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    #41
    Re: Oscilloscope CRT problem

    Jeez I hope the schematic is wrong- the 100uF/50v cap across the +12/-12v is backwards! The IC is a common OP-Amp. (741). Looks like the corresponding resistor to the 2120 is R952, 51ohms. Check it and the zener. H.V. for the anode is off of junction of R950, and HR959.
    Last edited by xtraelectronics; 09-10-2009, 12:27 PM. Reason: typo

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      #42
      Re: Oscilloscope CRT problem

      Well I have an update! I checked the -12v and +12v going into the high voltage circuit, and they're both there, and pretty close to their expected values. I was then looking around the schematic for where the 2kv line is (it's different from the 1.9kV line) and I found it. And the intensity knob is adjustable with 2 pots on the circuit board, so i fiddled with them, and now I get something on the screen I can see! First it was just the little blob moving back and forth, but then I fiddled with things more and now I can see waveforms! There are still a few problems though...
      1) the screen fades out from left to right
      2) there is a definite angle upward on the screen. I'm using the 2v p-p square wave it has built in, and if I set it so on the left side it goes from -1v to 1v, then by the time it gets to the right it looks like it's from about -0.5v to 1.5v

      any further thoughts?

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        #43
        Re: Oscilloscope CRT problem

        Awesome! There should be a"tilt" control on the unit somewhere, that copmensates for the earths magnetic feild and other stay feilds in the vicinity. I had to adjust mine while it was on the bench, and again after it was back on its shelf.

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          #44
          Re: Oscilloscope CRT problem

          >1) the screen fades out from left to right

          interesting.
          brightness(ie beam current) shouldn't be modulated at all, but it seems something is changing it in your case....
          could it be that another trimmer(or potentiometer) is borked from too much waiting...heh

          or the deflection is doing something it shouldn't (some extra wobbling or ringing when beam reaches right side) and that makes beam look dim...
          or is the same circuit(h. deflection) accelerating beam while it shouldn't...

          you know, that scope should have all lytics checked with esr meter.
          lytics drifting can cause such weird behaviour. better to say they're the first suspects.


          btw. could we wee some images of the trace now that you got something?
          don't put them inside the post, leave them as thumbnails if you're gonna post big pics.
          (ie just upload the pic to the post and that's it)


          oh yeah, well done!

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            #45
            Re: Oscilloscope CRT problem

            I found the 'trace rotation' pot and fixed it, so now the trace is level. I'll have a look at the schematic and play with the other pots to see if I can fix the fading out problem...I can change the X position setting, but that just chops it off at either end. I'll see if I can take a decent picture of it too...

            Thanks for you help everybody!

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              #46
              Re: Oscilloscope CRT problem

              btw. how old is that scope?

              i have pretty old hameg and it doesn't have those trimmer oxidation issues...
              http://picasaweb.google.hr/i4004b3/I...su_2008_10_19#
              its schematic is dated 22.5.1972.
              yay...older than me!



              Last edited by i4004; 09-10-2009, 05:27 PM.

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                #47
                Re: Oscilloscope CRT problem

                Well here's a kinda bad picture of it working. The trace on the screen is brighter than that, it's just my camera can't capture it properly....oh well


                that signal is the 12v output from my logisys with a 4A load on it...almost 150mV @ about 2kHz! Yikes!

                The problem of it getting dimmer on the right isn't too bad - I'll be able to live with it. Thanks everybody here for your help!
                Attached Files

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                  #48
                  Re: Oscilloscope CRT problem

                  fading to the right is bad filter or weak resistors for the "screens" (like G2) that let emissions to droop with each sweep.

                  Cheers, Wizard

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                    #49
                    Re: Oscilloscope CRT problem

                    Where might this resistor be? Here's the link to the schematic again:

                    Comment


                      #50
                      Re: Oscilloscope CRT problem

                      Hi Etnietering

                      Cool! you got an actual working scope AND an education about what makes it go.

                      That's almost like the really old ads in my Dad's really old electronics magazines.... "Build your own Oscilloscope and Earn while you Learn!"
                      I always wanted to get one of those but they were before my time.

                      That BK Precision scope looks an awful lot like the one CRT model they still sell at Fry's for $389 or so.


                      Astounded at the talent here,
                      Keri

                      PS. Someone once asked me what a scope does at Fry's once. He caught me setting a slow 2-channel sweep on it and wanted to know what those lines were.
                      Does "It lets you see what the electricity is doing" sound like an OK answer?
                      The More You Learn The Less You Know!

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                        #51
                        Re: Oscilloscope CRT problem

                        An oscilloscope CRT is just a graph illustrating voltage in the x-axis and time in the y-axis. It graphically represents voltage over time.

                        I think that's a simple enough answer.

                        But you can also drive the X and Y deflection amps independently and do some pretty cool things like the oscilloscope clock.

                        http://www.coolcircuit.com/gadgets/w...cope-clock.jpg

                        That's not my scope in the pic but I have the same exact model and have built this circuit.
                        Last edited by Krankshaft; 09-10-2009, 08:04 PM.
                        Elements of the past and the future combining to make something not quite as good as either.

                        Comment


                          #52
                          Re: Oscilloscope CRT problem

                          Ok let me break down the PSU circuit for you the main transformer not the HV one.

                          First since you have no ESR meter I recommend you replace ALL electrolytic caps in the unit but for the time being just try to replace the ones I circled in red.

