Need some help with a repair (blank green screen) (Fixed !!)

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Jackall
Posts: 6
Joined: Wed Feb 21, 2018 8:27 am

Need some help with a repair (blank green screen) (Fixed !!)

Post by Jackall »

Hi All,

I have a fully working dragon 32, and got another one off ebay, for spares, but it seems a pity to just dissemble it, so I decided to have a go at it.

The dragon powers up to a blank green screen (mainboard is an Issue 6 i believe).

IC 12 is missing PIN 19 (MC 6847), but this seems to have been done at the factory, since the Dragon was still sealed when I opened it.

I have swapped the cpu, both roms, the SAM (74LS783), with known good parts from the working dragon but the problem persists (the parts swapped also work on the "good" dragon).

Using the oscilloscope, I can see activity on all address and data lines, so it does not seem anything is stuck to ground, or to Vcc

Resistance between address lines / data lines to ground is more or less the same on all lines (very high).

Voltages on the RAM chips all seem OK too.

Also, the schematics I have, are barely legible, and was wondering if anyone can help me obtain a good set of schematics for the unit.


Thank you in advance

J
Jackall
Posts: 6
Joined: Wed Feb 21, 2018 8:27 am

Re: Need some help with a repair (blank green screen) (Fixed

Post by Jackall »

So The problem was resolved last night. It turned out the culprit was RAM Chip IC8 (4116).

For anyone facing similar issues I have documented the full troubleshooting process in a blog post here https://blog.johannfenech.com/?p=734

I cannot thank "sorchard" enough for making his hi res schematics, without which the repair would have been impossible

J
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robcfg
Posts: 1529
Joined: Sat Apr 04, 2009 10:16 pm
Location: Stockholm, Sweden
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Re: Need some help with a repair (blank green screen) (Fixed

Post by robcfg »

Glad you managed to get it working!

And thanks for your blog post, it’s very useful.
Jackall
Posts: 6
Joined: Wed Feb 21, 2018 8:27 am

Re: Need some help with a repair (blank green screen) (Fixed

Post by Jackall »

Thanks Rob,

Soon I will be posting insturctions about making a $2 dragon joystick using an "arduino" joystick, and a "cas" reader to load .cas files
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