Date   : Sun, 28 Sep 2008 12:20:23 -0500
From   : jules.richardson99@... (Jules Richardson)
Subject: Domesday video problems

Adam Sampson wrote:
> Hi all,
> 
> I'm currently trying to resurrect a Domesday system:
>   http://offog.org/stuff/domesday/25092008119.jpg
> 
> This belongs to the Computing Lab at the University of Kent, where I
> work. 

Heh - are Mark Wheadon & co still there? I was at ukc in the early 90s, and 
it's their fault I got into collecting old Acorn systems ;-)  (I unearthed an 
ARM Eval Kit and Teletext unit there; before that I'd been into Sinclair stuff)

> The system now mostly works: it'll boot off a Laserdisc, and you can
> navigate around happily. However, as you can see in the pictures,
> there's something wrong with the video output: the overlay graphics from
> the BBC are fine, but the genlocked video from the Laserdisc doesn't
> appear correctly:
>   http://offog.org/stuff/domesday/25092008116.jpg
>   http://offog.org/stuff/domesday/25092008118.jpg

Hmm, I don't have my service manual to hand (I'm in the US these days, and 
it's still stuck in the UK), but you say you have a copy now which should 
prove useful...

I seem to remember that there's a single board in these machines responsible 
for video mixing - it'd be worth poking around there (I can't remember the 
module number, but I think it's one of the three accessible from the underside 
of the player in the bottom 'slice'). The VP-415 players are *horrible* for 
suffering bad solder joints, and I've replaced a fair few capacitors in them, 
too. Fantastic design for the time, but the PCB and component quality tends to 
let them down.

Check in the service manual where the various video output taps come from, too 
- it may be you can tap into a sensible video signal 'upstream' of the video 
mix board which you can use to validate the video coming from the player.

Also check with the player's on-board diags. Details on how to kick the player 
into diag mode (and the list of fault codes) should be in the service manual. 
The built-in diags check the player functionality out pretty well.

Hazy memory (I don't have my AIV hardware with me, either) is telling me that 
there's an adjustment for video signal on the rear of the machine too (a pot 
accessed with a screwdriver via a tiny hole) - I think that's for final video 
output though (so you wouldn't see any sensible data coming from the BBC 
either) but it'd be worth a check.

> I believe the Laserdisc player was used as a video source for other
> experiments in the early 90s -- the SCSI ID was set differently, two of
> the Laserdiscs are marked "stills" and "video", and I've found
> framegrabs from them on other old machines. 

Hmm, on a personal note, can you track down anyone who was involved? I collect 
all sorts of Domesday system info (design details, marketing stuff etc.), so 
it'd be really interesting to hear more there.

 > It's therefore plausible
> that someone might have fiddled with other settings on the player -- or,
> since it's been sitting around for at least ten years unused, that a
> capacitor or belt needs replacing...

There's about twenty separate PCBs in the players, some of them connected via 
direct socket connections and some of them via multi-way cables, so there's 
plenty of scope for a dirty/intermittent connection. See comments above too 
about dry joints and failing caps.

Taking the machines apart is an art - I actually wrote down some notes along 
with photos of various stages of disassembly last time I did it which I'm 
going to put online, but the text is all in a notebook back in England right 
now :-(

If you really need to get your hands dirty, it's worth making up some 
multi-way cables/plugs - then you can hang individual boards out of the player 
and poke around with a 'scope whilst the player's running. It's really hard to 
get at everything otherwise.

I can't remember for sure now what happens if the player can't spin up to 
speed - my fuzzy memory's telling me that I did once try slowing the disc down 
to see what happened and it gave a loss of sync. It looks like your player is 
syncing properly, but is losing colour information somewhere in the mix stage.

cheers

Jules