Prime wrote:I've just finished building my version of this circuit and it's working, but have a couple of questions for Dave / Hoglet....
Prime wrote:I have not populated the area that replicates the two transistors in the Y line on the Atom board as I wanted to find out if I needed them before populating that area
These would be needed if you are driving it directly from the Y output of the 6847, but not if you are driving from PL4 pin 9.
If you are feeding directly from the 6847, and not using an inverting amplifier, the sync tip clamp circuit will not be working properly, and this might be why you are having trouble setting the levels.
The values of R1 and R2 and the orientation of D1 will clamp the negative going sync pulse to 1.5V (and the rest of the video signal will be higher than that). This is setup to work with the Y signal from PL4 pin 9 on the Atom.
Prime wrote:1) What position should the jumpers be when connected to the Atom, I've got them closed at the moment.
Closed is correct.
I'm not sure I ever properly tested V7 of the PAL that supports these links, but the changes were minimal so I hope it will work. Let me know if there are any problems.
Prime wrote:2) Should there be resistors in the SCART lead ? My colour board doesn't need them and my Beeb lead doesn't have them but I suspect this board (and the Acorn one) might need them.
I have found that the colours are I would say better than my board, but appear at times a little 'bright', also I find I have to tweek the pots when I change video modes, I found this problem with Arcadian's Acorn board too, which leads me to wonder about the resistors in the SCART lead...
As Mark rightly says, additional resistors are needed to drop the TTL levels down to SCART levels. At a pinch, you could just increase the 68R ones on the board to about 330R, but I don't think this is quite as good as having them at the monitor end of the cable.
Do you know whether your SCART inputs are terminated with 75R resistors (try measuring the resistance to ground)?
Regarding the adjustments between modes, do you have to tweak the sync pot or the luminance pot or both?
Mine are set as follows (for the signals on PL4)
Sync pot: 1.2v (pin 5 of IC3 to GND)
Luminance pot: 1.95v (pin 10 of IC3 to GND)
To set the Luminance pot, I use the following program:
10 CLEAR 4
20 COLOUR 0
30 FOR A=#8000 TO #8200
60 FOR A=#8C00 TO #8EFF
90 FOR A=#8F00 TO #91FF
120 FOR A=#9200 TO #94FF
150 FOR A=#9500 TO #97FF
200 GOSUB 240
220 GOSUB 240
230 UNTIL 0
240 FOR A=0 TO 500
250 IF ?#B001 & 128 = 0 THEN GOSUB 280
280 PRINT $7
300 UNTIL ?#B001 & 128 = 128
320 UNTIL ?#B001 & 128 = 0
330 PRINT $7 $7
350 UNTIL ?#B001 & 128 = 128
Shift moves from one mode to the next
The best mode to adjust in is the Colour Graphics mode with the alternative palette (and the white border). Rotate the pot until the orange bar turns black, then back the other way until it's just a stable orange.
I might be possible to adapt the sync tip clamp circuit to work with the Y output of the 6847. The purpose of this circuit is to set the DC level of the sync pulse to a defined level, regardless of the mode. It's currently set up for a negative going sync pulse, with normal composite sync levels. The problem is, the Y output of the 6847 is inverted, i.e. the sync pulses are the most positive part of the signal. The same circuit topology should work, but you would need to reverse the diode, and the use different values of R1/R2 so that the signal was clamped to 2V. i.e. increase R2 to ~680R (and the rest of the signal would fall below this). You would then need to put the links in the other position (to tell the PAL that the L and SYNC signals are inverted).
But I've not tried this myself.
I'd suggest getting it working first with the inverting amplifier.