[Techtoolslist] Atari Superman / Hercules Repair ~ Ram Read Fail

Tue Jul 1 08:05:08 EDT 2014

(This is a cut and paste, image heavy, original post here:
http://www.villagebbs.com/forum/index.php/topic,24850.msg205478.html#msg2054
78 )

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Symptoms were this: 

Didn't work , kept watchdogging... Hooked up Fluke, tried to do ram tests
failed., then did usual, use the logic probe test all the pins, everything
tested fine.

So, I removed all the 2101/9101/5101 ram, and just put sockets in and put
new ram in...

OK, let us test again.. Fail.. ERRR..

So, Manually write Address 0000, Read it, Pass, Try again, Pass, Tray again
pass, do a ram test at 0000, fail.. 

Scratch head some more.

OK, lets test some other things, maybe something else will make since, Test
ROM, Passes, flying colors, no issues... 

ok, let's make the LED(s) Light up ( D1/2/3/4 ) , write 0x1800 , 0x0f,
Led(s) come on, Write 0x00, leds still on, hit repeat, lights go off.. Huh?
Repeat rince.. Something Odd going on with /WRITE and /READ so poke around,
all the logic chips test fine... 

OK, time to pull out the big gun, ( see pictures above )

Signal I am looking for is in "RED" box.

http://coinopflorida.com/tech/atari_superman/3.PASS_1.JPG

Keep hitting "Repeat" ( Ie , Repeat Last operation on Fluke )

Bam! the /AUDWR0 did not go low.

http://coinopflorida.com/tech/atari_superman/4.PASS_2_F.JPG

Hmm... So, its very random.. 1 out of 5 write attempts pretty much always
fails.. But all the logic is correct.

So, Going farther back in the circuit i was not suspecting something to do
with Phase1 and Phase2 of the clock as you can see in the above picture it
changes.

Turns out on this board, Whenever "ROM" is being access, the CPU clock is at
1MHZ, everything else is 667Khz ( because the ram is slower and I/O is
slower ) 

So, i decided to check the Clock Signal in "High Speed" and "Low Speed"

High Speed: Nice Clean 1Mhz Signal

http://coinopflorida.com/tech/atari_superman/5.CLK_1MHZ.JPG

Low Speed: Hmm.. That doesn't look right, its 667Khz but the wave form is
all wacky.

http://coinopflorida.com/tech/atari_superman/6.CLK_667KHZ.JPG

So, the only part of the system that actually makes that clock signal is a
74ls160, so i remove it to test...

http://coinopflorida.com/tech/atari_superman/7.%2074LS160.JPG

Well, what do ya know, it fails...

Pop in a replacement, and give it a shot.

http://coinopflorida.com/tech/atari_superman/8.PASS_3_OK.JPG

yahoo!!! nice good signal, every write attempt passes, also all the signal
looks nice too, no shifting, exact same everytime.

So, lets try the ram test again :)

http://coinopflorida.com/tech/atari_superman/9.%20RAM_PASS.JPG

I then proceeded to swap out all the defective dipswitches, and test as much
of the hardware as i could without the machine, now off to the owner and
pray it works 

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