Having seen this fault on a phoenix pcb i repaired some time ago this will most likely be in the horizontal tv count circuitry,scoped the outputs on the 2 ls161's at location J2/H2 and they appeard ok.Followed these outputs to a ls86 and pin 11 output looked bad.
put in a new ls86 and .............................
looks as though that was the culprit,noticed there are blue specs all over the screen.Went over to the sprite line buffers with my scope and outputs were ok,looked at a ls273 location B7 and found a floating output on pin 16.
replaced the bad chip and now.............................
thats better! next job is to find the missing colours and this circuitry lies on the video pcb.scoped the inputs to the tile colour prom at location F4 and nearly all inputs were floating,these inputs to the prom comes from a ls174 at location B6.inputs to the ls174 seemed ok and clock pulse was present but outputs were floating,removed the chip and put in a new ls174 and powered on and..........................................
we have colour😌, next is to find the invisable bug and its critters. to be continued........................................................................🐞
put in a new ls86 and .............................
looks as though that was the culprit,noticed there are blue specs all over the screen.Went over to the sprite line buffers with my scope and outputs were ok,looked at a ls273 location B7 and found a floating output on pin 16. replaced the bad chip and now............................. thats better! next job is to find the missing colours and this circuitry lies on the video pcb.scoped the inputs to the tile colour prom at location F4 and nearly all inputs were floating,these inputs to the prom comes from a ls174 at location B6.inputs to the ls174 seemed ok and clock pulse was present but outputs were floating,removed the chip and put in a new ls174 and powered on and..........................................we have colour😌, next is to find the invisable bug and its critters. to be continued........................................................................🐞