Moderators: Dave Moll, Steve Anderson
Klaas Robers wrote:
Harry, I have seen that your horizontal scan line goes from the centre to the right. That indicates that it works any way, and it shows that the saw tooth goes only positive. The sawtooth that you have now, starts at zero and goes up positive. Two transistors and one resistor "shifts" all voltages about one volt negative and converts a high ohmic input to a low ohmic output. Exactly what you need in this case.
Klaas Robers wrote:Steve, would an emitter follower, NPN, resistor to - 12 volt, eventually a double emitterfollower (darlington) not solve the problem of uni polarity? Then the DC zero drops to - 1 volt, the impedance problem is solved, what do we need more? Steve, do you know the peak peak voltage of the sawtooth by heart? Is it easily adjusted to 2 volts? That would help Harry.
Klaas Robers wrote:Harry, thanks for the clear description and the photograph of the deflection yoke. This helps a lot.
First I should do nothing yet on the circuiting of the deflection coils.
Use Yellow to Green for one deflection (40 ohm) and
use Orange to Red for the other deflection (30 ohm),
leave the Blue output wire floating, not connected.!
If you used blue as one connection for the deflection this could be the reason that your deflection field was not a square / rectangle.!
If this works and you can make a square that is large enough, the corners touching the circular edge of the screen, leave it as is.
If you cannot stretch the deflection enough, then your voltage is too low, you have to switch one (or both) of the coil sets from series into parallel. But that is a next story. I had to do that for one of my coil sets, but I had just +6 and -6 volts. You have +12 and -12 volts, and that helps a lot.
[/quote]And there is no need to remove the resistors and the capacitor. The resistors are much larger in value than the coils where they are in parallel with, and the capacitor does no harm for the much much lower frequencies that we are using for SSTV.
Klaas Robers wrote:What I see is that the vertical scanning has a hic-up in the middle. This is to be expected, because the two output transistors have their emitters connected AND their two basis. This is a standard circuit for maximal cross over distortion. And that distortion is what you see. The vertical scan stops moving for one or two lines and then continue as it was going. So you see two lines on top of each other, and than one or two lines missing. This is why I have the 2k potmeter between the basises.
But I can advise you to do nothing to cure this at this moment, look over this effect and go on with the SSTV monitor. This is a known problem, that you may solve later.
By the way, in the horizontal line something similar happens in the middle, but you cannot see that so easily. You will see that later, when there is video and you see diagonal lines on the screen. Sam problem, same solution. Not yet.
Steve Anderson wrote:I would have thought that the negative feedback would have virtually eliminated that 'flat spot' in the deflection waveform. Perhaps there's less open-loop gain than I would have expected.
What i'm surprised about is why devices like a TDA2030/40/50 haven't been used here? Although touted as audio amplifiers what are they really? Power op-amps...designed to drive inductive loads (loudspeakers), so I would have thought ideal for this use. Maybe with slightly modified compensation due to the larger yoke inductance, though the current feedback resistor should help in taming that.
Their down-side is I find the quiescent current a little high (around 50mA). Not really a problem in an application like this, but in audio use they do run a bit warm even with no audio is being output. They've been around for many years and are cheap, I don't understand why less quiescent power-hungry devices been developed?
Steve A.
Klaas Robers wrote:Harry, if I look at the pencil drawn circuit diagram, I see that from the two connected bases of the final deflection transistors, there is a capacitor to ground. Can you look for the value of that capacitor? It should be as low as possible, I guess 1000 down to 100 pF. This capacitor is there only to keep the deflection amplifier quiet, not oscillating. I guess that at least in the vertical amplifier there is now a much higher capacitor mounted.
You can optimise it by trial and error, make it smaller and smaller, until the amplifier starts oscillating, but first mount a resistor of .... 470 ohm over the deflection coil. You will observe oscillation very well, because the line then gets "thick". If that happens, make the capacitor twice that value and keep it that value.
This aplies too for the horizontal deflection stage. If you obtained the capacitor value for the vertical stage, this can be used in the horizontal amplifier as well.
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