Moderators: Dave Moll, Steve Anderson
Steve Anderson wrote:Looking further down the line I've seen another error, and I'm sure it won't be the last. To be honest Harry, give up on this crap.
I have to temper that a bit as the idea of gating the sync pulse such that an erroneous bit of garbage doesn't cause a mis-trigger both in vertical and horizontal has much to be commended.
You can imagine the Editor, "Don't shoot me, I'm only the messenger."
Steve A.
Steve Anderson wrote:Moving on with this...I'm looking into the CRT circuit. You've already got the several kV you need for the final anode, now it's down to the few odd hundred volts for the grid/cathode/focus part.
Now I note that there's a remark on the Du Mont datasheet for this CRT (attached) about a 'beam alignment magnet'. See note 7 on the second page. No other datasheet for the 5AHP7 I have mentions this. So presumably it operates without one, but this fine-tunes the focus somehow. Something to bear in mind for later.
Harry, what's the no-load voltage of the power supply for this - the +360V or so rail used before for the 3BP1 deflection circuits amongst others? The load this time is going to be a lot less than before so I'm hoping it will come up to somewhere around +400V. It's not the end of the world if it doesn't.
Steve A.
Steve Anderson wrote:OK, +380V or thereabouts is going to be fine, I'll start pulling together the CRT bit. In this case you're not going to be needing the -300V, only the positive.
...hang on a sec! I had forgotten (again) you've got -320V (or so). Now that makes things a lot easier!
I wonder why Ralph only used half-wave rectification? If your transformer is centre-tapped as per Ralph's drawing adding two more diodes would boost the voltage (slightly) but with half the ripple. It's worth the extra cost of two 1N4007s.
Steve A.
Steve Anderson wrote:As for the half-wave rectification, maybe it was good enough in the application it was being used for, the extra few volts not required, and what ripple there was had no visible effect. But to add the extra diodes the transformer does need to be truly centre tapped as per the drawing.
The magnet thing is unusual in tubes of this size, it's a case of we'll see when the time comes - you may have to plunder the kids collection of fridge-magnets!
The warning regarding X-rays needs to be heeded, but generally you need voltages in excess of 10kV to produce any that could be of concern. At your 7kV all should be well. An a**e-covering exercise by the manufacturer I think.
Steve A.
Klaas Robers wrote:don't worry Harry, B&W TVs had some 10 to 16 kV, while colour sets had 25 kV. Both had a screen of glass containing quite some lead and that screened the X-rays sufficiently. More problem arised from the back cone of the picture tube, but nobody was sitting behind the TV.....
For the 5 to 7 kV it is no problem at all. X-rays go with the 3th power of the voltage, as far as I remember.
Steve Anderson wrote:The two presets are:-
VR101 allows you to set the sync-tips to 0V or a small offset if needed.
VR102 sets the overall video amplitude. I tend to think of SSTV as being 1kHz/V so 2.3kHz - 1.2kHz = 1.1kHz, so I chose 1.1V from sync-tips to white level as a standard here in the absence of any other. You can choose what you like, it's the FM sub-carrier that does have standards.
To test this out properly you'll either need an accurate audio oscillator, a PC-based version (a few freeware versions out there) or use some generated wave files. I have generated a bunch that go slightly beyond the normal bounds of SSTV (attached), this allows you to chart the 'S' curve above if you should wish to.
This just confirms or otherwise the linearity under static conditions, but it's a start.
Set-up should be quite easy. Set both presets to midway, feed in 1.2kHz and set the output with VR101 (point 1C) to 0V, there'll be a small amount of ripple, centre it about 0V.
Feed in 2.3kHz and adjust VR102 to produce 1.1V, go back to 1.2kHz and check it's still at 0V.
That's it! If you want feed in the other frequencies and note the output voltage without changing anything except the file playing. With those numbers you should be able to generate a chart (Excel) or a graph by hand. You should end up with something very similar to the above version.
Steve A.
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