Date   : Sat, 05 Nov 2005 02:27:01 GMT
From   : Pete Turnbull <pete@...>
Subject: Re: Parts blag

On Nov 4 2005, 15:59, David Hunt wrote:

> Don't forget this needs a heatsink!
>
> In my Issue 3 board [IC6] the chip is marked
>
> VTi 445 V 5304
> VC2069
> 201647
> (c) ACORN COMPUTER

That one *doesn't* need a heatsink.  It's only the original Ferranti
Video ULAs that need a heatsink.  The VTI VidProc does not.

Ferranti Video ULA type 5C094E part no. 201,601
VTI VidProc part no. 201,647

For the sake of completeness:
Ferranti Serial ULA type 2C199E part no. 201,602
VTI Serial Processor part no. 201,648

Both video chips were made in various revisions.  For practical
purposes, there is no difference between revisions of the Ferranti
video ULAs, they all need heatsinks and they all need the same value
bias resistor (the big resistor nearby) though the value of that was
revised on Issue 3(?) boards.

The first version of the VTI video processor does have a difference,
though.  Later ones use the same settings of link S26 for normal and
inverse video a the ULAs do.  If you find one of the first ones, it has
the number VC2023 (instead of the VC2069 mentioned above) and needs a
signal to tell when Teletext mode is in use.  For these, you have to
remove the jumper from S26 and add a wire, on the underside of the PCB,
between the centre of S26 and  pin 1 of IC10 (that's the /TTX signal,
driving one of the data buffers).

I was told a story about that heatsink.  Apparently inside the ULA
there's a chain of gates which have outputs somewhat akin to open
collector devices.  Because there are no additional pullups, the rise
times of the gates in the chain get progressively longer along the
chain, and this effect is worse at high temperatures.  The result is
that once the ULA has warmed up, the circuit does not operate properly.
 If you could drill a small hole in exactly the right place in the ULA
and poke in a wire in to add a pullup resistor to one of the gates in
the chain, the rise time problem would go away and you'd not need the
heatsink.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York