The Great CRT to Oscilloscope Project!

Discussion in 'The Projects Forum' started by Manticorp, Nov 4, 2012.

  1. Manticorp

    Thread Starter New Member

    Oct 18, 2012
    Hello, and welcome to my latest project - turning a small CRT monitor into a fully functional oscilloscope.

    The monitor in question is a combined colour TV, radio and cassette player that I hope to rip the innards out of and turn into a nice oscilloscope.

    Here's a pic of the device in question:


    which I want to turn into something that more resembles this:


    The device has quite a few inputs/outputs/controls, to name a few:

    • For Power, it can accept:
      • AC in from 110V to 240V (it has a selector on the side for selecting which)
      • DC in 12V (a rather large barrel plug which I measured to be 6.67mm in diameter)
      • A Proprietary rechargeable battery
      • A gagillion (read: 10) D cell batteries
      • A switch for going from charging the battery to operation
      • A Power switch
    • The TV part has:
      • Aerial in, selectable from the built in aerial or external.
      • Degauss button
      • V-hold (adjusts vertical scan rate I think)
      • Picture brightness (there's a screw hole next to this knob that might also adjust brightness or maybe focus??)
      • Contrast Adjustment
      • Colour Adjustment
      • A little switch that says AFC off/on which I'm not really sure what it does. Wikipedia says this is "Automatic frequency control"
    • The radio part has the following
      • Remote in (3.5mm)
      • Mic in (3.5mm)
      • Earphone out (3.5mm)
      • An XLR port labelled 'Rec/PB'
      • A switch labelled 'Beat Cut' with 3 positions, used to declick AM recordings, from what I understand.
    • And on the front panel, there's:
      • A Volume knob
      • Tone knob
      • Mixer for mic/radio ratio
      • Switch between casette, radio and tv mode
      • TV tuning with switch between UHF, VHF L and VHF H
      • Radio tuning with switch between MW, LW and FM
    • There's also a small light switch and a counter for the tape player, which has it's own usual play/stop/record controls

    I've managed to get my arduino to output stuff to the little screen using the great TVTuner library and hooking the arduino up to the Aerial, so at least we know it works:


    Because all analogue TV has been switched off here, I wasn't able to test it otherwise (and I don't have an old VHS player or anything handy!)

    I'm going to try and follow this great instructables post where he transformed a regular television into something quite similar.

    Improvements I hope to make to this would be to calibrate it vertically to get accurate voltage readings and to be able to adjust the timebase accurately, so maybe instead of having the flyback transformer driving the horizontal frequency, using a different circuit.

    It would also be great to switch from magnetic deflection to electrostatic deflection to be able to measure higher frequencies, although this is a big ask.

    Any help and advice would be much appreciated, I don't have much experience working with these high voltage/frequency circuits!

    Stay tuned for updates though!

    Here's some more pics of the item:


  2. spinnaker

    AAC Fanatic!

    Oct 29, 2009
    I think you should use the existing controls for your scope. The frequency control could be used for time division. :)
  3. Manticorp

    Thread Starter New Member

    Oct 18, 2012
    Yeah, it looks like that would be entirely possible to do, maybe if I replace the single switch with a rotary and a different pot I'll be able to get better control.

    I need to whack the thing open first to see what the circuits are like on the inside. It's a shame there's no horizontal frequency control :/

    [edit: Actually scratch that, as I'll be switching the horizontal supply and vertical supply around putting the inputs on the vertical part, that makes little to no difference :)]
  4. MrChips


    Oct 2, 2009
    This is a wonderful project and very doable.
    But before you begin, you should understand the limitations of a TV screen.

    I will not go into the details at this point but a TV screen is not exactly the same as a CRT in an oscilloscope.

    A TV screen uses magnetic deflection to create a raster scan on the screen.

    A CRT uses electrostatic deflection and can handle much higher speeds than a TV screen.

    There are options around this which can be discussed later.
  5. Manticorp

    Thread Starter New Member

    Oct 18, 2012
    Indeed, as I said:

    I'm not quite sure how I would drive the electrostatic circuit, though I managed to find a great place with lots of schematics for CRO's that I could trawl through, though it'll take a while.

