Date : Sat, 28 Jul 2001 15:03:48 +0100
From : Mike Tomlinson <mike@...>
Subject: Re: Help with BBC B fault
In article <20010727042234.MNMQ18810.mta01.mail.mel.aone.net.au@...>,
mikef@... <mikef@...> writes
>I've been given a BBC B which I'm trying to get working again. It's suffering
>from the dreaded single
>continous beep fault.
[...]
>Can anyone suggest other things to try that may solve or narrow down the
>problem? Could it be a faulty
>SA5050?
Unlikely.
>Also, what happens between the machine issuing the first and second
>beeps?
The first beeeeeeeee noise is random data on the slow data bus (hanging
off the system 6522) being interpreted and amplified by the 76489 sound
chip. It stops once the OS has configured the system VIA. If the OS
cannot start, or configure the hardware, the noise never stops - this is
what you are hearing.
The second beep is the control-break type reset beep (VDU 7).
>Something else I forgot to mention in my last post in that I get slightly
>different pitched sounds each time I switch it on. Pressing the break key also
>causes different pitches and causes different LEDs to turn on. Sometimes the
>tape relay clicks on as well.
In order:
Take out the user 6522 (this is below the 6502 and provides the user and
printer ports)
Take out the system 6522 (between the 6502 and the 8271) - the machine
won't boot properly without it, but you should see a cursor on the
screen if everything else is working.
Take out the 8271 and the DFS ROM.
Make sure you have an OS ROM in the leftmost ROM socket - this should be
marked PB04, I think - the OS and BASIC ROMS are PB04 and PB05. I can't
remember for sure which is which at the moment (too warm here today.)
Check IC14, 74LS245 - to the right of the 6502. If this is socketed,
try replacing it. It's a common failure point. Do not use a substitute
- e.g. 74HC245, 74ALS245.
Was the machine known to be working previously?
--
"Security-wise, NT is a server with a 'Kick me' sign taped to it."
- Peter Gutmann