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    Oscilloscope CRT problem

    Hello,

    I just purchased a BK Precision 2120B oscilloscope, to help me with my projects and such. I got it from ebay, the seller had a 95+% positive rating, and said that the unit had been used for about an hour, and is as good as new. It arrived today, and needless to say, I was very excited. I couldn't wait to avoid more homework, because I got my new toy I plug it in and turn it on, and the CRT does not turn on. I have no idea how to even begin diagnosing the problem, but I did take the cover off (after unplugging it and letting it sit for a while, of course), and looked for burns/fried components/bad caps. I found none, and apart from a few cobwebs and some dust, the unit looks pretty good on the inside. Some of the caps are from a brand called Engel (ever heard of them?) and the rest have a logo which is a capital A in a rectangle.
    They made good use of screws and take-apart-able connectors, so disassembling the unit won't be too much of a problem, I'd just like some advice on what to do or how to start.
    I tried to get some pictures of the circuit boards without disassembling it.








    I searched online for a schematic and couldn't find one (I did find the manual though). The power light comes on, so I know something is working. I'd be happy to take more pictures, and any advice anybody can give is welcome

    Thanks!

    Eric
    Attached Files

    #2
    Re: Oscilloscope CRT problem

    Originally posted by etnietering
    Hello,

    I just purchased a BK Precision 2120B oscilloscope, to help me with my projects and such. I got it from ebay, the seller had a 95+% positive rating, and said that the unit had been used for about an hour, and is as good as new. It arrived today, and needless to say, I was very excited. I couldn't wait to avoid more homework, because I got my new toy I plug it in and turn it on, and the CRT does not turn on. I have no idea how to even begin diagnosing the problem, but I did take the cover off (after unplugging it and letting it sit for a while, of course), and looked for burns/fried components/bad caps. I found none, and apart from a few cobwebs and some dust, the unit looks pretty good on the inside. Some of the caps are from a brand called Engel (ever heard of them?) and the rest have a logo which is a capital A in a rectangle.
    They made good use of screws and take-apart-able connectors, so disassembling the unit won't be too much of a problem, I'd just like some advice on what to do or how to start.
    I tried to get some pictures of the circuit boards without disassembling it.
    I searched online for a schematic and couldn't find one (I did find the manual though). The power light comes on, so I know something is working. I'd be happy to take more pictures, and any advice anybody can give is welcome

    Thanks!

    Eric
    95% positive? Ouch!!! Usually I won't trust a seller who has less than a 98% rating. Well, that's water over the dam. Next step, was it guaranteed to be working or insured?

    Assuming the above are not going to be helpful, here's some general ideas. Approach the problem from the shipping damage angle. Make sure all connectors are firmly seated. Does the filament in the CRT glow? If not, pursue that trail.

    What happens when you hit the beam finder button. Adjust the brightness. check trigger mode, etc.

    PlainBill
    For a number of reasons, both health and personal, I will no longer be active on this board. Any PMs asking for assistance will be ignored.

    Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic.

    Comment


      #3
      Re: Oscilloscope CRT problem

      Go back to auction pics and set the controls just like the pic that shows them close up.

      Anything? Power light on? As PB said, CRT filament on?

      Toast
      veritas odium parit

      Comment


        #4
        Re: Oscilloscope CRT problem

        PB: no, it wasn't guaranteed nor insured, and the seller won't take returns.
        I can't see anything glowing in the CRT tube, but most of it is covered up with metal. Where would the filament be? All of the connectors appear to be seated properly.

        Toast: the power light does come on. It also has a calibration feature that outputs a 2v p-p square wave across two terminals on the front, and when I put a red LED across these it glows dimly, which is what I would expect. I've played with all the knobs and switches and nothing changes anything.

        There was one thing I did though. The board at the back of the CRT was only seated partially, so I shoved it back in (protecting myself from it, of course). It seems odd to me that this board is supposed to be at an angle (see picture). I have a feeling that it's supposed to be level...what would the risks be in spinning it?

        edit: normally I'd go for a seller with a higher feedback, but I got this for $120 shipped, whereas a new one would be $350-400...
        Last edited by etnietering; 09-08-2009, 05:08 PM.

        Comment


          #5
          Re: Oscilloscope CRT problem

          Originally posted by etnietering
          PB: no, it wasn't guaranteed nor insured, and the seller won't take returns.
          I can't see anything glowing in the CRT tube, but most of it is covered up with metal. Where would the filament be? All of the connectors appear to be seated properly.

          Toast: the power light does come on. It also has a calibration feature that outputs a 2v p-p square wave across two terminals on the front, and when I put a red LED across these it glows dimly, which is what I would expect. I've played with all the knobs and switches and nothing changes anything.

