Date : Sun, 24 Feb 2002 10:07:33 +0000
From : John Woodgate <jmw@...>
Subject: Re: BBC Micro repairs
I read in !bbc-micro that Mike Tomlinson <mike@...>
wrote (in <YHNFFYAkVBe8EwLW@...>) about '[BBC-Micro] BBC Micro
repairs', on Sat, 23 Feb 2002:
>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.
Thanks for prompt and helpful response. I don't have an ADFS ROM.
>
>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.
The DISCopy Fascinating Manual doesn't explicitly say what it is for! I
used it to transfer files on DFS discs to IBM 360K discs.
>
>>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!"
No tone at all.
>
>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.
Yes, the -5 V is present.
>
>>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.
Yes, the 16 MHz is there, as are its sub-multiples, but I haven't
checked that they are all *everywhere* that they should be.
>
>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.
Noted.
>
>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.
No Econet.
> 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.
I will try that.
>
>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.
>
OK, I'll look at that as well.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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