Skee-Ball Model S - help needed (SOLVED!)

[MrGorf]

Can you take a picture of the power supply part of the board. I'm a little confused.
Looking at the schematic posted, it shows pin 1 as input on both regulators, pin 2 as output, and pin 3 as common.
According to the data sheets of both regulators, pin 2 is ground and pin 3 is output.
Just want to make sure they are wired correctly or was it an error in the schematic.

 

[Lescell]

Sure, here they are. You are right, that is strange how they numbered them like that..

 

[MrGorf]

Would you be able to remove the output pin of the 5V regulator and measure the voltage unloaded?

 

[Lescell]

Unloaded 5V output of U17..

R32

Voltage IN = 16.2
Voltage OUT = 16.2

U17

Voltage IN = 16.2
Voltage OUT = 5.0

U16

Voltage IN = 16.0
Voltage OUT = 15.0

 

[MrGorf]

So it sounds like with no load the 5V is fine. You said you removed the chips one at a time and the 5V was still loaded down.
Are all of the chips inserted correctly? There is a notch at the top of each chip, and that should correspond with the notch at the top of the socket.
What happens if you remove ALL of the chips. Is the 5V still low?
Just trying to see what is loading down the 5V line.

 

[Lescell]

Yes, all chips were verified to be correctly installed. I then removed one at a time until all socketed chips were removed and the 5V was still loaded down.

 

[MrGorf]

The only other thing I can think of is to check the small (0,1uF?) caps at each chip. They usually go between +5 and GND, so if one is shorted that could cause your loading problem.

 

[JakesArcade]

+1 on the bypass caps, pull them all and see if the voltage comes up, if it does replace them all. I have a gameplan mpu-2 that I just replaced far too many chips before realizing the problem was with the caps pulling the voltage down on the ram chips causing the entire data bus to go all wonky.

 

[Lescell]

For the bypass caps, are these the caps with a Cxx number by them, or the ones that have CX or CY next them?

 

[MrGorf]

 

[Lescell]

All bypass caps removed (x9) along with chips U2, U5, U6, U8, U9, U12, U14. No change in voltage, still dropping to 2.8VDC at the output leg of U17.

 

[JakesArcade]

In that case there isn't much left, I'll review the schematic tonight but I would try and just leave it on, see what chip or part gets really hot.

 

[Lescell]

There are 2 components that get super hot, U17 and R32 (5V section). Does the fact that U17 staying cool when the output leg is disconnected, tell us that U17 isn't the issue? Or could U17 still be the problem?

 

[JakesArcade]

If you have a spare 7805 it wouldn't hurt to try and replace it, however it's to be expected that both of those components get really hot in the state that it's in right now.

The way a 7805 works (broadly) is that it converts any voltage above 5v into heat, so say you're drawing 1A at 5v and the input of the 7805 is 10v, it would convert the 5v drop into 5W of heat. The resistor is doing something similar, where its job is to drop some of the voltage before the regulator to put less work on the 7805.

Are you sure nothing else is getting hot? Or even past mildly warm?

What I would do now is check 5v and ground at each chip remaining on the board, since it seems like a decent amount of current is flowing through that resistor/regulator you might be able to see a noticeable voltage drop the closer you get to the short. if the whole board more or less has the same voltage I suspect something very close to the regulator is having issues.

If that does not show anything carefully inspect the board top to bottom looking for any potential shorts from things like bad solder joints or metallic debris shorting some pins.

It also would be pretty easy to pull D29, see if that has any effect.

 

[Lescell]

Looks like D29 might be bad. Pulled it out of circuit and tested, it is reading .003V in both directions.

 

[JakesArcade]

bingo, with it removed are you getting 5V? It should be safe to power up without it. Looks like it's just there to protect any potential over voltage from a back feed.

 

[Lescell]

Yes, 5V at the output pin! Now, is 14V too much for input voltage? The regulator is still getting hot, but maybe that's normal hence the heat sink attached to it. R32 is dropping the voltage a bit, but not a ton. Just making sure it's normal.

 

[JakesArcade]

Yes the regulator and resistor getting hot is normal, that's just them burning off the excess power as heat to regulate it to 5v. once you reinstall bypass caps and chips I would suspect the voltage to drop a bit more on the resistor and for that to get hotter.

Note I would not connect this to the machine until you order a replacement diode, it's most likely there to shunt any back feeding that may happen.

I quickly looked through the manual but didn't see the parts list, perhaps someone else can chime in if you can't find the correct replacement part.

 

[JakesArcade]

14v is normal, looking at the schematic the transformer is feeding both the 12v and 5v regulators.

 

[Lescell]

Thanks for all the help, I have connected all chips and bypass caps, everything seems ok. The diode is a 1N5908 that I should be able to source.

Once again, thanks a bunch for your knowledge!