Horizontal scanning redution

Discussion in 'General Electronics Chat' started by Thevenin's Planet, Aug 30, 2012.

Thread Status:
Not open for further replies.
  1. Thevenin's Planet

    Thread Starter Active Member

    Nov 14, 2008
    183
    1
    Hello
    I am wondering if the Horizonal scanning frequency (in a T.V.) can be reduced to a lower sawtooth wave frequency and what would be the simplest method ?
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    What type of TV?

    In general, no you can't significantly change the horizontal scanning frequency without modifications to the circuit.
     
  3. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    In CRT TV's the horiz freq is also used to drive the main flyback transformer to produce the high voltage for the CRT. It's all in sync and there are numerous feedback systems to detect errors and shutdown the set. You can't mess with that.

    In the OLD days I saw a magazine article with a black and white TV converted into an oscilloscope, they had to competely re-wire the circuit and use a separate driver for the H-yoke to adjust the horizontal scan speed.

    These days it is still doable but it probably easier to get a EHT supply for the picture tube and just make HV amps to drive the horiz and vert stages separately, instead of trying to "mdoify" a TV. Also black and white TVs have a much simpler CRT and use a much lower (safer) EHT voltage, maybe 8kV not 24kV.
     
  4. dataman19

    Member

    Dec 26, 2009
    136
    29
    Simple answer - yes (but there is a catch).
    Practical answer - "are you crazy?" Change the horizontal oscillator frequency and you decrease the TV scanning bandwidth. The symptom here is a smaller picture - with part of the picture folded over the picture (Due to decreased oscillator frequency - the amount of fold over is determined by the osc frequency, the size of the viewed picture is the result of the drive amplifier bandwidth, along with the voltage applied across the deflection yoke coils).
    ..
    Incidentally, the new LCD TV's are not prone to this problem, and no - you can not decrease the oscillator frequency in a new LCD TV/Monitor (at least no beyond what the LCD is designed for), as this would cause a fault and the set would shut down.
    ...
    Dave
    Phoenix, AZ
     
    martyrcomms and #12 like this.
  5. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    Yeah, what they said. You can do it right after you redesign the whole thing.
     
    R!f@@ likes this.
  6. Thevenin's Planet

    Thread Starter Active Member

    Nov 14, 2008
    183
    1
    TA1223AN (video,Audio,Deflection),IC.Model Toshib CF13g22,CL14G22 and Chassis TAC970 ).
    At the Horizonal output at 1.3 volts....is this the peak of the linear rise of voltage in the deflection coils or the plates of the yolk. Theorically, it seems that R-411(560 ohms) and R-410 (390 ohms) could be considered as an output of local oscillator or but Iam not sure do mixing a sawtooth ramp wave to produce a slower rate of frequency. I am not sure how does a frequency is changed to slow its rate...maybe by harmonics,multiplier....or turning the odd or even pulses off at the perfect moment. This is what am working with schematic.
     
  7. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    If you look at the circuit,you will see that the signal coming out of pin 32 is the horizontal drive signal.
    You will also note that it isn't a sawtooth.
    The shape of the waveform is so it can ultimately produce a sawtooth of current in the horizontal scanning coils.

    The period of this signal for both PAL & NTSC is around 64usecs.
    The 10usecs label on the drawing is evidently the time/div setting suggested for an Oscilloscope observing that point.

    You will notice that the waveform is changed by passing through Q402 & T401--this also helps give the waveform the required shape to produce a linear current sawtooth in the scanning coils.

    The standard TV systems,with the horizontal frequency around 15kHz,allow the horizontal deflection system to be.effectively,a tuned system,with all the circuitry optimised for operation at that frequency.

    Having to generate a fair amount of power to perform the horizontal scans,makes it convenient to also derive the EHT for the CRT from the same source,hence the use of the "Horizontal output".or "flyback",or "EHT" transformer ( All names for the same device).
    Again,this is a tuned device,optimised for 15kHz.

    There have been multi-standard TVs made which normally operated on the PAL 625 line system,but could be switched to work on the old British 405 line system which had a horizontal frequency of around 10kHz.

