Date   : Thu, 13 Jun 1996 21:13:37 +0100
From   : Mike Tomlinson <mike@...>
Subject: Re: An Issue issue

In message <9606111308.ZM10612@...>,
davidl@... writes
>I wonder if anyone would be kind enough to point out the differences between an
>Issue 4 and an Issue 7 BBC? Ideally, I would like an exhaustive list, but any
>important ones that spring to mind would be appreciated.
>
>Thanks,
>
>Dave.
>

From the hardware manual:

Changes from issue 4 to issue 7 (5 and 6 never went into production):
======================================================================

R114 changed to its present value of 18 ohms 1W, and C42 changed to
33pF.

R75 went to its final value of 82k.

The diodes and resistors on the ROM select circuitry which could be used
to select 1Mhz operation were omitted.

Links S18 and S19 are made with tinned copper wire.

When the VidProc ULA was replaced with the first set of custom designed
ICs, a modification was necessary - S26 was left open and a wire link
was made from the TTX-VDU line of IC2 to the invert input of the VidProc
(ic6 pin 7). Later versions of the custom IC made this modification
unnecessary.

All other changes from issue 4 to issue 7 were cosmetic changes,
including some thickening up of the tracks to improve power supply
distribution.

====================================================================

Speaking from experience, as a hardware engineer repairing BBCs for over
five years at an Acorn dealer, there was little difference in
reliability between the issue 4 and issue 7 boards. Issue 4s usually had
yellow screenprint, and issue 7s white.

The issue 2 and 3 boards were pretty foul. I only ever saw one issue 1 -
I understand only 1000 were produced.

The most common faults on any BBC were:

- blank screen on startup apart from flashing cursor.
keyboard cable loose/disconnected/poor contacts

- Return/Copy/Delete/cursor cluster keys stop working. 
broken tracks on RHS of keyboard, usually caused by enthusiastic
pounding of the Return key during games playing. 

- Dead, comes on if left for a few minutes. 
C9 in power supply. Replace with 105degC replacement

- Mode7 faults (absent, twinkling characters).
Heatsink on VidProc missing or dislodged

- No sync on RGB output.
ic48, 74LS86, faulty

- Continuous Tone
This happens when the system ROM cannot initialise the hardware.
Unfortunately can be caused by pretty much anything, but try
substituting the two 6522 VIAs, the 6502, ic14 (74ls245) first.
Do not use a 74hc245 or a 74als245 - they will not be reliable.

The machine will boot and work without the user VIA fitted (ic69), but
will not print and the user port will be disabled. It will also boot
without the system VIA (ic3) fitted, but will not run properly.

Always suspect any ROM boards or other expansion cards, particularly
those made by Watford Electronics. Remove and reinstate default links to
see if the problem disappears.

Hope the above is useful.

-- 
Mike Tomlinson
constructive email to mike@...               ; flames to /dev/null.