Date   : Mon, 28 Nov 2005 15:13:40 +0000
From   : Richard Gellman <splodge@...>
Subject: Re: BBC B black text in Mode 7

Brian Widdas wrote:

>On Mon, 28 Nov 2005, John wrote:
>
>  
>
>>Recently recovered BBC B, boots up ok, but text in Mode 7 and hence start up
>>screen, is black.
>>
>>Types OK, have visible text in other modes. Cannot change text colour in
>>Mode 7.
>>    
>>
>
>Mode 7 is generated by a separate chip, the SAA5050. Is it possible this
>(or something which feeds it a signal) is on the blink?
>
>Brian
>  
>
The SAA5050 is fed no signal (aside from its 6Mhz clock). The CRTC 
generates addresses in the same way it deoes for modes 0-6 (although 
mangling circuitry adjusts these into teletext row/cell addresses), and 
the resulting data bus value is picked up by the SAA5050 (along with the 
row address) and it generates RGB out values which are passed through 
VIDPROC to the video circuitry. These leaves the following possiblities 
for faults:

Teletext chip faulty, or not receiving 6Mhz clock signal.
Teletext chip not connected to data bus/mangled address bus (teletext 
character row).
Teletext chip not connected to VIDPROC
Teletext address mangling circuit faulty.
VIDPROC faulty

Its unlikely that the connections have failed (although a visual 
inspection of the board for any cracks is worth doing), or that a 
specific part of VIDPROC is faulty (if VIDPROC had a fault, it would 
most likely affect the whole chip, rather than just the TTX pass-through 
circuit). This suggests that either the teletext generator chip is in 
need of replacing, or that the address decode chips that convert the 
CRTC addresses into RAM address+row address are not operating. I would 
suggest the former is the likely cause.

-- Richard