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PostPosted: Thu Sep 28, 2017 6:00 am 
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Joined: Sun Jun 12, 2011 12:06 pm
Posts: 254
Location: Poland
I had this 350-in-1 multicart:
Image Image Image Image Image

It does not work at all in most of my famiclones, except one, on which it starts without problems on 8/10 powerups, on 1/10 it has glitched menu graphics and on 1/10 some random game is started instead of menu.

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I suspected reset circuit to be cause of problems and examined signals on them - indeed, reset, fed to latches is oscillating (version with 74LS02). I replaced the gate with different LS02 chip but same result - and I tested both gates on logic tester and they are OK. The interval of oscillations is 50 Hz (20ms) so probably some noise from mains.
Image

Then I replaced LS02 with HC02 and it works fine:
Image

Any idea what is cause of troubles? Does thus LS gate have some kind of retardance when sourcing surrent?


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PostPosted: Thu Sep 28, 2017 8:24 am 
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Joined: Wed Apr 07, 2010 1:14 am
Posts: 484
Location: Iran
The diode seems to be inverted.
This works perfect for me :


Attachments:
reset schematic.PNG
reset schematic.PNG [ 1.15 KiB | Viewed 263 times ]
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PostPosted: Thu Sep 28, 2017 10:08 am 
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Joined: Sun Apr 13, 2008 11:12 am
Posts: 6503
Location: Seattle
krzysiobal wrote:
Any idea what is cause of troubles? Does thus LS gate have some kind of retardance when sourcing surrent?
74LS (also letterless, F, S, L, ALS, a few others) logic is current mode; inputs have to source current in order for an output to be able to signal a logic "0". For 74LS parts, that's up to 400µA.

I have to guess that the external 2.2kΩ resistor in parallel with the 74LS input pullup was just barely enough to change the time constant to something too short to work consistently. Removing the 2.2kΩ resistor ought to be equally functional as replacing the part with a 74HC part.

(edit: I have to admit that I'm more than a little surprised that the difference between 2300µA (5V/2.2kΩ) and 2700µA would be enough to make a difference)


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