Date   : Sun, 30 Mar 2008 11:58:33 +0100
From   : ken.lowe@... (Ken Lowe)
Subject: Reviving a dead BBC+

Ken Lowe wrote:

> I'm also a bit puzzled about the arrangement of the CRTC PCB track on the
> B+. I believe pin 4 of IC39 (&FExx decode circuitry) should be connected
> directly to pin 25 of IC78 (6845). This isn't the case. I've checked the
> equivalent circuit on the original BBC and there is a direct connection
> between IC26 pin 12 (&FExx decode circuitry) and pin 25 of IC IC2 (6845). 
> I
> suspect there may be a broken track or bad via on the B+ board. 
> Unforunately
> it must be underneath one of the soldered ICs making it very difficult to
> track down. In the interim, I've temporarily installed a wire jumper but
> again this has made no difference.

Ok, having removed an IC to see what happens to the track underneath the IC, 
i've worked out what's going on here. Unlike my working beeb, on this B+ the 
CRTC line from the decode circuit (pin 4 of IC39) passes through a de-glitch 
circuit (pin 3 (in), pin 2 (out) of IC32 which is a 74LS374N) before being 
fed into the CRTC (pin 25 of IC2). This doesn't help much though; the decode 
circuit is still being adversely influenced by A9 & A10.

Ken.