Date   : Sat, 23 Feb 2002 22:19:16 +0000
From   : Mike Tomlinson <mike@...>
Subject: Re: BBC Micro repairs

In article <r5AP4rAhn7d8Ew2g@...>, John Woodgate
<jmw@...> writes

>The result is that the Beeb with the Watford DFS and the drives now
>work, BUT the discs with the critical information on were made with the
>Acorn 1770 DFS, which is in the other Beeb and that one doesn't work.
>The discs may be  Acorn 1770 DFS or IBM360K format,

Discs formatted and written using Acorn's 1770 DFS are physically
compatible with the earlier 8271 DFS, so they should be readable with
the earlier DFS, *unless* you also have an ADFS ROM in the machine with
the 1770 daughterboard.  If the disc(s) have been formatted with the
ADFS ROM, they will be double density and unreadable with the earlier
8271 DFS.

8271 disc controller (chip)/Acorn DFS 0.90/Acorn D(N)FS 1.20/Watford DFS
1.43 and 1.44 write discs in single density.

1770 disc controller (module)/Acorn DFS 1.26 up write discs in single
density, in a format compatible with the earlier DFS.

1770 disc controller/Acorn ADFS 1.30 up/Watford DDFS write discs in
double density, NOT readable by the earlier 8271 chip.

> which is what I
>bought the 1770 mod and DISCopy for. With the working Beeb, they give
>the 'Sector not found' error message.

I don't know what DISCopy is, but it sounds like it has written the
discs in double density.  (This is likely if fitting the 1770 module was
a requirement for it to work.)  IBM's 360k format is also double density
and will not be readable by the 8271 chip.

>I am reluctant to swap the 1770 DFS into the working Beeb, because it
>has a Watford ROM expansion board, which is physically incompatible with
>the 1770 board.
>
>The non-working board has the +5 V supply at all three feed points on
>the board, but there is no tone on switch-on and no LEDs light.

No tone at all, or a continuous tone?  Usually a hardware failure on a
Beeb results in the OS being able to initialise, and you will get a
continuous "beeeeee...." where the sound is the same pitch as the first
part of the normal raucous startup "beeee-BEEP!"

Do you have -5v on the purple wire from the PSU? (Not needed for the
machine to boot, but it's used by the sound circuitry, so you will get
no sound if it's not connected or working.)  It's also used by the
serial port.

>I am an electronics engineer and I have lots of test equipment, but I
>don't 'do' computers, so I'm a bit in the dark in trying to diagnose the
>fault. I have the Beeb circuit diagram. I suppose I should look at the
>power-up reset first, around IC16.

If the machine is totally dead, (no sound at all) and you are sure the
power supplies are present and correct, first thing to check is the
16MHz crystal (easier if you have an oscilloscope.)  ISTR it's somewhere
near the 6502.  This signal is divided down by the Video ULA to provide
8MHz, 4MHz, 2MHz and 1MHz signals around the board and is essential for
the machine to work.  It seems to fail more readily if the machine has
been stored in a damp location.

Normally a reset line fault (there are both RESET and notRESET, where
notRESET is logically inverted) will result in a continuous tone on
power up.

The machine will happily boot with the following ICs missing:  user 6522
VIA (ic3, below the 6502), 6850 ACIA (serial), 68B54 ADLC (econet), all
user ROMS including basic (but the system ROM in the LHS most ROM
socket, marked PB04, must be present, of course),  and the 8271/1770
FDC.  It will sort-of boot but will not be usable with the system 6522
VIA missing (to the left of the 6502) - you'll get a screen with a
flashing cursor but will not be able to type anything.  So a valid
diagnostic technique with a faulty Beeb is to remove all the above
chips, see if the machine boots (if not, the fault is severe and needs
professional attention) - if it does, the chips can be replaced one at a
time, powering up in between until the faulty component is isolated.

Finally, if ic14 (74LS245, to the right of the 6502) is in a socket,
this is a high-failure component and replacement is worth a try.  but
the replacement must be a 74LS245 and not an ALS245, HC245 or other
alternative.

M.

-- 
     Security-wise, NT is a server with a 'Kick me' 
     sign taped to it.         -- Peter Gutmann