Hello All,
I am currently trying to repair an old ADDS 4000 CRT terminal. It powers on, but there is a consistent hissing sound coming from somewhere around the analog part of the board and no picture. So far I have removed, cleaned, and socketed one chip (MC74F38N) that had visible corrosion/residue surrounding many of the pins and cleaned the whole PCB (it was very dusty). I have also replaced all capacitors that were visibly leaking, and reflowed all the solder joints on the flyback. I do have all the necessary electrolytic caps in case I should need to replace all of them. The terminal showed no change after doing these things.
The board has several bodges that were already there when I got it. I am not sure if these were done at the factory or if someone did them later. It appears to have been powered on for a long time as there is some serious screen burn on the tube and there are several brown spots on the board around power resistors. The heater on the tube appears to glow, and the 5V logic seems to come up as well (it beeps after a few seconds). I also disconnected the deflection and focusing coils and ruled them out as the source of the hissing noise.
I suspect that the flyback transformer is bad, however it is hard to pinpoint where the sound is coming from and I cannot see any visible arcing, even in the dark. The flyback has no visible cracking or bulging. I have not done any live probing yet over saftey concerns, but I have discovered that the horizontal output transistor (Sanyo C3175 NPN) is shorted across pins 1 & 3 which I believe are the base and emitter, which could suggest a shorted primary in the flyback that took the transistor down with it. During a few powerons, however, I did notice that static was building up on the front of the screen, so the flyback could still be working to some extent.
I have also noticed there is 15
across the 5V rail for the terminal logic, however as I said I have not done any measurements of this voltage while it was on.
There is very little information about this terminal online, and even less about this specific flyback transformer which says it's a 52500027 / 350-09800. I found one website which still sells them:
https://www.donberg.ie/catalogue/line_output_transformers/hr_diemen_line_output_traformers/hr_42079.html however the minimum order amount is 50 euros so I would rather not buy one if I don't have to.
Any help on where to go from here or how to further debug this safely would be greatly appreciated.