Sega Turbo shows all 8's in score display??

Robran68

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I finally got this thing up and running but the sound AMP was out. I installed an aftermarket sound AMP and have sound now but now the scores all show 8's! I don't see how the AMP board has anything to do with the LED's but not sure. Anybody ever have this issue? The scores were working yesterday so they were working. Also I have three functional boardsets and all three have the same issue.
 

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Update, took one of the short ribbon cables of a boardstack and attached the LED board with one of those and no change. Tried isolating the CPU board from the video and sound down to connector A and D only...still no change. Did a continuity check on the wiring and all looked good. I'm just about out of ideas...anybody have any other suggestions? :confused:
 
I looked at the schematic for ya. The score display is trivial; just 7-seg and all the drivers are on the CPU board. Assuming you were only touching that hacked-up sound wiring then:

- Recheck +8 unreg that might feed into CN6? Blown fuse or no power?
- Recheck the others: +5v, +12v, -12v. Bet they're working if the rest of the game is
- Check the base of the drive transistors on CPU board (sheet 5 of 7).

I was guessing the 8v is missing so the base is 0v and the segments are all driven.
 
I looked at the schematic for ya. The score display is trivial; just 7-seg and all the drivers are on the CPU board. Assuming you were only touching that hacked-up sound wiring then:

- Recheck +8 unreg that might feed into CN6? Blown fuse or no power?
- Recheck the others: +5v, +12v, -12v. Bet they're working if the rest of the game is
- Check the base of the drive transistors on CPU board (sheet 5 of 7).

I was guessing the 8v is missing so the base is 0v and the segments are all driven.

Thanx Joey, I will check for the 8 volts and double check the transistors.
 
Just to clarify, no reason to believe every transistor failed simultaneously. What I mean is check the voltage on the cpu board feeding the base of those transistors (refer to schematic).
 
Just to clarify, no reason to believe every transistor failed simultaneously. What I mean is check the voltage on the cpu board feeding the base of those transistors (refer to schematic).

Not that good at reading schematics but from what I see you're talking about the 8v and -+ 12v lines.

I followed the wires back to a very strange looking transistor, its number on the schematic is diode bridge 4BIOY (Nihon Inter). It feeds the +8v unreg to the CPU board, maybe that's the issue??
 
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So one tap (~10v AC) of the transformer is fused (4A) and feeds to a bridge rectifier (the thing you didn't recognize) which converts the AC to rough DC (~8v DC). Easier to understand with this picture on wikipedia. Typically there's also a capacitor to smooth things out but I don't see it on the wiring diagram or schematic.

No guessing if it's the issue -- use your DMM, measure, and trace the circuit forward. Check:

- For 10vAC off the transformer
- Continuity across the fuse
- For ~8vDC past the bridge rectifier
- For ~8vDC reaching the gameboard (see sheet 5 of 7)

That'll prove my guess correct or wrong.
 
Ok, just thought I'd update this thread being I resolved this issue. Ended up hooking the +8v for the "D" connector on the CPU board straight to the switcher +5v(2 Orange) and GND (2 Wht\Blk). Now the displays work perfectly. :)
 
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