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JamesD wrote:Early Color Computers had the same problem. I think an older version of the SAM was to blame but it's been a long time since I last heard about it.

I tried swapping the CPU and SAM chips a few days ago without any luck.


BTW, I also have a very early issue Dragon 32 (piggyback board type) which has a video fault that causes a very bad flickering of the picture when 2 colour modes are used in combination with SCREEN 1,1. Four colour modes work perfectly, and 2 colour modes in combination with SCREEN 1,0 also work without problems.

Can anyone help with this one?

zephyr wrote:BTW, I also have a very early issue Dragon 32 (piggyback board type) which has a video fault that causes a very bad flickering of the picture when 2 colour modes are used in combination with SCREEN 1,1. Four colour modes work perfectly, and 2 colour modes in combination with SCREEN 1,0 also work without problems.

Can anyone help with this one?

I desoldered the LM1889N mod chip (IC9) and I'm left with a black and white only display, but the flickering problem is gone. Can anyone confirm that the LM1889 chip could be causing the flickering problem?

Update: I replaced the LM1889N chip with a new one, but it did not fix the problem.

Back to the hot Dragon, it's just an idea but perhaps a previous owner tried to overclock it. Other than changing the crystal I don't know how you would go about it on a Dragon, but it may be worth looking around that area.

Thanks Alastair, but that machine has gone much worse. It now only works for about 30 seconds before filling the screen with junk and freezing solid.

zephyr wrote:

zephyr wrote:BTW, I also have a very early issue Dragon 32 (piggyback board type) which has a video fault that causes a very bad flickering of the picture when 2 colour modes are used in combination with SCREEN 1,1. Four colour modes work perfectly, and 2 colour modes in combination with SCREEN 1,0 also work without problems.

Can anyone help with this one?

I desoldered the LM1889N mod chip (IC9) and I'm left with a black and white only display, but the flickering problem is gone. Can anyone confirm that the LM1889 chip could be causing the flickering problem?

The upper part of the video section (IC9,IC10,IC11,IC14,IC21 and IC23) generates the color burst. It looks like there is some faulty connection. First of all I would suggest checking all the lines as well as the power inputs for each IC.

Thanks for helping with this! I'll attempt to repair it based on your information.

I have a Dragon 64 with a partially working keyboard. The following keys are not working: [12349 Break QRTYI ASDJKL ; Clear ZCB ,]. I have tried swapping the keyboard and examined the circuit board for any obvious problems with the tracks to the keyboard plug (no luck). Looking at the circuit diagram now leads me to assume that the problem is caused by IC5 (the MC6821 PIA chip) but I would like to be 100% sure before attempting the nightmare of replacing this non-socketed chip. Can anyone confirm that the PIA chip would be causing this problem, or is there something else to try.

You can view the relevant part of the circuit diagram at dragondata.co.uk:
http://dragondata.worldofdragon.org/tec ... 4-1-IO.jpg

If a simple continuity test from the keyboard connector to the appropriate PIA lines shows no breaks, I can't think of anything else it could be.