    In France,they made multi-standard sets which operated on SECAM 625 line,& the weird 819 line system they had for a while.

    Both of the above were in the days of discrete circuits,or very early special ICs,& needed some pretty clever switching to insert extra inductors & capacitors to change the operating frequencies of the Horizontal Output transformers & scan coils.

    I believe you would not be able to substantially change the horizontal scan frequency of a more recent CRT TV,as the 15kHz line rate was standard for many years,after the older systems became obsolete.
     
  8. Rleo6965

    New Member

    Jan 23, 2012
    9
    0
    Why do you want to reduce the horizontal frequency? Your picture display will be distorted or roll due to out of sync.
     
  9. Thevenin's Planet

    Thread Starter Active Member

    Nov 14, 2008
    183
    1
    Time base variable for various frequencies to be measured.The horizontal sawtooth wave at 15,750 Hz is the Time base for the Horizontal scanning ? Since the T.V.scanning is a locked position that was verified from one the AAC posts and threads,that is, it produce the raster that comes from a free running oscillator,even if T.V. station signal is not transmitted.Giving the correct balance for vertical and horizontal correlation for timing or synchronize to place the pixel or whatever in correct position.It would not be appropriate to measure a 60 Hz with a time base 15,750 Hz. So this sawtooth wave which is actually the time base for the Television but is a locked Frequency rather then like the Oscillator variable to measure different frequencies to keep equal distance for time on the X axis and Y axis.That sawtooth wave could be disconnected from the from the Horizonal driver,T401,Horiz output and replaced with a circuit that will begin conversion of the ramp Horizontal with the same ramp speed but different frequencies(variable), as with the oscilloscope.Remember I am not using the T.V. section that's processing the T.V. station signal. Only the Horizonal out put at pin #32 of TA1223AN IC, is to use not the standard 15750 Hz that can be use at higher frequencies,but lower frequencies.What should I use to convert thus frequency to lower frequencies is a puzzle to me.
     
  10. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    The above is almost unreadable.
    Please tell us what your basic purpose is.
    Are you trying to use the TV as an oscilloscope?
     
  11. Thevenin's Planet

    Thread Starter Active Member

    Nov 14, 2008
    183
    1

    That is one of my intention is to use the yolk of the deflection coils as an oscillscope.Yes I noticed the square looking signal at #32.Since it's not shaped that should be easier to shape,make the frquency variable with some type of circuit there ? Why would they produce such a wave signal? Q402 is just amplifiering the signal from #32 ? Are C-416, R-416 and T401 shaping the sawtooth wave ? Because the the drawing pointing at C-417 and C-463 don't look as though the signal has been shaped to a sawtooth wave,or it's shapped after Q404 which is not shown.
     
    Last edited: Sep 4, 2012
  12. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    The horizontal scanning coils (yoke) are inductive,& in order to produce a
    sawtooth current,they need the waveform shown.

    Deflection in a Electromagnetic deflection system is proportional to the current through the coils,not the voltage across them.

    Again,as I said several times already,the horizontal scanning system is optimised for around 15,000Hz,& will not operate correctly at other frequencies.

    Please read all the answers in this thread again,as you keep asking similar questions over & over!

    I Googled for "TV converted to Oscilloscope" & found the following:-

    http://www.dansworkshop.com/2008/03/homebuilt-oscilloscope/
    There may be others,but what this guy has done is similar to your idea.

    Many years ago,a magazine in Australia did a mod which was quite different to what you propose.
    They sampled the signal they wanted to observe,turning each sample into a pulse which they fed to the CRT cathode.
    This appeared on the screen as a series of white dots,offset across the screen with each line,so as to ultimately show a full waveform.
    The normal scan was left untouched,although I think they rotated the yoke 90° like Dan did.
     
    Last edited: Sep 4, 2012
  13. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    And keep in mind with the cheap price of nice digital 'scopes these days people are selling old 'scopes on ebay all the time for $50 to $80. That's for a proper 'scope with good high freq performance and linear scales etc, not a dodgy modded TV that will only work OK if it works at all.
     
