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Television Standards Converters, Modulators etc Standards converters, modulators anything else for providing signals to vintage televisions.

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Old 2nd Sep 2022, 2:26 pm   #1
FERNSEH
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Default A TV Pattern Generator. P.T. June 1958.

This constructional article appeared in the June and July editions of the 1958 Practical Television magazine.
TV receiver design and construction articles were the most ambitious projects to appear in the magazine. The design of this 405 line pattern generator must be one of magazine's most complex projects so far.
For starters it would be interesting to learn how the master 10Kc/s oscillator operates. Its purpose is to create the 100microsecond duty cycle line sync and timing pulses. C1 (500pF) must be the timing capacitor. Obviously, it charges though R1 and R3, but how is it discharged? Grid current the left section of V1 perhaps. A similar circuit can be seen around V4.

DFWB.
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Old 2nd Sep 2022, 4:01 pm   #2
spamfritter
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Default Re: A TV Pattern Generator. P.T. June 1958.

Interesting. Very simple but as you say, how did it work ? Never seen this one but I do have my later PTV cross hatch generator board somewhere still.
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Old 2nd Sep 2022, 7:45 pm   #3
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Default Re: A TV Pattern Generator. P.T. June 1958.

You are correct David, it is grid current in V1 that discharges C1. The circuit is arranged to provide a higher (common) cathode voltage when V2 is conducting than when V1 is conducting.

Assume C1 is discharged, V1 is off, V2 is on. So when C1 charges up to equal the voltage on V2 grid, V1 conducts, V2 turns off and the (common) cathode voltage falls, causing V1 grid to be forward biased and discharge C1. When C1 voltage falls to below that of V2 grid, V2 conducts once more and the oscillation cycle repeats.

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Old 2nd Sep 2022, 9:27 pm   #4
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Default Re: A TV Pattern Generator. P.T. June 1958.

Spamfritter wrote:
Interesting. Very simple but as you say, how did it work?
Hi Ron,
in the text the designer of the pattern generator refers to an article about the multi-vibrator circuit in the December 1955 Practical Television magazine.
Originally intended as a sawtooth generator. "It was chosen for the application for the following reasons:
(a) It has fast rise and fall times, giving a good rectangular output at V2 anode.
(b) Its output is positive going at V2 anode.
(c) It is exceptionally stable, when used in this circuit."


DFWB.
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Old 2nd Sep 2022, 10:47 pm   #5
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Default Re: A TV Pattern Generator. P.T. June 1958.

Correction: the article about the multi-vibrator is in the December 1955 Practical Wireless magazine.

DFWB.
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Old 3rd Sep 2022, 8:10 am   #6
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Default Re: A TV Pattern Generator. P.T. June 1958.

Is the synch switch selecting between odd and even frames?

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Old 3rd Sep 2022, 9:54 am   #7
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Default Re: A TV Pattern Generator. P.T. June 1958.

"Is the synch switch selecting between odd and even frames?"
Hi Peter,
the switch selects positive or negative modulation. Something I'd thought of as being unnecessary for 405 line TV.
The designer believed a proper interlaced TV signal to be not important for the purposes to device was to be used for.
From the text. "First, as to interlacing. It would be wise to have it, certainly: but to introduce it would have meant four extra stages, and for this reason it was reluctantly decided to make do without. After all, few receivers get a good interlace, and on a good many de-focusing or "whites" is so bad that it is hardly useful when it is attained"
Surely, if the master oscillator was adjusted to 10,125c/s good interlace would be possible anyway?
The method to make the frame sync pulses is interesting. To create the frame sync broad pulses, the line sync is simply flipped upside down for a duration of four lines. The result is four 80microsecond pulses instead of eight 40microsecond pulses. Some sets may not accept that type of frame sync, the Murphy V310 and V410 for example.

DFWB.

Last edited by FERNSEH; 3rd Sep 2022 at 10:11 am.
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Old 3rd Sep 2022, 10:17 am   #8
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Default Re: A TV Pattern Generator. P.T. June 1958.

Thanks David,

Flipping the line pulses is what you get from the graphics card if you don't add my monostable frig.

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Old 3rd Sep 2022, 11:26 am   #9
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Default Re: A TV Pattern Generator. P.T. June 1958.

The RF modulator. The EF50 has the suppressor grid characteristics to accept the baseband video. Alternative valves might be the 6AS6 or the Mazda 6F33 and the older 6F32.

DFWB.
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Old 21st Sep 2022, 11:48 am   #10
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Default Re: A TV Pattern Generator. P.T. June 1958.

Returning to the design of the PT pattern generator. If the master oscillator frequency is doubled it would be possible to produce 2H broad pulses for the frame sync. To do this, however, would require many more valves.
Makes one realise it wasn't easy to create high performance circuits during the valve era.
Just have to take a look at the circuit diagram Murphy TPG11 to find out what is needed to produce proper sync pulses and a simple test pattern.

DFWB.

Last edited by FERNSEH; 21st Sep 2022 at 12:06 pm.
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