Question about D32 circuit diagram

[zephyr]

On the SA D32 circuit diagram is an X and the text "disconnect for b/w" on the track from pin 6 of IC9 (LM1889) to R7. Cutting this track on my Dragon seems to result in the TV displaying a rather poor quality green screen display with no colour. Can someone please explain exactly what this is for, and why it is mentioned on the circuit diagram?

Here's a copy of the diagram at http://www.dragondata.co.uk.

http://dragondata.worldofdragon.org/tec ... -sound.gif

[rolfmichelsen]

I did a similar trick back in the good old days. I simply removed LM1889 from its socket. This disabled color, but also resulted in a much steadier monochrome display. I did it on both my D32 and D64 with good results. LM1889 has been socketed (not soldered) on all Dragon models that I have seen so it was easy to remove.

[Vato]

Have a look at page 20 of Dragon User, September 1988.

Vato

[zephyr]

Thanks for the information, but this does not explain the reason for the "disconnect for b/w" on the original circuit diagram.

[Rinconete]

The VDG (MC6847, IC12 on CPU schematic) generates the appropiated brightness and syncronizing signals to create a B/W composite video signal. The colour burst is generated by means of a LM1889 and added to the B/W composite video at the final stage. If the colour burst is not present you see a B/W image. That is the reason why disconnecting the LM1889 output turns the colour off.

You may find a detailed explanation here

http://www.maxim-ic.com/appnotes.cfm/an_pk/734

[zephyr]

Thanks for the feeback, but the problem is that cutting the track shown on the circuit diagram does not result in a clear b/w display.

[Rinconete]

...displaying a rather poor quality green screen display with no colour

mmm, green?, can you see a B/W image?... it should be B/W. I have removed the LM1889 in my Dragon 64 and I can see a perfect clear image. However, the D64 video amplification stage is different to the D32 video amplification stage.

Could you see a perfect clear colour image before disconnecting the LM1889 output? Which IC and pin number are you disconnecting?

[zephyr]

...displaying a rather poor quality green screen display with no colour

mmm, green?, can you see a B/W image?... it should be B/W. I have removed the LM1889 in my Dragon 64 and I can see a perfect clear image. However, the D64 video amplification stage is different to the D32 video amplification stage.

Could you see a perfect clear colour image before disconnecting the LM1889 output? Which IC and pin number are you disconnecting?

Yes, the colour display is very good. Desoldering the LM 1889 chip, or disabling the colour crystal (0.1uf capacitor in parallel as described in Dragon User, September 1988) results in a nice clear b/w display, but cutting the track from pin 6 of IC9 (LM1889) to R7 does not.

http://dragondata.worldofdragon.org/tec ... -sound.gif

[Rinconete]

Yes, the colour display is very good. Desoldering the LM 1889 chip, or disabling the colour crystal (0.1uf capacitor in parallel as described in Dragon User, September 1988) results in a nice clear b/w display, but cutting the track from pin 6 of IC9 (LM1889) to R7 does not.

http://dragondata.worldofdragon.org/tec ... -sound.gif

that's it!! it is not pin 6 is pin 13!! Please check the D64 schematic in the upload subforum.

[zephyr]

Thanks! Perhaps you could help with my 'flickering' video fault?

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