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Re: Andy Rea's original ULA design blog/schematic.
Posted: Fri Oct 21, 2022 3:15 pm
by Moggy
Having been in recent touch with Andy I can confirm what Paul has stated that Andy no longer takes part in the zeddy community, sadly.
Re: Andy Rea's original ULA design blog/schematic.
Posted: Tue Nov 01, 2022 8:54 pm
by DaveR
Does anyone know why the video output mixer/combiner uses 160 and 750 ohms resistors?
I tried 4 different TVs and a composite video to HDMI convertor box and could not get a stable picture.
I changed the resistors to 470 and 1k ohms - as used on the Arduino (and probably other!) composite video tutorials - I now have a perfectly stable picture.
Thanks,
Dave
Re: Andy Rea's original ULA design blog/schematic.
Posted: Tue Nov 01, 2022 10:05 pm
by 1024MAK
Maybe those values worked okay with the display that Andy used.
Each different TV or monitor is slightly different, so what works fine on one may not work okay on another. Especially as the ZX81 can never produce a broadcast standard signal.
Mark
Re: Andy Rea's original ULA design blog/schematic.
Posted: Tue Nov 01, 2022 10:53 pm
by DaveR
Maybe I have just been unlucky - but also surprised that several google searches have found no-one else with the same issue.
My photo of the board has a date stamp of August 2018 but I have only recently been motivated enough to find a fix.
I also had to blow a new EEPROM using an Arduino to get the machine running. Would have been easier with a Mega but only had an Uno at the time. But it seems to be working fine now.
Regards,
Dave