation, it had a very sharp trace, and nice controls, so I would like to get it going...
    It'd be nice to have a scope both at home and at work.
    Although I think my wife's a bit sick of having the BWD in pieces on my desk in the living room. :)

    But, then again, I don't leave lifestyle and design magazines all over the house either. :)

    Speak soon, and thanks again.
     
  13. spec

    spec Well-Known Member Most Helpful Member

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    Good news Josh- glad the Tek scope fault was just the mains filter.

    No probs about help- I always learn from peoples questions.

    All the best.

    spec
     
  14. joshAU

    joshAU Member

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    Thanks spec.
    I'm very happy about it. :)
    Thanks again, and if you ever have any computer issues - eg. malware, data recovery, or whatever else that you need help with, let me know, I'd be glad to help.
    josh
     
  15. Mark Holian

    Mark Holian New Member

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    Interesting discussion.
    I have recently acquired a BWD-521,
    with its own peculiarities.
    I could not get a trace, except when connected to the calibration square wave on another scope.
    Also I don't get a vertical deflection on the "upper" trace. It also does not adjust position vertically,
    either of the Vertical amplifier plug ins give a square wave if plugged in the other slot.
    I already have another scope in pieces, (hitachi v-152) so not to pull this apart at the moment.
    Mark
     
  16. spec

    spec Well-Known Member Most Helpful Member

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    Hy Mark

    Welcome to ETO. I see you are another member from down under- which part?

    It would be better, if you wanted to discuss your scopes, to start a new thread of your own. By all means refer to this thread though.

    spec
     
  17. Mark Holian

    Mark Holian New Member

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    Hi spec et al
    I am in Redcliffe, 30k north of Brisbane
    Yes I will start a new thread, when I have done some footwork on the problem
    8)
    Mark
     
  18. joshAU

    joshAU Member

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    Hi Mark, and hi again spec et al.
    Hope you're all well.
    I haven't had a chance to get back to my one yet, too busy trying to finish a kitchen & bathroom.

    In case you haven't already found it Mark, the service manual is here:
    http://elektrotanya.com/bwd_521_oscilloscope_5_inches_30mhz_sch.zip/download.html

    spec and the others on this forum will be happy to help, I'm sure .
    I am too, but I suspect I won't be anywhere as much help as they can be. :)
    Good luck with it.
     

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