Dragon 64 repair - a little help?

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gareth
Posts: 7
Joined: Sat Jun 07, 2014 10:55 pm

Dragon 64 repair - a little help?

Post by gareth »

Hi there,

Just joined the site, having just got my 64 down form the loft, not having used it for 10+ years.
Is there anyone out there who could repair/suggest someone who could repair it for me at a reasonable charge? On powering up, I get the green screen, with a bit of the copyright message, but mainly a screen full of zeros. The screen will let me type a few letters, but on the 2nd and 3rd row of the keyboard, the letters you type come up as something different on the screen. It makes no difference whether the Dragon DOS is plugged in or not.

I have got my 32 working, but always had a softer spot for my 64. Can anyone help?

Thanks.
zephyr
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Joined: Mon Jul 21, 2008 1:18 am

Re: Dragon 64 repair - a little help?

Post by zephyr »

I don't know of anyone who could repair it for you. I can only suggest that you try changing the SAM (MC6883/SN74LS783) chip, followed by the CPU (MC6809E) and VDG (MC6847) chips if SAM chip swap doesn't fix the problem. The SAM and CPU chips should be quite easy to replace because they are usually socketed.

Please make sure to carefully remove the mainboard from the case before attempting to change any of the chips. It is possible to remove the chips from their sockets with a screwdriver if you don't want to buy an IC removal tool.

e.g.
http://www.youtube.com/watch?v=I1SNRC9_D8g

Regards,
Steve
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tormod
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Re: Dragon 64 repair - a little help?

Post by tormod »

Yes, even just taking out the chips and reseating them might help. Oxide layers can grow and cause contact problems. Some people recommend polishing the pins with a rubber. Unless you had an accident (short-circuit, hot-plugging cartridges, etc) chances are good that your chips are fine inside. Be careful when handling the chips, don't bend the pins and avoid static electricity.

In my own experience, it is worth checking out the voltage levels if you have a multimeter. I though my VDG was broken but it turned out to be one bad capacitor on the power board.

For someone to repair it depends on where you are. I would have loved to take a look, but you could rather have bought a new one for the shipping cost it would have incurred. If you have a vintage computer club nearby (or other electronics club) there are sure some helpful people that will find your Dragon fascinating to debug.
zephyr
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Joined: Mon Jul 21, 2008 1:18 am

Re: Dragon 64 repair - a little help?

Post by zephyr »

gareth
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Joined: Sat Jun 07, 2014 10:55 pm

Re: Dragon 64 repair - a little help?

Post by gareth »

Thanks to all above. I have never tried this sort of thing before, but I'll have a go at removing the chips one at a time, checking the contacts and replacing. I thought they were all soldered in, but from the comments, it looks like a bit of gentre screwdriver prising might work. I'd be surprised if the chips had blown, as it was fine when last used (albeit a long time ago). It's possible the heat/cold got to it in the loft, but then again, the 32 has survived!

If this fails, if anyone else reads this and fancies fixing, please post here. I will pay postage in the uk and any reasonable charges to repair. I don't want to have to bin the old girl!
gareth
Posts: 7
Joined: Sat Jun 07, 2014 10:55 pm

Re: Dragon 64 repair - a little help?

Post by gareth »

Hi all,

I've swapped the SAM and CPU chips one at a time with my 32. As suggested, this was quite straightforward with a screwdriver. I don't think these are the problem, as the 32 works fine with them in. (I'm assuming both these chips are the same for a 32 and 64 - is this correct?) However, the 64 is now all but dead, even when I swapped them back, with no display at all now and the DOS doesn't power up either. From the suggestions above, that leaves the VDG. I am loathe to have a go at this, as it is soldered in. I have another 'spare' 32 I could swap with this to give it a try, if someone has some advice about getting it out, without me overheating solder, damaging the board somewhere else, etc.

Any help most welcome.

Gareth
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tormod
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Re: Dragon 64 repair - a little help?

Post by tormod »

IMO it would be a pity to "destroy" your spare 32, rather get a new VDG. But did you check all the supply voltages yet?
gareth
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Joined: Sat Jun 07, 2014 10:55 pm

Re: Dragon 64 repair - a little help?

Post by gareth »

Hi Tormod,

I will try. How do I do this?
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tormod
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Re: Dragon 64 repair - a little help?

Post by tormod »

See the "Power Regulator Board to Main Board Ribbon" on http://dragon32.info/info/psu.html

Measure using a voltmeter between Ground and respectively -12, 5 and 12V. The voltages should be close to the nominal value, say within 5%. If the voltages are not OK, disconnect the ribbon cable and see if the voltages from the power board are still not OK. In that case it would sure be something wrong on the power board.
zephyr
Posts: 1474
Joined: Mon Jul 21, 2008 1:18 am

Re: Dragon 64 repair - a little help?

Post by zephyr »

gareth wrote: I don't think these are the problem, as the 32 works fine with them in. (I'm assuming both these chips are the same for a 32 and 64 - is this correct?)
Yes. They are exactly the same chips.
gareth wrote:From the suggestions above, that leaves the VDG. I am loathe to have a go at this, as it is soldered in. I have another 'spare' 32 I could swap with this to give it a try, if someone has some advice about getting it out, without me overheating solder, damaging the board somewhere else, etc.
The VDG chips can easily damaged by over-heating them with a soldering iron. It would be better to buy a new VDG chip and socket to match. Carefully remove (desolder) the old VDG chip from your D64 mainboard and solder the socket in its place. This way you won't need to desolder the chip if it ever fails in future.


BEWARE:

Desoldering those chips without damaging the PCB tracks can be quite difficult if you have never done anything like this before. Please seek some expert guidance before attempting to desolder it.

Do not attempt to swap the power regulater board with the one from your Dragon 32. They are NOT compatible.
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