Date   : Sun, 06 Apr 2008 17:30:13 +0100
From   : acorn@... (Andy Ford)
Subject: Help with 'B' video output (only mode7 working)

Many thanks Peter.

I looked and it is a 2023, so I removed the jumper and temporarily 
connected the middle pin of S26 to pin 1 of IC10. Result = Perfect 
picture! :)

I have just made a small wire (soldered a jumper to one end for just 
clipping onto the middle of S26. I wonder if you know whether I should 
solder directly to the IC leg (does not seem wise to me, but I can 
solder carefully) , or whether there is somewhere nearby that I could 
use. I notice that the track for pin 1 of IC10 is not on the top.

Would you happen to know a sensible point to connect the mid pin of S26 
to ? Or is "onto the IC" the best idea you think ? Space is a little 
tight around that area.

Thanks :)

Andy Ford.






Pete Turnbull wrote:
> On 06/04/2008 16:26, Andy Ford wrote:
>   
>> Additional info...
>>
>> I have just this moment tried swapping IC5 and IC6 with ones from a 
>> issue3. This "spares" issue3 machine does work (it has some faults but 
>> the video is ok)
>>
>> But there was no improvement. I don't mind desoldering if needed but I'd 
>> like to be (as much as possible) that I would be doing the right 
>> component if someone thinks it is item X etc.
>>     
>
>   
>>> I have here a nice (tape only) issue 4. However, only mode7 appears to work.
>>>       
>
>   
>>> When tested with the composite video (either with or without the colour 
>>> jumper enabled) , I do get a very vague picture (it is offset to the 
>>> left and almost inverse)
>>>       
>
> About the only differences between Mode 7 and the other modes, is that 
> different address bus drivers control the RAM access, the video 
> processor gets bit data directly from the memory instead of from the 
> SAA5050 - and if you see some sort of picture, however distorted, that 
> much is probably working - and some versions of the video processor 
> control the polarity of the video signal differently. It sounds like the 
> video processor is not setting up its outputs correctly in Modes 0-6, 
> giving an inverted picture, so check whether IC6 is a Ferranti ULA or a 
> later video processor (2069), or an early video processor (2023).  If 
> it's a ULA or later vidproc, there should be a jumper on S26 and no wire 
> linking S26 centre (or IC6 pin 27) to the inverted TTX signal (usually 
> from one of the bus buffers, IC10 pin 1).  If (and only if) it's an 
> early vidproc, there should be no jumper on S26 and that wire should be 
> fitted, which is a modification specifically for early VTI chips.
>
> It sounds as if somebody has got a mismatch between the video processor 
> and the modification.
>
>