Hi everyone, thanks for accepting me into your community.
A very short background, I'm UK based and restore vintage colour television from 1968-1988 and have a collection of 50+ sets. Many of the later sets having teletext. I also have a private museum where these are displayed. I built an analogue UHF distribution signals system that feeds all the sets, this includes patterns gens and two teletext systems, one based on VBit, the other on IMOGEN. Both teletext sources work perfectly, all capable TV's able to decode and display pages.
This leads on to my question and problem. As part of celebrating Teletext 50, I decided to restore a BBC model B and its accompanying Teletext adapter, to have it set up as part of a display along with other text sets and very rare early teletext set top decoders. The restore of the micro went well, both units appear to function as I would expect, i.e. software loads and run correctly, well all except one.
Connecting the telextext adapter to the UHF system worked perfectly, the box acquired a stable 100% signal. However, if only that were the end of it. Although for the most part teletext is working, I appear to have an odd issue which I'm struggling to tie down. I have what appears to be random page corruption, I've asked elsewhere, it was suggested to me there might be a video problem. I scoped the video output of the TDA2541 and into the SAA5030, it's as expected and stable. Also, it seems a little too specific for it to be a video fault (see attached examples below).
For example, take home index page 100, the corruption is just at the top portion. However, go to page 459, the world clock, it's OK, the same with page 200 Finance. This leads me to wonder if there is a fault in the teletext decoder. Perhaps acquisition and control, the SAA5030? Or maybe more likely the timing chain SAA5020, which handles the sync between teletext and the video signal?
I have tested without other ROMS installed, without the Watford expansion board, with just O/S, basic and the Teletext ROM in the outermost slot of the four original ROM slots. Same odd corruption scenario each time. So I'm left thinking, as stated above, I have a fault in the Teletext adapter, but where is the question. Other than this fault teletext is working, I can select pages numbers, all function keys work etc. I have included photos that hopefully give an idea of what I'm trying to explain. I have also included a ROM list etc
I'm new to the BBC micro but as detailed above have a background in vintage electronics, so any tips to point me in the right direction would be greatly appreciated.
p.s.
If this thread is in the wrong place, please accept my apologies and relocate accordingly.
A very short background, I'm UK based and restore vintage colour television from 1968-1988 and have a collection of 50+ sets. Many of the later sets having teletext. I also have a private museum where these are displayed. I built an analogue UHF distribution signals system that feeds all the sets, this includes patterns gens and two teletext systems, one based on VBit, the other on IMOGEN. Both teletext sources work perfectly, all capable TV's able to decode and display pages.
This leads on to my question and problem. As part of celebrating Teletext 50, I decided to restore a BBC model B and its accompanying Teletext adapter, to have it set up as part of a display along with other text sets and very rare early teletext set top decoders. The restore of the micro went well, both units appear to function as I would expect, i.e. software loads and run correctly, well all except one.
Connecting the telextext adapter to the UHF system worked perfectly, the box acquired a stable 100% signal. However, if only that were the end of it. Although for the most part teletext is working, I appear to have an odd issue which I'm struggling to tie down. I have what appears to be random page corruption, I've asked elsewhere, it was suggested to me there might be a video problem. I scoped the video output of the TDA2541 and into the SAA5030, it's as expected and stable. Also, it seems a little too specific for it to be a video fault (see attached examples below).
For example, take home index page 100, the corruption is just at the top portion. However, go to page 459, the world clock, it's OK, the same with page 200 Finance. This leads me to wonder if there is a fault in the teletext decoder. Perhaps acquisition and control, the SAA5030? Or maybe more likely the timing chain SAA5020, which handles the sync between teletext and the video signal?
I have tested without other ROMS installed, without the Watford expansion board, with just O/S, basic and the Teletext ROM in the outermost slot of the four original ROM slots. Same odd corruption scenario each time. So I'm left thinking, as stated above, I have a fault in the Teletext adapter, but where is the question. Other than this fault teletext is working, I can select pages numbers, all function keys work etc. I have included photos that hopefully give an idea of what I'm trying to explain. I have also included a ROM list etc
I'm new to the BBC micro but as detailed above have a background in vintage electronics, so any tips to point me in the right direction would be greatly appreciated.
p.s.
If this thread is in the wrong place, please accept my apologies and relocate accordingly.
Statistics: Posted by crustytv — Tue Oct 22, 2024 6:50 am