Date   : Mon, 02 Oct 2006 16:33:50 +0100
From   : "David Hunt" <dm.hunt@...>
Subject: Re: One more BBC B to fix

> Hi,
> 
> I've got an Issue 4 Model B here which won't start up. The 
> best I can get it to work is in 16k mode (jumper 26 south). 
> In this setup, most of the time when switched on a random 
> configuration will occur. Usually the RGB output is 
> completely white and the Caps LED comes on then goes off. 
> Usually, but not always, I can press Ctrl-Break after this 
> and different LEDs will light up and sometimes the RGB output 
> goes black. Also, on only a small number of occassions the 
> machine will boot to a prompt but then lock up a few seconds 
> later. The keyboard is useable until the lock up.
> 
> I've swaped over all the usual socketed ICs, the keyboard and 
> the PSU. I've replaced the 555 timer and IC32 but I still 
> have the symptoms described above.
> 
> Can anybody shed any light on what the problem might be? I've 
> not come across it before and can't tell if it's a bad 
> component or a broken track somewhere.
> 
> This is 1 of 2 Model Bs out of 20 which I can't get working. 
> Thanks for any help.

I've had a few Beebs with these kind of problems, I've done the following;

- change the video ULA for known working one
- change the System VIA for known working one
- change the 6845 for known working one
- change the 6502 for known working one
- change the PSU for known working one
- make sure the NMI line is jumpered according the options on the board,
e.g. disc/econet
- push the ICs firmly down in their sockets
- push down in various places on the board switching on the Beeb to see if
it starts up when a particular site is pressed down
- have a look over the board on the reverse side for any sign of dry solder
joints or broken tracks

In the end most of the problems I had have been down to the PSU's
electrolytics drying up (rare) or dry solder joints/broken tracks (common).
The symptoms essentially manifest themselves as dead Beebs/Masters.

Heating up the Video ULA solder pads with a soldering iron and a little
liquid flux fixed two dead(ish) Beebs I had been given both Iss.4. They kind
of worked but displayed garbage on the screen and locked up after a random
amount of time, it looked as if the repeated heating and cooling of the old
video ULA and some unidentified liquid (possibly part of the heatsink
compound) had gathered around the pins causing corrosion. A quick clean and
a resolder and all's well again.

Wandering off topic slightly, Acorn didn't put enough physical support under
the motherboard so when addons such as Solidisk's SWR128 that went into the
rightmost ROM socket was inserted and then pushed down (often in a rage) the
circuit board tracks and solder stressed and often parted company resulting
in an intermittently crashing Beeb.

Dave ;)