  14. Thevenin's Planet

    Thread Starter Active Member

    Nov 14, 2008
    183
    1


    That signal at # 32, horizontal Drive signal,is it 15,750 Hz ? It's not Sawtooth wave, so what is it ?
    "The scope need to be synchronized it's sweep with a rising or falling edge of the signal under test,so that the trace does not crawl across or race across the screen," said Danworkshop. Is this the reason that the horizontal frequency is 15,750 Hz ?
     
    Last edited: Sep 6, 2012
  15. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    I think the horizontal scanning rate of 15750Hz and vertical at 60Hz was used for old black and white transmissions and TVs in North America. Colour TV used a multiple of 3.58xxxxMHz which was the colour subcarrier frequency and is a little different.

    Why are you playing with an obsolete CRT TV?
     
  16. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    No,they are only talking about their modification at that point.
    If you look at the drawing on that website,they are using the vertical sweep to move the trace horizontally.
    in other words,they have turned the yoke 90°.
    All the original horizontal circuitry does with their mod,is to provide EHT for the tube.

    The signal at pin 32 is not a voltage sawtooth,but it is a modified rectangular wave,which is required to cause a sawtooth current in the horizontal scanning coils.
    This in turn,is so the trace will proceed at a uniform speed from one side to the other of the screen.

    The reason for 15,750Hz is due to the NTSC Television standard.
    In countries which use the PAL standard,it is 15,625Hz.
    In order to have a viewable picture on the screen,it is normal to have the horizontal sweep happening at the same time as the vertical sweep,so that each scanning line across the screen finishes lower down the screen than where it starts,so forming a "raster".
    As Audioguru says,when the colour TV system was first devised,it became necessary to shift the horizontal frequency slightly from 15,750Hz,to avoid some interference effects with the colour subcarrier.
    This was also done for the PAL system,but the change is so slight that people normally still use the old figures unless they are working to very high degrees of accuracy.

    A TV converted to an Oscilloscope will basically be a "fun" project,but will not be a practical instrument,as the deflection yoke will not give equal deflection to both high & low frequencies.

    I would suggest you look for some books or websites giving a basic outline of how analog TV works,so that you don't have to ask stuff which is hard to answer in one short posting.
     
  17. Thevenin's Planet

    Thread Starter Active Member

    Nov 14, 2008
    183
    1
    It's good way to learn.Besides the T.V. is very close associated to the O-scope.It seem that it's not to much change in the principles of electronics only the substances of the materials. The circuits seem to do the same but they are in IC's. I figure the scanning is the same principle only that Horizontal scanning is 15,750 Hz and in O-scope it is changable for what is called T or period.Like on the schmatic, the O-scope was set at 10 usec.Which is about 100,000 Hz, to measure square wave at #32 on the schmatic.The time division control on the O-scope adjust the Horizontal speed or period..?
     
  18. Thevenin's Planet

    Thread Starter Active Member

    Nov 14, 2008
    183
    1
     
  19. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    Not really a good thing to learn though!

    CRT technology is absolutely obsolete, and is being dumped at the scrapyards at a scary rate (go to your local dump and have a look at the TV mountains!).

    All new TVs and new 'scopes are flatscreen LCD technology. If you really want something to leard then don't waste time with obsolete high voltage CRT technology, it was the last "vacuum tube" to bite the bullet, and it did last a while, but it is OVER now.

    Learn things that will be useful in 10 or 20 years from now, not things that were useful 20 years ago!
     
  20. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    The point is,that he is playing around with an old B&W TV,& wants to make it into a (sort of) Oscilloscope.
    However,he knows NOTHING!
    In order to even attempt to do what he wants to do,he has to find out how the thing worked in the first place.

    A lot of information which was useful 20 years ago will still be useful in 40 years time,& much of the "latest & greatest" from now will be obsolete.

    Forget the tubes!
    There is a lot of circuitry in analog TVs,covering a wide range of circuit techniques,which is still useful in other,totally different applications.
     
Loading...
Thread Status:
Not open for further replies.