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  • Joined: 14 Jun 2022
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sms down player 1 loop
Post Posted: Tue Jun 14, 2022 5:19 pm
Hi guys, I'm French and I'm coming to you because my master system 1 remains with the bottom activated
without cartridge or controller, the hang-on game passes all levels in a loop
I tested ports, links, capacitors and emi filter
nothing
I also redid the questionable welds
I changed the console to switchless 50/60 but it's not the first time I've done it, and the frequency change works correctly
just the down of player 1 running in a loop... any idea? ic4 sega 315-5216? the bios?
thank you in advance for your help
VID_20220611_211458.mp4 (6.73 MB)
video of down loop

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Post Posted: Tue Jun 14, 2022 9:10 pm
Hi @bruce7491!

You say it's like the down button is being pressed in controller 1 even without the controller plugged in, so my initial reaction would be to check for:

- A short somewhere between pin 2 of the controller port and gnd. You should be able to eliminate this by turning the power off to your master system and using a multimeter to test for continuity between pin 2 of controller port 1 and gnd. You can also test for continuity between pin 2 and all the other pins, there shouldn't be any.

- Some fault in IC4 which I believe should have an internal pull-up on pins 25-31 and 34-40. Again with a multimeter measure the resistance between each of those pins and Vcc (pin 32). The resistances should all be the same, I'm not sure exactly what but something of the order of 4-10k I'd guess. Pin 26 is the controller 1 Down button. If the resistance between pin 26 and pin 32 is substantially lower than that between the other pins and pin 32 then it might mean there's a fault in IC4.

- You might also want to confirm continuity between pin 2 of the controller port and pin 26 of IC4 (they should be at 0 ohms) but I honestly don't think that would be responsible for the symptoms you're seeing.

I initially wondered whether there could be a BIOS issue relating to port 3F but actually the down button shouldn't be affected at all by port 3F so I think it must be an electrical issue.
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Post Posted: Tue Jun 14, 2022 9:59 pm
hi will
thanks for all the leads...i'll give it a try...
I had already tested a possible defect between the pins of connector 1 and the ground without finding anything...
but i will do it all again
thank you
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Post Posted: Wed Jun 15, 2022 6:47 pm
hi
there is no short circuit between pin 2 of the connector and ground as well as in all the other pins of the connector
the continuity between pin 2 of the connector and pin 26 of IC4 is ok
there is infinite resistance between vcc 32 and all inputs of ic4
the strange thing is that at the beginning of my tests I measured twice a resistance of 12k between pin 26 and vcc then nothing... infinite

cheers
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Post Posted: Wed Jun 15, 2022 7:49 pm
Ah, okay, in that case sounds like the gatearray doesn't have passive pull-ups, sorry I don't have a working system to compare with, perhaps someone else does?

The fact that you did see some strange behaviour from pin 26 might suggest a fault with the chip.

If you're feeling particularly brave you could power up the system and this time carefully measure the voltage between those pins and gnd. If there are active pull-ups on those pins then they should all read +5v with no controller attached.
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Post Posted: Wed Jun 15, 2022 10:03 pm
let's try to take the measurements under voltage...
I communicate the result
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Post Posted: Fri Jun 17, 2022 12:06 pm
back on the tests ... everything is ok ....
voltage, welds, continuity, passive components...
I now suspect a problem between ic4 and the z80 via the data bus... the addressing bus on 3 pins also coming from the z80
common data bus between z80 ic4 rom ram....
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Post Posted: Fri Jun 17, 2022 12:26 pm
Good that everything seems to check out.

The only reason I didn't originally suggest a data bus problem is that you're not seeing any visual defects at all. Let's say you had a problem with one of the data bus signals (D1 I think in this case), you'd presumably find that all the tile and palette data (not to mention things like register settings) you sent to the VDP would be corrupted, which would almost certainly make your title screen look worse than it does.
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Post Posted: Fri Jun 17, 2022 1:02 pm
so maybe an internal fault in ic4...?
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Post Posted: Fri Jun 17, 2022 1:54 pm
Yeah it's definitely a possibility unfortunately.

I was really expecting you to find that there was a fault with the pull-up on pin 26 but oh well, our hunches aren't always right!

I suppose that IC4 is soldered directly to the board rather than in an IC socket, so it's not easy to take it out and do more tests?

Ideally you want to have exhausted all other possibilities before you think about the chip itself, but sounds to me like you've done all the testing of external components you can think of.

If you're reading +5V at pin 26 both with and without the controller connected then I can't imagine there's much more investigation on that input you can do. Possibly I'd wonder about trying to tie pin 26 manually to +5V (either with a jumper wire for a quick experiment without a controller attached or via a resistor as a potential permanent solution) whether that would make a difference, maybe the internal pull-up is only applying weakly, but seems unlikely.

You could test your theory about the data bus, by measuring the voltage of D1, that's pin 7, as the console's running and seeing if it's reading constant 0V. With a digital voltmeter it should be reading somewhere between 0V and +5V indicating fluctuating value (of course if you've got an oscilloscope you could actually see the value directly). But again, I think the data bus is unlikely because you're not seeing visual defects. You can also check continuity between pin 7 of IC4 and pin 15 of IC1 (the Z80).

Wonder if anyone else out there has any suggestions?

EDIT: thinking a little more, if you get to a point where you're thinking about removing the chip and trying to replace, before you do that, if I were you I'd try soldering the legs of a 10k resistor between pin 26 and pin 32 (Vcc) and just seeing if that makes a difference.
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Post Posted: Sat Jun 18, 2022 1:55 pm
small fix
under voltage it has 0v on pin 25 of ic4
all other pins are 5v
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Post Posted: Sat Jun 18, 2022 2:54 pm
bruce7491 wrote
small fix
under voltage it has 0v on pin 25 of ic4
all other pins are 5v


Aha!!!!

That makes a lot of difference! I just tried a copy of Hang On myself, and can confirm, what you are seeing with the Level going down 3..2..1..3..2..1.. etc. actually does mean that it's the UP button which has the problem, not the down button!

Okay, so you're measuring a constant 0V on pin 25 (the up button) even when the controller is disconnected?

If so, I think you should definitely try soldering a 10k resistor between pins 25 and 32. If it's just the active pull-up inside IC4 that's failed an external resistor will maybe do the job instead.

If that doesn't solve the problem then it means there's either a hard short or some problem deeper in the IC, and in that case I think you're looking at a new gate array chip unfortunately.
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Post Posted: Sat Jun 18, 2022 3:11 pm
yes the controller is disconnected
I will solder a resistor to see the result
I looked for a circuit course on the up player 1 line from the connector pin to the z80 via ic4 .... nothing
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Post Posted: Sat Jun 18, 2022 9:55 pm
Great ! a big thank you to you, adding the resistor fixed the problem
thanks again for your help
a saved sms!

https://www.youtube.com/watch?v=vnVLXdrEemY
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sms down player 1 loop
Post Posted: Sun Jun 19, 2022 3:37 am
Nice work.

Adding an external pull-up resistor to IC4 does not always fix this kind of fault depending on how the pin is faulty inside.
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Post Posted: Sun Jun 19, 2022 10:02 am
Hey, @bruce7491, amazing, so glad this sorted your problem!!

@async yeah felt like a bit of a punt but guess the pull-up must just have gone quite high Z or something, if it had been a short to gnd then obviously no amount of external pulling would have fixed. Felt like a low hanging fruit to try anyway before embarking on a desoldering mission!

Happy gaming bruce7491!
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Post Posted: Sun Jun 19, 2022 7:02 pm
thanks a lot

cheers

stef
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