Date   : Sat, 06 Jan 2007 12:13:14 -0600
From   : julesrichardsonuk@... (Jules Richardson)
Subject: Acorn Atom

Michael Foot wrote:
> The Atom has fully populated RAM and also a Colour card (202,006) 

Wow. I've been looking for the colour board for many years and have never seen 
a single Atom with one (just photos). I've got the technical manual for the 
board so did think about building one, but decided that it was more trouble 
than it's worth (it would be fun for the sake of it, but there are too many 
other things that are more important :-)

> Upon plugging it in, I cannot get it to display anything either via the 
 > UHF modulator or the colour card output. I believe the machine is working
 > since presing Ctrl-G gives a beep, typing P. <Return> makes no sound and
 > typing rubbish <Return> gives a beep also. The PSU output reads 11.20V
 > but I assume it is unregulated.

Even Acorn say in the manual that if you buy an Atom with the colour board, 
you should concentrate on getting the Atom working without the colour board 
fitted first :-)

Now, fitting the colour board involves bending C2 out of the way on the Atom 
main board to expose a through hole beneath C2; the PCB track leading to this 
is cut, severing the normal output to the modulator. The colour board then 
plugs into PL4 (2 x 5 pins) on the Atom, and the colour board's output is a 
wire which is soldered to the "through hole side" of the cut made.

In other words, desolder that wire, unplug the colour board, and re-make the 
cut connection below C2 on the Atom and you'll be back to a 'stock' machine 
again. If you still get no output that way, then at least you know it's not 
the colour board at fault.

Those modulators are becoming notorious for 'mechanically' breaking inside 
around the output socket, incidentally; pull the lid on it and just check it's 
not a simple problem like that. Composite video (at 1V) is available at pin 9 
of PL1, with return on pin 10:

   +--+     +-------------------+
   |IC|     | 2   4   6   8  10 |
   |54|     | 1   3   5   7   9 |
   +--+     +-------------------+
                     PL1

... just in case you want to rule out the modulator easily.

> Do I need to use a higher rated PSU because of all the add-ons?

Hmm. Acorn quote 750mA for a stock 2KB Atom, then another 750mA for the 10KB 
of extra RAM. Bus buffers / VIA chip take 100mA if you have those fitted (not 
all machines will). Unfortunately I don't have power consumption figures, but 
I doubt it's more than a couple of hundred mA at most.

In other words, you're probably reasonably close to the limit, but if the 
machine seems to be capable of some basic stuff then it can't be *that* upset.

The 2114 RAM chips used in the Atom are well known for failing. There's also a 
lot of socketed chips in an Atom - I'd work my way through those, re-seating 
them just in case it's a bad contact between chip and socket that's causing a 
fault.

cheers

Jules