Fault: lines through some of the sprites and part of the dragon on the title screen. This fault occured about 2 minutes after the first power up, thereafter about 5-10 seconds after each power up when 'warm'.
Seems to be a common fault with these bootlegs in the sprite generation circuit,many slightly different variations can occur.
This fault is on the bottom/video board,and I'd learnt from previous repairs that there are 2 identical circuits involving a long thin 6116 ram a ls273 and 2 ls257(on this PCB). Normally if the ram has gone the garage door has lines that allow you to partially see the car through it when it's closed. This however didn't and some of the sprites were actually still complete which I hadn't seen before, which to my mind also ruled out the ls257's.
So this really left the ls273's, which I had a probe about on. To my surprise I found that on 1 of them as the fault occured after a warm power up 8Q was going hi causing 8D to go hi and pin 14 on it's corresponding 6116 ram also. Seemed weird so not being one to desolder chips easily I cut pin 14 on the ram and tried another power up and same thing apart from pin 14 on ram which I'd cut. Realising that pin 14 on the ram was connected to 8D on the ls 273 anyway I cut that and reattached pin 14 on the ram. This time I powered up and pin 14 on the ram stayed low but 8Q still went high??? Wow, ls273 breaking down internally? Replaced this and fault cleared.
Sorry no pics didn't think about it until after I'd changed the ls273.
Got another one to do with no sound at all but the amps working,rather hoping it's going to be an ls138.
Seems to be a common fault with these bootlegs in the sprite generation circuit,many slightly different variations can occur.
This fault is on the bottom/video board,and I'd learnt from previous repairs that there are 2 identical circuits involving a long thin 6116 ram a ls273 and 2 ls257(on this PCB). Normally if the ram has gone the garage door has lines that allow you to partially see the car through it when it's closed. This however didn't and some of the sprites were actually still complete which I hadn't seen before, which to my mind also ruled out the ls257's.
So this really left the ls273's, which I had a probe about on. To my surprise I found that on 1 of them as the fault occured after a warm power up 8Q was going hi causing 8D to go hi and pin 14 on it's corresponding 6116 ram also. Seemed weird so not being one to desolder chips easily I cut pin 14 on the ram and tried another power up and same thing apart from pin 14 on ram which I'd cut. Realising that pin 14 on the ram was connected to 8D on the ls 273 anyway I cut that and reattached pin 14 on the ram. This time I powered up and pin 14 on the ram stayed low but 8Q still went high??? Wow, ls273 breaking down internally? Replaced this and fault cleared.
Sorry no pics didn't think about it until after I'd changed the ls273.
Got another one to do with no sound at all but the amps working,rather hoping it's going to be an ls138.