Dragon 64 fixing...

From: Jules Richardson <julesrichardsonuk_at_yahoo.co.uk>
Date: Tue Jul 20 04:56:37 2004

On Mon, 2004-07-19 at 22:43, Tony Duell wrote:
> >
> >
> > Finally found the five minutes to see if my Dragon 64 works. It doesn't.
> >
> > I get a white screen with a black border (which seems healthy enough),
>
> Unusual, I would have expected something in green (but see below!)

LM1889 chip it was - I now get coloured crap on the screen :)

The only spare LM1889 I had was in the Dragon 32, so I desoldered that
one... breaking one of the pins off right at the chip package in the
process - grrr! Out with the scalpel to cut away enough of the IC
plastic to expose metal and I've got it working... but bugger!

> It'll give some kind of (Microsoft) BASIC sign-on message. I am not sure
> what the exact wording is. It should be black characters on a green
> background.

Interesting. When the random characters stay together in the middle of
the screen I get a couple of A's and a D in the right place that made me
wonder if it was trying to say "Dragon Data", and then presumably
waiting for a keypress before dropping to BASIC (and giving the same
sort of text as the Dragon 32 does). Maybe that's just coincidence,
though.

> > is screwed - but why the random characters should jump positions
> > sometimes I don't know (unless the design of the reset circuitry is not
> > very good)
> >
> > One obvious fault I found was that pin 6 of the 40 pin 6847 IC was
>
> DD3 -- Data bus bit 3. The machine will behave oddly without that!

Absolutely. I'm confident that I've sorted that one though, although
I'll double-check (and make sure that the solder attempt by the previous
owner hasn't shorted anything it shouldn't)

> The only 18 pin chip I cna think of in the Dragon is the colour encoder,
> presumably an LM1889 or something. The fact that it's not there could
> explain the lack of colour on the screen!

Yep :) Now running with a 17.5 pin chip... :-)

> > None of the DRAM is getting warm (suggesting failure), and the fact that
> > there's something approaching normality on the display suggest that the
> > power rails are OK and the CPU's at least operating.
>
> The design is similar to the CoCo, and is close to the classic Motorola
> application circuit for the 6809/6883/6847. I would start with data
> sheets on those chips. Check the power lines (I think all you need is +5V
> in the 64), check the reset pin (is it stuck active?), check for CPU
> clocks (E and Q, sourced from the 6883 SAM), check for memory address
> activitiy, and so on. There's a lot you can do without a schematic.

ta - will do. I've grabbed the 6847 datasheet, and I should have the
6809 one already. Spare 6809's I have. The 6883 is socketed on both
Dragon 32 and 64 so if that's dead I could do a swap. Don't think I have
a spare 6847 though, and the one in the D32 is soldered. Of course, it's
likely to be something simpler I expect...

cheers,

Jules
Received on Tue Jul 20 2004 - 04:56:37 BST