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  • Joined: 21 Jan 2022
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Master System 2 Help
Post Posted: Fri Jan 21, 2022 1:03 am
Hi

First time poster but I have an issue that I was hoping to get some advice on.

I am having an issue with a master system 2 that i have just purchased. Basically, Button two is not working. Have tried and bought new controllers to rule that out. Continuity between pin 9 on the controller connecter is okay and pin 9 to resister R6 is also okay and leg 20 on the IC4 chip to R6 is fine. I have de-soldered and re-soldered the controller connecter just to ensure a good connection. Could the ic4 chip be dead? I really want to solve this for my master system 2.

I would really appreciate any help/advice.

Thanks.
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  • Joined: 14 Aug 2000
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  • Location: Adelaide, Australia
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Master System 2 Help
Post Posted: Sat Jan 22, 2022 1:47 am
From what you described it looks like that input is dead on IC4 like you said.

If you haven't already, maybe check the resistance between ground and those points you measured continuity for just to sure there is nothing external to IC4 pulling the circuit to ground.

If you have a Megadrive controller, you could see if that makes button 2 work. Because the Megadrive controller multiplexes it buttons, the button 2 input is driven high and low instead of just passively pulled low. I’ve seen it once before where a Megadrive controller made an dead input work. It’s a longshot, but good luck.
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  • Joined: 21 Jan 2022
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Post Posted: Sat Jan 22, 2022 11:21 pm
Thank you. Just great to have theories bounced off someone. I have a friend with an old mega drive so ill give that a try otherwise, I may just use this machine for parts and pick-up another unit.

Thank you very again, very appreciated!
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  • Joined: 05 Sep 2013
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Post Posted: Mon Jan 24, 2022 10:59 am
asynchronous wrote
If you have a Megadrive controller, you could see if that makes button 2 work. Because the Megadrive controller multiplexes it buttons, the button 2 input is driven high and low instead of just passively pulled low. I’ve seen it once before where a Megadrive controller made an dead input work. It’s a longshot, but good luck.


It's TH not TR (button 2) that gets driven high and low in a Megadrive controller.

If you get continuity and you don't have any shorts to ground from the pad port to the IC4 leg you might have a faulty chip, but still the short is the most likely option.
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  • Joined: 14 Aug 2000
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Master System 2 Help
Post Posted: Mon Jan 24, 2022 1:50 pm
@sverx I'm not talking about the pin which the console drives high and low. I'm talking about the pins on the console which the Megadrive controller drives high and low.

The 4 outputs of the 74HC157 inside the (3-button) Megadrive controller are connected to the Left, Right, Button1 and Button2 pins of the controller port on console.
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Post Posted: Mon Jan 24, 2022 1:56 pm
well I doubt it would work - if it can't read the status of an SMS controller button 2 it won't read status of an MD controller button C and start I suspect :|
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