    I think first I'll try and get it working with the magnetic deflection if possible, then go on to converting it to electrostatic.
  6. MrChips


    Oct 2, 2009
    You cannot convert the TV screen to electrostatic deflection. You would have to replace the whole tube and oscilloscope CRTs are much longer in length.

    You have some options:

    1) Accept the low frequency limitations of the TV screen
    2) Keep the raster scan and create a digital scope
    3) Replace the TV with an LCD graphics display - but this gets into a whole new kettle of fish.
  7. Manticorp

    Thread Starter New Member

    Oct 18, 2012
    Hmm...I've just opened up the thing and the inside is SO packed, it's going to be hard to modify this thing very much. I should have prepared myself for that really...

    Here's a pic of the inside:


    Option 2 is looking like an attractive option at the moment!

    Can I ask, why is the longer tube needed? Surely having a shorter tube and an amplified signal, even when using electrostatic deflection, would have the same effect?
  8. SgtWookie


    Jul 17, 2007
    MrChip's last post is exactly what I was going to write.

    Since you already seem to have the capability to display information from the Arduino to the TV screen, I suggest you go that route. It'll be difficult to display letters & numbers due to the relatively poor resolution (as compared to a real O'scope CRT or an LCD or LED graphics display).

    You might be interested to know that there are still quite a few old HP network analyzers around (VERY expensive when new) whos' CRT displays have grown too dim with age to be used anymore; and another company came up with a color LCD retrofit so that these great old pieces of equipment could continue to be used.
  9. SgtWookie


    Jul 17, 2007
    The electrostatic deflection plates in an O'scope CRT are very close to the electron beam; but even then it takes quite a bit of voltage to move the beam away from the center of the screen.

    With the TV CRT, the tube is relatively very short, and you would not be able to place the electrostatic deflection plates near enough to the beam to move it sufficiently.

    If you tried to use really high voltage, you would have a big problem just finding components rated for it, not to mention trying to charge/discharge the deflection plates quickly enough. It's not worth the effort.
  10. spinnaker

    AAC Fanatic!

    Oct 29, 2009
    Now that is innovative. Is it a direct replacement? Or is a major hack of the scopes board needed.

    I think what you are suggesting is to use a LCD display instead. I was going to suggest that but I thought it would defeat the whole purpose of the OP's original project idea.

    Besides wouldn't an LCD of that size carry a fairly hefty price tag for just a fun project?
  11. Manticorp

    Thread Starter New Member

    Oct 18, 2012
    Hmm this is interesting, I see that using electrostatic deflection requires very high voltages, which yes, as you say will require very expensive equipment...

    However, with the magnetic coils, I can most certainly use signals up to 15kHz without any problems, as this is the rate of the signal used anyway.

    Because it's such a small little screen (about 10cm by 7cm) I wouldn't mind guessing that I could 'analyse' higher frequencies without running into too much trouble with the inductance of the coils...

    Indeed this is true, I'd like to stick with the CRT if possible. If I can't make an oscilloscope out of it, I might just stick with the audio-visualiser side of things.
  12. John P

    AAC Fanatic!

    Oct 14, 2008
    You do have the option of taking the input and creating a digital "screen" which you then rasterize and send out to the monitor at whatever frame rate that screen uses--if you say "programme" I assume it's 50Hz. Your resolution would be whatever the screen has--do say it's not 425 lines! It would be like pointing a TV camera at a scope screen; I can imagine some nasty beat-frequency effects.
  13. MrChips


    Oct 2, 2009
    There is another solution, one that I have done in the past before low cost graphics displays became available.

    We rotated the CRT yolk 90 degrees so that the horizontal scan lines went vertically from bottom to top. Then we added digital circuitry to display dots on the screen and were able to create an X-Y graph (or I should say Y vs Time). This would be very easy to do with a microcontroller.
  14. crutschow


    Mar 14, 2008
    The deflection amplifiers for the magnetic coils are resonant type circuits and would need to be extensively modified to provide the varied frequencies required by a general purpose oscilloscope. I recommend not trying to make an analog oscilloscope and go with some simpler application that uses the built in, fixed frequency raster deflection circuits such as a display for a digital oscilloscope front end.
  15. Manticorp

    Thread Starter New Member

    Oct 18, 2012
    Hmm yeah this is starting to sound like the sort of thing that is gonna make this an impossible project. I think you're right, pretty much the only way this is gonna work for any sort of useful signal is to hook up a DSO to it and just use it like a regular display.

