Re: SEGA G80 - Star Trek Draw Problem..

I have a stack of these that all report u27 errors with known good rams, so
yes, I suspect that ram u27 is often a false positive with this hardware,
and there's actually something else upstream causing the issue.

If the lines are mostly at 45 degrees, check the connector between the two
boards. Try a different ribbon cable.
On Apr 9, 2012 9:57 PM, "Arcadius Stempak" <arcademanx@yahoo.com> wrote:

> Well, first of all ' Thank you ' I appreciate any and all reaponse
> regarding this ' odd ' problem....
> Ok here is what I did (including my results)...
>
> Verified Continuity between Pin 8 (2114 RAM) U27 and U31 Pin 8 and U46 Pin
> 4 - Continuity is ' Good '.
> Replaced LS244's at U48 and U49 - no change, problem still the same.
> Replaced LS374 at U13 - no change, problem still the same.
> Replaced LS245 at U14 - no change, problem still the same.
> Replaced all 6 Diodes (with new ones) - no change - problem still the same.
> Replaced LS139 at U46 - no change, problem still the same.
> Replaced U1 to U5 - again (since I had socketed these) - with no change.
>
> Nothing looks correct anywhere on the screen - random lines being drawn /
> blanked / drawn all over the place.
> (I call it repeated ' misdraw ' )
>
> So here is what I tried - out of curiousity - .
>
> Pulled Each Ram (2114) one at a time - then initiated CPU - Self Test (Red
> Button) -
> (As one would figure - CPU Always comes up (initially - OK - since it has
> its own RAM)... however....
> The end Result of Removing Each and Every Ram (2114) chip is that no
> matter what or which one is removed
> (including U27) the Test Always (even with All Ram chips removed) will
> begin to Fail and get Stuck at U27.
>
> It's as if this Control Card was ' stuck ' on U27 and ignores All other
> Faults, including its own Ram
> (that is unless the System is designed to Read - Ram - starting at U27? -
> first?)
>
> I also say the above, since even after pulling (for Test purposes)....
>
> *2114 (Ram) between U24 - U31 <--- with Memory removed there is no
> picture, but U27 still indicates Fault.*
> ***LS244's at U48 and U49,*
> *LS245 at U14,*
> *LS283 at U33, *
> *
> *
> At this point I am open to try just about anything, in order to try to
> get this thing going.
> * *
> * Only thing I did not do yet is hook up to a Scope.... but if anyone
> thinks that's a good idea I will do this.*
> *
> Thanks,*
> *
> *
> *Regards,*
> _Arcadius
> *
> *
> *
> *
> *From:* Douglas Gauck <douglas@gauck.com>
> *To:* vectorlist@vectorlist.org
> *Sent:* Monday, April 9, 2012 7:59 PM
> *Subject:* Re: VECTOR: SEGA G80 - Star Trek Draw Problem..
>
> You know all this from the schematics, but chip select is pin 8 for 2114
> RAM, and that's selected by the LS139 at U46. Should have continuity
> between U27 pin 8, U31 pin 8 and U46 pin 4.
> Of course it could be the LS244s at U48 and U49 that interface video RAM
> with the address bus, or the LS245 to the data bus, or maybe the LS374 you
> put into U13 is bad? Then you'd have more bad RAM (which it sounds like you
> do) that the self test may not have indicated because I bet it stops at the
> first error it encounters.
> But the faults you're getting now are with the diodes in place? Getting
> anything on screen that looks right? Difference between screen and scope?
>
> -DrG
>
> On Apr 9, 2012, at 11:04 AM, Arcadius Stempak wrote:
>
> All,
>
> Apologize for the slooooowww response, been busy ....
>
> Ok things have gotten a bit worse since my last update.....please read
> ..Thanks..
>
> So I went on to replace U13 - unfortunately still no change....
> Ok so I said why not pull D2 and D4 - (associated with Green Output -
> connecting to Pin 6 and Pin 8 of U1.
> And in fact in doing so I found one bad Diode (I forget which one - but
> one of then indicated a Reading in both directions).
> However this is where I made (my fault) a * Big * mistake (well from my
> perspective)
>
> Even though I was going to proceed with Replacing the 1N4148 - Diode -
> (which I should have just done)
> In my infinite wisdom (or lack of) I lifted one of the Diodes (Iegs) of D2
> and D4 and plugged in / powered on Control Board.
> Since that time, I now have the following Fault(s)....
>
> - Unit Powers up - OK - but Screen is now filled with Random / all over
> the place - Vectors being Drawn -
> - Control Card - continues to Fail - U27 Memory Chip
>
> If I press the ' SELF TEST ' Switch on the CPU - I get the following
> Results -
> - (CPU Led indicates One Long Flash - 2 short flahses- One long pause - 7
> short flashes - Repeat) -
> - I Replaced U27 - with no change -
> - I Replaced All Memory Chips - with no change -
> - I replaced U27 socket (which was not required - but I was desperate) -
> with no change -
>
> If I swap the Timing Card with a Known Good set of Timing/Control Cards (I
> have two sets) it works OK.
> If I swap the Control Card with a Known Good Set of Timing/Control Cards -
> same (U27) problem - (as listed above) -
>
> So the problem is isolated to the ' Control Card ' but I am not sure what
> could be causing a ' Mis-read ' of U27 ?
> Does anyone happen to know what (on the Control Card) controls Memory
> Access / Self Test / Verification?
>
> Thanks.... appreciate any and all input.....
>
> Regards,
> _Arcadius
>
>
>
>

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Received on Tue Apr 10 08:17:40 2012