          There was one thing I did though. The board at the back of the CRT was only seated partially, so I shoved it back in (protecting myself from it, of course). It seems odd to me that this board is supposed to be at an angle (see picture). I have a feeling that it's supposed to be level...what would the risks be in spinning it?

          edit: normally I'd go for a seller with a higher feedback, but I got this for $120 shipped, whereas a new one would be $350-400...
          A skewed socket board is not a problem - it's supposed to be keyed, and will only fit in one position. The filament is very close to the base of the CRT. You should be able to see it glowing, but try it in a dark room.

          PlainBill
          For a number of reasons, both health and personal, I will no longer be active on this board. Any PMs asking for assistance will be ignored.

          Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic.

          Comment


            #6
            Re: Oscilloscope CRT problem

            Yeah after I wrote that I looked and it in indeed keyed...I should have looked before I asked! I do see a very small yellow-orange glow coming from what looks like a filament in the base of the CRT. No picture on it though...

            Comment


              #7
              Re: Oscilloscope CRT problem

              Originally posted by etnietering
              Yeah after I wrote that I looked and it in indeed keyed...I should have looked before I asked! I do see a very small yellow-orange glow coming from what looks like a filament in the base of the CRT. No picture on it though...
              That's a good sign!

              Now, you need to figure out if there is high voltage to the tube. On a regular CRT monitor, if you place your hand on the glass surface in front, you can feel the static build up. On such a small tube, I don't think you can fee that.

              Do you know what a Flyback transformer is/looks like?

              Comment


                #8
                Re: Oscilloscope CRT problem

                I've taken apart regular CRT monitors...are the flyback transformers the giant things with the huge wire coming out of the top? I don't see anything that looks like that, or even like a general transformer, other than the main power transformer. It might be hiding under the box where it says 'H.V. DANGER!!' on the circuit board...I'll take a look.
                Also, it has a plastic protector in front of the screen, so I won't be able to tell if there's any static on the screen.

                Thanks for the help! I'll check for the flyback transformer.

                Comment


                  #9
                  Re: Oscilloscope CRT problem

                  I have an update - I believe what's under that box is the flyback circuit. It has a transformer and some blue capacitors and such.
                  Also - I found what I believe is a schematic. According to some people on the electro-tech-online forum, it's from a different oscilloscope but the schematic is the same. Here's the link: The part of it that's under that box is in the bottom corner of page 6 - the stuff connected to the MV950. Is this the correct place to look?

                  Comment


                    #10
                    Re: Oscilloscope CRT problem

                    Yes that is the correct location but take great caution the flyback voltage is about 12000 volts, if you are not sure on what to do take it to someone who knows like a tv repair shop,again be very careful.

                    Comment


                      #11
                      Re: Oscilloscope CRT problem

                      Originally posted by etnietering
                      I have an update - I believe what's under that box is the flyback circuit. It has a transformer and some blue capacitors and such.
                      Also - I found what I believe is a schematic. According to some people on the electro-tech-online forum, it's from a different oscilloscope but the schematic is the same. Here's the link: The part of it that's under that box is in the bottom corner of page 6 - the stuff connected to the MV950. Is this the correct place to look?
                      That's it. It's unlikely that the current would be high enough to kill you, but 12KV would be enough to alter your outlook. A simple neon lamp should glow if held close to the transformer. I'd also check the output voltages of the power supply.

                      Again, do this with caution.

                      PlainBill
                      For a number of reasons, both health and personal, I will no longer be active on this board. Any PMs asking for assistance will be ignored.

                      Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic.

                      Comment


                        #12
                        Re: Oscilloscope CRT problem

                        How did you pay for this? PayPal? Credit card (I hope!)?

                        I'd complain to the seller through the eBay message system (so there is a record). The item is NOT as advertised. i.e.- working display

                        You'll have to ship it back and pay for that, but you can file with PayPal or your credit card for defective merchandise.

                        'As Is' sales showing a working devices that are not working upon receipt are valid claims for returns & refunds. No damage to the shipping box, I trust? Well packed?

                        Keep everything until this is resolved.

                        Fight!

                        Toast
                        veritas odium parit

                        Comment


                          #13
                          Re: Oscilloscope CRT problem

                          Thanks for the tips guys, I'm not going to try it until tomorrow though, it's too late for me to being working with 12kV...
                          I do have a question though: what is the best way to find out if it's working? I don't have a neon lamp, and my cheapy voltmeter only goes to 500v.
                          From what I can tell, by looking at the schematic and the traces on the board, the output from pin 10 of the transformer doesn't go anywhere. the trace is connected to a pad that doesn't have anything soldered to it. There's space for something with 2 leads into the board, 'IN' and 'G'. Could this be the MV950 thing? and it's just not there? Seems odd that it's not there...and it doesn't look like it was once and it was removed...hmmm....