    Oh well, game over for the oscilloscope conversion project...until I can afford a DSO that is...question is though, do I just take the thing apart and scavenge for parts (the mini CRT tube might make a nice ornament? ;) ) OR do I try and breath some other sort of new life into it, like the oscilloscope clocks you sometimes find?
  16. smokingwheels

    New Member

    Aug 20, 2011
    Try and find a circuit for this TV to CRO adapter may still be possible to get the parts.
    Its a Electronics Australia kit from 1981.

    You can also get usb cro adapter from china but there limited to 0-5 volts input not much chop.
  17. Manticorp

    Thread Starter New Member

    Oct 18, 2012
    Okay, so I managed to get a very basic Oscilloscope function running on the screen from software. By using a buffer I'm able to get adjustable response speeds.

    The way I have it hooked up is to have the input read from analog port 0 and 1. on port 0 is the sensor output and on port 1 is a capacitor used to adjust the size of the buffer.

    The limiting factor at the moment is the TVout library, which is very slow on the arduino and has a maximum resolution of 128 pixels wide (the library says 160 but I can't get that to work...)

    I don't know what frequency this thing is capable of working it...but to hazard a guess I'd say very low, somewhere around 60Hz with a buffer size of 1 and around 1920Hz with a buffer size of 32.

    So, in short, to arduino software approach isn't really viable.

    Here's pic of it in operation:


    and a short video. It's just hooked up to an LDR at the moment because that's the only thing I had laying around!

    Here's the link:

    This is as far as I'm willing to go with this, after hearing lots of people talk about it online it just seems too complicated. However, it is nice that I got some working functionality from it, even if it was limited.
  18. Manticorp

    Thread Starter New Member

    Oct 18, 2012
    Here's the code I was using for the arduino:

    Code ( (Unknown Language)):
    1. #include <TVout.h>
    2. #include <fontALL.h>
    4. TVout TV;
    6. float val;
    7. float mval;
    8. int BUFFERSIZE=16;
    9. float buffer[16];
    11. int i;
    13. void setup() {
    14.   TV.begin(PAL,128,96);
    15.   TV.select_font(font6x8);
    16.   TV.clear_screen();
    17.   TV.fill(BLACK);
    18. }
    20. void loop() {
    21.   for(i=0;i<=BUFFERSIZE-1;i++){
    22.     val=analogRead(0);
    23.     mval=map(val,0,1024,0,48);
    24.     val=map(val,0,1024,0,1000);
    25.     buffer[i]=mval;
    26.   }
    27.   toTV(buffer);
    28.   TV.set_cursor(2,88);
    29.   TV.print((val/200));
    30.   TV.print(" v | BFS: ");
    31.   TV.print(BUFFERSIZE);
    32.   BUFFERSIZE=analogRead(1);
    33.   BUFFERSIZE=map(BUFFERSIZE,0,1024,0,(TV.vres()/2));
    34. }
    36. void toTV(float BUFFER[]){
    37.   for(i=1;i<=BUFFERSIZE;i++){
    38.     TV.set_pixel((TV.hres()-i),((TV.vres()/2)-BUFFER[i-1]),1);
    39.     TV.set_pixel((TV.hres()-i),(TV.vres()/2),1);
    40.   }
    41.   TV.shift(BUFFERSIZE,LEFT);
    42. }[/i]
  19. spinnaker

    AAC Fanatic!

    Oct 29, 2009
    What is the interface to the TV? Is that also on the arduino?
  20. Manticorp

    Thread Starter New Member

    Oct 18, 2012
    Yes indeed, using the TVout library, which can be found here. It says it works with RCA plugs, but it also works with standard aerials.