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Thread: Help me diagnose a Sega Genesis Model 2

  1. #16
    Electronics Engineer Road Rasher db Electronics's Avatar
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    Quote Originally Posted by slivercr View Post
    @db Electronics: The nTIME signal you refer to is the one going into PIN75 of the ASIC I am guessing? Ill check for continuity and try bridging it if necessary. I doubt this is the problem though, since it doesn't explain the SegaCD's faulty sound when it boots up. Also, out of curiosity since you seem to be well versed, do you know how to probe test pins 81-84 of the ASIC (and what to expect back)? Or what pin 58 (labelled SOUND) does?
    Can you link the schematic you are referring to? My model 2's are both "PC BD MD2 VA0 USA", I'm not sure how many differences there are between VA0 and your VA1.8. I have VA0 and VA1 schematics, not the VA1.8 .However the pins you'r describing seem to match my schematic. I have ZERO clue about the 4 test pins or the sound pin, they may be JTAG or something similar to test the VDP but anyone's guess is as good as mine.

    You're right that this doesn't really explain your sound issues; you may have two separate issues on yours hands. Here's a schematic someone traced of how nTIME is used on the S&K cart. They only labelled it as Pin31?, I guess they didn't know what it was.
    http://www.emulationzone.org/project...atics_full.gif

    For sound, since you've replaced caps already, the best I can recommend is to troubleshoot the LM324 opamps (IC9 and IC10 on VA1 schematic), do you have an oscilloscope? I would simply follow the audio signals through each component until I find something odd. The CD audio (SL2 & SR2) gets mixed with the Genesis audio in IC10. Actually there's quite a bit of mixing which occurs here. If you have a 32X it would be interesting to see if it's audio (SL1 & SR1 through the cartridge port) sounds distorted when output through the Sega CD RCA jacks.

    Quote Originally Posted by KillerBean2 View Post
    I beg to differ. The contacts might still be the culprit, even if they are clean. Have you made sure that none of them have been bent out of shape? After all it's a simple matter of metal touching metal. If there's just the slightest bit of air between the two, the contact fails.
    If dirty contacts were an issue, seems to me like ALL games above the 16Mb (i.e. using Address[23]) boundary would not boot (probably due to the checksum being wrong). However, silvercr says all games under 16Mb always boot (therefore Address[22 to 1] are making good contact) and that only some games larger than 16Mb don't boot - coincidentally it seems to be only games that use the nTIME signal to activate different features such as a mapper or save file. I would be very surprised to find selective dirt or contact bending depending on the nTIME usage of a game.

    EDIT: Sorry my mistake, most significant bit of address for games is Address[21], not Address[23]. Don't know why I made that mistake
    Last edited by db Electronics; 03-16-2014 at 11:32 AM. Reason: spelling, sorry I'm french!
    www.db-elec.com
    Code:
    do {
        Genesis();
    } while (Nintendont());

  2. #17
    Master of Shinobi MaxWar's Avatar
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    Quote Originally Posted by db Electronics View Post
    If dirty contacts were an issue, seems to me like ALL games above the 16Mb (i.e. using Address[23]) boundary would not boot (probably due to the checksum being wrong). However, silvercr says all games under 16Mb always boot (therefore Address[22 to 1] are making good contact) and that only some games larger than 16Mb don't boot - coincidentally it seems to be only games that use the nTIME signal to activate different features such as a mapper or save file. I would be very surprised to find selective dirt or contact bending depending on the nTIME usage of a game.
    When it comes to cart contacts, the PCB thickness can also have an influence. Thicker cart PCB will result in more pressure on the contacts.

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    @db Electronics: Im using schematics for VA1. As far as I can tell the only difference between that and VA1.8 is in the version of the ASIC. I checked continuity of B31 with PIN75 and it was perfect so no luck with your theory EDIT: I got schematics from here

    I then reflowed all solder joints of the cart slot, still no luck. Same problems going on.

    For the time being Im sticking with my theory of the FM Synth (or something related to it) being the culprit

  4. #19
    Electronics Engineer Road Rasher db Electronics's Avatar
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    Quote Originally Posted by slivercr View Post
    @db Electronics: Im using schematics for VA1. As far as I can tell the only difference between that and VA1.8 is in the version of the ASIC. I checked continuity of B31 with PIN75 and it was perfect so no luck with your theory EDIT: I got schematics from here

    I then reflowed all solder joints of the cart slot, still no luck. Same problems going on.

    For the time being Im sticking with my theory of the FM Synth (or something related to it) being the culprit
    Continuity won't tell you much. Ideally you want to have a look at it with an oscilloscope on the culprit games.
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    Code:
    do {
        Genesis();
    } while (Nintendont());

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    That'll have to wait until Monday then, when I have access to the lab.

  6. #21
    Electronics Engineer Road Rasher db Electronics's Avatar
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    I experimented a bit with my VA0 and Sonic 3 by putting a scope on the nTIME signal. Seems that every time I put my scope on it and powered on the console I would lose my save data. Be careful!
    www.db-elec.com
    Code:
    do {
        Genesis();
    } while (Nintendont());

  7. #22
    Master of Shinobi MaxWar's Avatar
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    Quote Originally Posted by db Electronics View Post
    I experimented a bit with my VA0 and Sonic 3 by putting a scope on the nTIME signal. Seems that every time I put my scope on it and powered on the console I would lose my save data. Be careful!
    Is your probe on high resistance setting?

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    I'd say the probe has high capacitance and wrecks the signal edges. Bigger games not working means A20 or A21 are bad. A22 and A23 aren't used except by the 32X (and the other MD components onboard)

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    Electronics Engineer Road Rasher db Electronics's Avatar
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    Quote Originally Posted by Jorge Nuno View Post
    I'd say the probe has high capacitance and wrecks the signal edges. Bigger games not working means A20 or A21 are bad. A22 and A23 aren't used except by the 32X (and the other MD components onboard)
    I know it's A21 or A20, stupid me . Somehow when I was typing I was thinking most significant bits (of address lines for games) but still managed to put down A23 instead of A21 - A21 is clearly the bit which changes from '0' to '1' when crossing the 16Mb range...
    www.db-elec.com
    Code:
    do {
        Genesis();
    } while (Nintendont());

  10. #25
    Take it apart! WCPO Agent MEGADRIVE Jeroi's Avatar
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    I would try to test with multimeter if asic is dead. Turn game on and watch asic pin with volt meter. Does it stay GND all the time. If so the asic transistor is broken and shorted. Ofcourse continuety in this is useless as you will see continuety from asic pin to cardridget slot connector.
    #MEGADRIVEJeroi @ Quakenet irc server.
    Be true to yourself. GFX doesn't matter, the game does. If you are intrested to donate one NTSC Genesis for hardware testing purposes, please pm me.

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