                          Edit: toast, the pictures don't actually show it working...here's the link: http://cgi.ebay.com/ws/eBayISAPI.dll...=STRK:MEWNX:IT all it says is 'like new condition' and I payed with paypal
                          Last edited by etnietering; 09-08-2009, 10:56 PM.

                          Comment


                            #14
                            Re: Oscilloscope CRT problem

                            Ok. Thought it was this one:
                            http://cgi.ebay.com/ws/eBayISAPI.dll...m=190331062094

                            Look at that one and try setting the controls like this picture:
                            https://www.badcaps.net/forum/attach...id=11537&stc=1

                            Yeah, I think he carefully worded that to NOT say it was working, just that it was Like New.

                            I'd still bitch. You at least might get some $$ back.

                            Furthermore, always use a credit card with PayPal. If they stonewall you, you still have recourse through the credit card.

                            Toast
                            Attached Files
                            Last edited by Toasty; 09-09-2009, 12:06 AM.
                            veritas odium parit

                            Comment


                              #15
                              Re: Oscilloscope CRT problem

                              OK i'll send them a message tomorrow - I need to get to sleep!
                              Also, now that it's completely dark, I turned it on with all the lights off, the power LED is definitely on, there is definitely an orange glow in the back of the tube, but not so much as a hint of illumination on the screen.
                              It still strikes me as odd that that part is missing...I'll try to take a picture tomorrow.

                              Comment


                                #16
                                Re: Oscilloscope CRT problem

                                MV950 appears to be a voltage multiplier block. One end is the anode to the CRT, the other to the flyback, and ground.

                                When you take the pics, would you take top down shots of the whole chassis? Or, maybe 2? 1/2 the front the other 1/2 the back?

                                Is the HV anode lead connected to the CRT?

                                If that part (MV950) was there and removed, the anode lead went with it. If the lead to CRT is missing and there is a hole on side of tube where it should be connected, this guy ripped you off. You'll have good reason to complain and void the whole deal and get him to pay return shipping.

                                But, I might be getting ahead of myself.

                                Toast
                                Last edited by Toasty; 09-09-2009, 02:26 AM.
                                veritas odium parit

                                Comment


                                  #17
                                  Re: Oscilloscope CRT problem

                                  Wow no HV anode lead geez.

                                  Basically you need 5 things for a CRT to display a sharp image filament voltage, high voltage anode (electron accelerator voltage), screen voltage, focus voltage (both derived from the flyback via a resistive divider in most cases), and deflection drive (to deflect the beam into something useful).

                                  Even without deflection you should still a get a green dot without focus a blurry green dot .

                                  This is why I went with a Tektronix 475 they're hard to kill and are built like rocks built in the late 70s - early 80s in Oregon no made in Taiwan here . I'd be careful with the dials on this unit especially the volts / div and time / div they don't look as solidly built.

                                  If that pic on e-bay is of the scope working and the anode wire is missing this is textbook fraud. No way you're getting ANY image without anode voltage.

                                  The HV anode feeds the acceleration grid in the tube if the electrons don't strike the phosphors hard enough they emit no light.
                                  Last edited by Krankshaft; 09-09-2009, 05:13 AM.
                                  Elements of the past and the future combining to make something not quite as good as either.

                                  Comment


                                    #18
                                    Re: Oscilloscope CRT problem

                                    Is the timebase running? Is the trace centered? It's best to check the basics before looking for complicated faults.
                                    It is a good shrubbery. I like the laurels particularly...

                                    Comment


                                      #19
                                      Re: Oscilloscope CRT problem

                                      I have almost the same scope. 2120. (no B) It's a bit older, but when I went to use it today, I was surprised to find the same sympton as you. No trace. I appears HV is missing. The scope acted "funny" the last time I used it, about a month ago. I will test some components, and keep you updated on this so maybe we can get to the bottom of this.
                                      Dan

                                      Comment


                                        #20
                                        Re: Oscilloscope CRT problem

                                        Ok, found R549 in the flyback cct open. It is a 47 ohm, 2 watt resistor. The scope now has HV and a trace. Tested the small caps in the area, they are marginal. Probably tolerated by the cct, but I think I will replace them anyway, just to be safe. There are a few others elseware in this scope that are def. bad. Hope this helps you
                                        Dan

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