Date   : Tue, 24 Oct 2006 21:30:56
From   : Michael Foot <mjfoot@...>
Subject: Re: One more BBC B to fix

In message <58e9a734e%mjfoot@...> you wrote:

> In message <4e6fa4f7f2info@...>
>           Sprow <info@...> wrote:
> 
> > In article <672022E79B6349DBA63CA47178833796@...>,
> >    Andrew Chesterton <andrew.chesterton@...> wrote:
> > 
> > > in the process of loading the OS and is now accessing the wrong
> > > locations in the memory map. 
> > 
> > The OS isn't loaded per se, it's just there. I agree with the
general
> > suggestion that the address decode is a likely suspect, the OS needs
RAM to
> > work everytime it uses a JSR/RTS pair,
> 
> I'm still wotking on this problem. So far I've replaced a few ICs with
IC30 (a 74LS74) being one of them. This has helped a bit. Now in 16K
mode I can usually get the machine to boot (bur-beep) but then it locks up
instantly. The screen has the BBC 16K header but the screen then fills up
with BASIC keywords and other junk. Usually it shows STR$ on alternate
lines down the screen. When I press Control-Break I get random charcters on
the mode 7 screen. It still does not boot in 32K mode or with the jumper
off.

I've been mucking around with this machine a bit more and here's where
I'm I'm at. I can get the machine to boot pretty much every time but only
when:

S25 is south (16K mode) and the power leads that plug in near the RAM
are unplugged. I also removed all non-essental ICs except the VIAs and have
closed S9 to stop NMIs.

While the machine boots, it locks up soon after if the keyboard is used.
Usually a blank line and then another BASIC > prompt appears before the
lock up.
If I start the machine in mode 7 it looks correct. If started in mode 6
or 4 I get orange vertical blocks on every second character. If in mode 5
I get orange writing. I thin the orange colour is supposed to be yellow
but with low intensity. Usually I can use the keyboard for a little while
too.

I have checked out the keyboard, power supply and all standard socketed
ICs. I have also checked out ICs 20,30,31,32,33,34,37,39,40,43,44,45,74
and the 555 timer. The crystals are good also.

I've not seen anything like this before. The strange screen display
indicates a RAM or addressing problem but having the power leads removed
could indicate a voltage problem.

If you have any more ideas please post them. Any help is appreciated.

Cheers,

Mike.


-- 
Michael Foot <mjfoot(at)paradise(dot)net(dot)nz> *
http://homepages.paradise.net.nz/mjfoot/
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