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 Mon Apr 9 20:59:12 2012
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