Dragon 64 fixing...

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Mon Jul 19 17:43:52 2004

>
>
> 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!)

> then in the centre of the screen a few random characters (some with
> inverse video). Occasionally after a reset the random characters don't
> stay together but occupy random locations toward the middle of the
> screen.
>
> I assume (having never seen an operational D32 or 64 before) that it
> should say "Dragon 64" or something, implying that character generation

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.

> 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!

> broken and had been badly repaired in the past (such that I don't think
> the joint had held), but fixing that hasn't made a difference. The fact
> it was ever broken at all suggests that someone's been fiddling inside
> the machine though...
>
> I notice IC17 (a socketed 18 pin DIL) is missing - should it be?

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!

>
> 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.

>
> Of course presumably a RAM fault could quite easily explain random
> characters and locations though, but doesn't explain the stable white
> screen / black border (unless that's done in low level video hardware).

It is. Look at the 6847 data sheet.

-tony
Received on Mon Jul 19 2004 - 17:43:52 BST