Date   : Sun, 14 Aug 2011 16:33:58 +0200
From   : rick@... (Rick Murray)
Subject: Advice on restoring a dead Beeb?

[repost to list, bloody wonked headers...]


On 13/08/2011 19:08, Peter Coghlan wrote:

> Dunno. I'm just making it up as I go along!

;-)


> I thought there was always a VDU7 beep at power on as this is part of
> the "BBC Computer 32K" string being displayed?

I meant optional as in the sense of it not being a *required* part of 
the reset sequence (resetting after power-up without holding Ctrl, for 
instance).

I wonder if there is *any* possibility of a power-up that omits the 
second beep? Auto-boot for example?


Thought: Does your keyboard have any links made in the location (usually 
bottom right near the end of the space bar). Maybe it is hanging waiting 
for a disc drive to spin up? [though there ought to be something 
on-screen...]


> what happens at startup and in what order. However, we don't know
> for sure which version of the OS is involved and we are missing some

If it is a mask programmed ROM type HN613128 and (C)Acorn, it is 99% 
going to be MOS 1.2.


> everything is connected in parallel to the data bus, almost anything
> could be the source of the problem, whether it is accessed or not.

Take the CPU chip out, you'll see the difference.

By the time we turn on the keyboard LED, we've zeroed and counted 
memory. It isn't a random behaviour, the LED is set from an addressable 
latch, which ought to default to "all off" at power up.


> By reconfiguring jumpers, I think it is possible to have the OS in
> four parts in what would normally be the paged ROM sockets and have
> only one language ROM available in what would normally be the OS socket.

Yes, I believe that was for development when MOS zero-point-zero was in 
a bunch of 8K EPROMs.

Not entirely certain why this option would be continued into production 
hardware. I guess it is one of the many little quirks of the BBC (like 
why didn't composite video output colour in the first place?).


It might be useful to grab a copy of the annotated MOS disassembly from:
   http://bbc.nvg.org/doc/OS-disasm.zip

The reset vector code starts at &D9CD which is about three quarters of 
the way down the file "OS4".


It is a shame the original poster "reseated" the various things. I'd 
have thought powering up first would have been a good idea, and only 
mess around inside if it is non-functional.
[caveat: well, the very first thing is to give it a gentle shake while 
rotating it to check nothing got in, and nothing (video ULA heatsink?) 
has fallen off]


Best wishes,

Rick.

-- 
Rick Murray, eeePC901 & ADSL WiFI'd into it, all ETLAs!
BBC B: DNFS, 2 x 5.25" floppies, EPROM prog, Acorn TTX
E01S FileStore, A3000/A5000/RiscPC/various PCs/blahblah...