                          Now onto the circuit breakdown off the unregulated 140 volt winding (used by the CRT) it's bridge rectified and filtered by C901. Its regulated by a discretely built voltage regulator comprising Q901 and U904 this produces a regulated 140 volts.

                          The +12V, +5V, and -12V supplies are fed off the same winding and rectifier.

                          The transformers 32 volts output is rectified and filtered by C902 and C903.

                          The +12V source is further filtered by C906 after the regulator. The -12V source is filtered by C908 and the +5V source by C907.

                          They use IC based regulators here since they're not in weird voltages like the 140V source.

                          A thing that gets me though is that the winding should be putting out 32 volts.

                          At center tap according to the schematic you have 16 and 16. To use a bridge rectifier you need to use both ends of the secondary winding to use the center tap you would go with a full wave 2 diode rectifier.

                          Looking at the input cap it's only rated for 25 volts when unregulated the transformer can put of 32 volts !

                          Oh man I get it (2) 25V caps are in series. Are you fricking kidding me?

                          This is just sloppy as hell. 2 caps in series are the sum of their working voltages so 25+25= 50V rating.
                          Attached Files
                          Last edited by Krankshaft; 09-10-2009, 08:32 PM.
                          Elements of the past and the future combining to make something not quite as good as either.

                          Comment


                            #53
                            Re: Oscilloscope CRT problem

                            The problem here is if one cap is slightly leaky it will transfer it's voltage to the other cap which could exceed its voltage destroying it in a cascading fashion. This is why this setup annoys me.

                            Also on a related note there is a POT for HV adjust (VR963) on the scope by messing with POTs randomly you may have inadvertently compenated for a sagging supply voltage from the main transformer.
                            Elements of the past and the future combining to make something not quite as good as either.

                            Comment


                              #54
                              Re: Oscilloscope CRT problem

                              Hi Krankshaft

                              That's a really neat trick... to put a clock on the scope.

                              That sounds like a kinda complicated "circuit" to me.
                              Wouldn't it involve making the scope trace out a raster pattern?
                              Is some kind of video IC involved?


                              Tried to build a digital clock project once that was supposed to go bing-bong-bing-bong. But it didn't.
                              Keri
                              The More You Learn The Less You Know!

                              Comment


                                #55
                                Re: Oscilloscope CRT problem

                                Yeah the circuit is anything but simple.

                                It's pretty advanced actually with an OSD to configure the clock and how it looks.

                                http://www.dutchtronix.com/ScopeCloc...1-Standard.htm

                                It consists of a Clock IC PCF8563 a Digital / Analog Converter AD7302, and a microcontroller to tie it all together so yeah this advanced version is a bit more complex. It even has a battery to keep the time just like a computer.

                                It also has automatic screen repositioning so you don't need to worry about burnin.

                                If it looks cool then it's probably complicated .
                                Last edited by Krankshaft; 09-10-2009, 08:56 PM.
                                Elements of the past and the future combining to make something not quite as good as either.

                                Comment


                                  #56
                                  Re: Oscilloscope CRT problem

                                  >That BK Precision scope looks an awful lot like the one CRT model they still sell at Fry's for $389 or so.

                                  yeah, it seems they're stil making 2120b model.
                                  http://www.bkprecision.com/products/...s/?model=2120B


                                  >Does "It lets you see what the electricity is doing" sound like an OK answer?

                                  quite right.
                                  you can also do "how electricity looks inside the wire"..hehe...

                                  scope-clock
                                  http://www.dutchtronix.com/ScopeClock.htm
                                  it uses a freakin mcu to put clock on scope display...
                                  talking about idle mind and devil's playground...hehe...

                                  another one
                                  http://www.franktechniek.nl/Kits/osc...scopeclock.htm

                                  >It also has automatic screen repositioning so you don't need to worry about burnin.

                                  clock should be on all the time, so worn cathode is thing to worry...hehe...
                                  i wonder how many days could it do....hm...
                                  i wonder can that tube do 3k hours...
                                  Last edited by i4004; 09-10-2009, 08:57 PM.

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                                    #57
                                    Re: Oscilloscope CRT problem

                                    Dang it I'm highjacking the thread sorry about that.

                                    In summary check the 140V source (C904), +12V (C906), -12V (C908), and +5V (C907) right on the output filters after the regulators. Measure right across the output caps they're easy enough to find.

                                    Are they all normal?
                                    Last edited by Krankshaft; 09-10-2009, 08:58 PM.
                                    Elements of the past and the future combining to make something not quite as good as either.

                                    Comment


                                      #58
                                      Re: Oscilloscope CRT problem

                                      im thinking of perhaps doing one of these:

                                      sigpic

                                      (Insert witty quote here)

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                                        #59
                                        Re: Oscilloscope CRT problem

                                        ^^
                                        i thought about something simillar before i got scope.

                                        but all i've found was a way to mod the zx spectrum to do scope.
                                        imagine what sort of scope would that be.....hihi...
                                        even worse than that one on telly!

                                        Comment


                                          #60
                                          Re: Oscilloscope CRT problem

                                          >>im thinking of perhaps doing one of these:<<

                                          By hijacking this thread?

                                          Krankshaft is known to digress...

                                          Best to start a thread on what you'd like to do. It's easier to keep things clear for everyone who is contributing to the threads' subject matter and for the OP to follow.

                                          It's also polite.

                                          Toast
                                          veritas odium parit

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