Date   : Wed, 21 Oct 2009 23:18:28 +0100
From   : pete@... (Pete Turnbull)
Subject: BBC Model B Issue 3 Circuit Diagram

On 21/10/2009 20:54, Mark Haysman wrote:

> Does anyone have a scan of the Circuit diagram for an Issue 3 Board? I 
> specifically need to know the parts around the Econet area, and where/how it 
> allows you to add an onboard clock, as well as the jumper settings.

I don't have suitable scanner access right now, but I do have the 
diagram, and I've done these upgrades a few times myself.  The clock 
parts are:

IC97 - half of a 74LS74
IC90 - CD40178
IC92 - 74LS74
IC93 - half of a 75159

The other half of IC97 is used for the switch that controls interrupts, 
so it should already be there.  The other half of IC93 is the data line 
driver, so it too should also be there already.

R25 - 3K3
R19 - 3K3
R26 - 100R

None of the resistors need be better than 10%

C4 - 47pF plate ceramic

S3 sets the clock speed.  Set only one position:
  1: 1250K
  2: 909K (if I read the fuzzy part correctly)
  3: 714K
  4: 500K
  5: 500K
  6: 428K
  7: 375K
  8: 333K
  9: 300K

S5 enables the clock when set North

S6 North divides clock by 2, South divides by 4

The terminator parts (it's a proper active terminator) are:

R54 - 100K
R56 - 100R
R42 - 56R
R43 - 56R
D3 - 1N4148 (with the cathode connected to the junction of R56/C22/R42)
C19 - 10?F electrolytic
C22 - 10?F electrolytic

The resistors should be 2% or better.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York