On Thu, 1 Sep 2011 21:55:49 +0100 (BST), ard at p850ug1.demon.co.uk (Tony
Duell) wrote:
Well, another
step forward. Thanks to Tony's advice, I continued
investigating the video output, and found another dead IC (74LS51 this
time). I'm beginning to wonder how a machine could end up with so many
faulty ICs; that's two in just the video circuitry. Maybe the original
ICs do
fail, but I wonder if this board has been mistreated in some way.
That's my guess. Physically it looks spotless - not even any dust, it looks
near-new. Another nice thing is that at some stage, someone has replacd the
ROM sockets with nice turned-pin ones, so I probably don't need to worry
about bad sockets there.
One thing that is odd is that there's a white dust on the other sockets,
around the legs of the ICs. I was highly suspicious of this initially, but
on close inspection with a magnifying glass, the sockets seem OK (if a bit
cheap), and measuring them indicates they're making good contact. I'm still
a bit nervous about this though. Many of the legs of the ICs have
discolouration, often ending up black. Cleaning them seems to sort that
out. I'm wondering if some of the machine's issues are due to it being
stored in a mildly corrosive environment. But, there's no trace of
corrosion on any other surfaces, only the IC legs. Further issues in this
machine could be from ICs with dirty legs, I suppose.
> PSU short to ground did something horrible. I now
get a clean video
> display (which rolls on the Monitor ///, but that's not a big surprise).
> At this point though, I really need to replace the
crystal. Holding it
> so the broken leg makes contact with one hand while powering the machine
> up with the other really isn't a good way of
going about things; this is
why the
picture's still rolling, I can't adjust the hold on the monitor
and keep the crystal steady enough with the other hand!
Can you not solder the
wire back to the crystal, at least as a temporary
fix?
The broken leg came off almost perfectly flush with the bottom of the
crystal can. Believe me, I've tried to solder something to the stub, but
there's just not enough there to make a connection with acceptable
strength. I do want a crystal of the original frequency; this mod looks
like it was a period thing (quite a standard procedure though, from your
earlier post), so I'd like to keep it as part of this machine. I wouldn't
really feel bad about returning it to NTSC-ish behaviour though. I'll drop
in to RS on the way home from work today and grab a 14.318MHz crystal. That
should allow me to work on the thing this weekend.
> Silly question of the day: I'm guessing no-one
would have any idea where
I'd find a
14.25045MHz crystal in New Zealand...
At one times there were companeis who would
make any crystal you wanted
(within reason), the cost went up spectacularly if you wanted it very
accurate, or wanted it quickly. But if you could wait a few months for
the part, it was affordable. Radio amateurs used ot get odd crystals made
ot put ex-PMR radios on the amateur bands, for
example, but I guess
that's not done much any more.
Anyhow, there may still be such compaines around,worth a look anyway.
It may be the only way to get hold of a crystal like that. On the other
hand, Rakon (fairly large crystal manufacturer) is just one suburb over
from where I live. Maybe they'd be helpful. Will
see what happens.
Cheers,
Mike