Bar Generator

Discussion in 'General Electronics Chat' started by capnray, Mar 15, 2013.

  1. capnray

    Thread Starter Member

    Jul 3, 2010
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    I just dug out a "Bar Generator" manufactured by RMS. I'm guessing it is a color bar generator. It is old, very old, has a 12AT7 as an active component. I have yet to connect it to a TV to see what it actually does.

    Has anyone ever seen one of these?
     
  2. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Not a valve one!

    If it's old enough to have a 12AT7 it is NOT colour but would be a B&W (greyscale) bar generator. It's a museum piece really.

    Photos would be nice. :)
     
  3. Ron H

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    I don't think that color bars could be generated with a single 12AT7, but I don't understand your "old enough" comment.:confused:
     
  4. capnray

    Thread Starter Member

    Jul 3, 2010
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    I have attached photos.
     
  5. #12

    Expert

    Nov 30, 2010
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    Apparently it wants to be fed to an analog tuner, usually channel 2 or 3. The only thing I can think of using it for is pincushion adjustment, and it will not work on a modern digital tuner. You will need an antique TV to test it with.
     
  6. Ron H

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    The capacitors are probably bad.

    I would really like to see the schematic. Horizontal or vertical bars and a VHF oscillator with a twin triode and a handful of parts is pretty impressive.
     
  7. THE_RB

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    No it's just a single bar I think Ron. It's basically an oscillator that produces one pulse in sync with the horiz sync or one pulse in sync with the vert (which is much slower freq obviously).

    And looking at the output wires (only 2) and 25/75 Hz range on ajustment pot, it does not look even to be synced it's just a free running adjustable freq oscillator with a 2 position range switch!

    And even though it has a flat RF cable lead on the output it's not RF modulated, it would just be to clip leads to clip directly into the luminance circuit and inject the "bar" there.

    It really is ancient! I'm guessing 1950's design?
     
  8. Ron H

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    Vertical could be synced to the mains. Do you think the tunable inductor is part of the horizontal oscillator?
    If you look closely at the Bar Adjust on the front, it has four labels: 0, 25, 50, 75. That doesn't sound like a horizontal frequency adjustment. Percentage, maybe? And if that's a pot, it's not like any I ever saw, and I'm old.
     
  9. crutschow

    Expert

    Mar 14, 2008
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    Perhaps its a variable capacitor, not a pot.
     
  10. Ron H

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    That's what I was thinking, but the only variable caps that I remember from the '50's were the ones with the multiple meshing aluminum plates, and the little ceramic trimmers with mica insulators.
     
  11. BReeves

    Member

    Nov 24, 2012
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    I think it's a pretty cool find but being an old goat may have something to do with my thinking.

    From the looks of the coil and twin lead output I would bet money it was designed to connect to the antenna input on the TV. Wouldn't be too difficult with a twin triode, one section is the RF oscillator and output, the other handles the modulation.

    That is a variable cap, sorta like the ones with mica insulators but with a knob which looks to be broken off. You can see the threads where the adjustment closes or opens the leafs on the rear view.

    Worthless today except for the novelty aspect.
     
  12. Ron H

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    Let me preface my comments by saying that I'm not a hard-core RF guy.
    The RF oscillator would have to be in the 60MHz range, so the inductor would have to be < 1uH, even if the tank capacitor is < 10pF. I guess that inductor could be in that range, as it doesn't appear to have too many turns on it. The variable cap, if that's what it is, doesn't appear to have anything to do with the carrier frequency, since it is labeled "bar adjust".
    It seems to me that there has to be a horizontal oscillator, in order to generate vertical bars. If that's true, taken all together it seems like a tough task for a dual triode.
    I must be missing something.:confused:
     
  13. THE_RB

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    Yep. :) It won't generate the VHF RF for a black and white TV as it is much too simple to make the VSYNC and HSYNC pulses and also make luminance pulses, and modulat all to RF.

    The knob looks to be an old carbon pot with ceramic body, and the "knob" part is missing from the shaft obviously.

    You are right the pot marking is likely 0-100% on the freq adjust which is just an arbitrary marking I think to make the box look good.

    The switch selects two frequencies, vert and horiz. Then I expect the free running oscillator is made from two triodes as a hysteresis osc to make fast transisitons from black to white levels.

    In use you would just turn the pot until it gets roughly in phase with the horiz freq, making one or more vertical bars which are drifting across the screen (to do pincussion or yoke adjustments etc). Likewise a misadjustment in freq would give some diagonal looking bars.

    Same usage in vert mode but at a lower freq. It might be synced to mains as you said but likely is not, as it's actually more useful unsynced and adjustable. :)

    It's so simple because it is just a free running oscillator with a range switch and a freq pot.
     
  14. Ron H

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    Roman, what do you think the adjustable inductor is for?
     
  15. capnray

    Thread Starter Member

    Jul 3, 2010
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    Thanks for all the comments. I have yet to determine if the caps are good/bad. I will attempt to generate a schematic. YES! The adjustment is a capacitor, multi plate as suggested.
     
  16. BReeves

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    Nov 24, 2012
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    Was thinking it wouldn't be real difficult to draw a schematic of the thing. You can see how the coil is wound with an ohm meter and the rest is easy. Guessing the coil is actually an RF transformer with the primary going to one of the plates and the secondary going to the twin lead output.

    Will be interesting to see who made the best guesses.
     
  17. debe

    Well-Known Member

    Sep 21, 2010
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    I would bet its a VHF osc fed into the tuner of an old B&W tv to give a crude greyscale bar. Its very simmilar to a B&K television analyst i used to have. It had a similar set up to generate a colored bar for very early american coloured TVs, it was more a coloured rainbow than a colour bar.
     
  18. debe

    Well-Known Member

    Sep 21, 2010
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    This is the colour part of the circuit, Will start a thread on the 1960s Television analyst.
     
  19. THE_RB

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    I don't think it's "adjustable" as such, just a convenient former to wind a small coupling transformer. The transformer is likely used in the oscillator between the two triodes to give positive feedback for fast switching speed, as the oscillator needs to have fast transitions from black to white level.

    I really don't think it's RF output! RF output has to be fed into the tuner of the set, and has to have VSYNC and HSYNC pulses to be of any use normally.Generating VSYNC and HSYNC needs more valves, at least one more 12AT7 for the two other oscillators (H and V).

    Maybe Debe is right and it is RF output, but it was common in the B/W days to inject signals direct into the TV set circuitry when doing tests. We shouldn't just expect an RF output. These days we're used to the convenience of plugging in to an RF port but in the old days a TV tech would always have the set's back cover off and there were test points to inject signals.

    Injecting direct into the luminance stage would give a very crisp bar and be best for functionality for yoke and pincussion adjustments.
     
  20. capnray

    Thread Starter Member

    Jul 3, 2010
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    OK, seems last post didn't fly. The front panel variable component is indeed a capacitor with aluminum plates. The internal adjustment is the output transformer.

    I have generated a schematic which is attached.

    I have taken some voltage measurements and it seems the rectifier and electrolytic capacitor are still functioning. With the switch in VER, I get a spike at 249 Hz.
     
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