Experiences with a composite video mod for a Sinclair ZX81

[Rod Smallwood]

On 03/01/2016 18:32, Eric Christopherson wrote:
> On Sun, Jan 03, 2016, Adrian Graham wrote:
>> On 03/01/2016 02:06, "Terry Stewart" <terry.stewart296a at gmail.com> wrote:
>>
>>> It's interesting that none of my present TVs would tune properly to the RF
>>> of the ZX81 but they will with some of the other RF only computers like the
>>> Coco 1 and Atari 400?  More circuitry in the latter perhaps...
>> Going from an old, old memory from when TVs became auto-tune rather than
>> dial-in or 'press-a-button-and-hold-until-something-happens'
> What's this about holding a button?
>
>> I recall it was
>> to do with the 'black level' from the ZX81 in that it was too great and
>> caused the autotune mechanism to skip it altogether as it wasn't seen as a
>> useable signal.
>>
>> Hopefully someone else can elaborate on that.

I dont know abourt the ZX but when I worked on VDUs and wanted a quick 
comp output.
I seem to remember you could connect a 560R to positive going TTL video 
and a 390R
to negative going TTL mixed sync and you would get 1v comp into 75R.

Rod