Fixing 32 chip Atari RAM problems

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PhilC
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Fixing 32 chip Atari RAM problems

Post by PhilC » Mon Jun 17, 2019 5:40 pm

Hi All,

I wanted to document a few things whilst diagnosing and repairing a C070789-001 REV.D1 board

This board would not boot but would show the typical white border with black background on the screen, which is usually an indication of a ram failure.

Doing things the old old fashioned way would result in you tracking down which ram chip it was by either trial and error or having to scope out all the address lines, then working out which chip is on which line etc etc.

So to track the problem down I used the V4.4 Diag cart, which when booting an ST with a ram problem outputs useful text to the serial port (presuming that works of course!) Below is an example of the kind of output that you can expect from the diag cart.
ramdiag.PNG
ramdiag.PNG (1.93 KiB) Viewed 444 times
What this essentially means is that one or more Ram chip wasn't working correctly. In this case it was U39. I simulated this in this case by removing the chip to create the error. The difficulty as a novice (which included me before I learned how to diagnose the output earlier) is to know which chip is failing.

So to interpret the Diagrom output you have to look at all the 1s and 0s and learn how to associate them to the actual chips on the motherboard. And this (presuming I get it right) is how to do that. :-)

I1 Ram/data line error 0000000000000000 0000001000000000 000008

In the case above "I1 Ram/data line error" is the diagnostic code which you can look up in the service manual for the ST. Basically its telling you that it has detected a ram fault whilst checking the ram on bootup of the Diag Cart.

The first 16 O's or 1's relate to the memory in region 513 - 1024kb and the second set of 16 0's or 1's relate to the lower banks of ram, ie 0 - 512K. To break down the digits even further, the first 8 0's or 1's are the left 8 chips of each bank of ram and the last 8 are the right set of 8 chips of Ram.

So in the example above, we can see that there are no 1's in the first 16 binary bits, so we can assume that the upper bank of Ram is ok. Next looking at the second set of bits, we can see that the first 6 are 0's but the 7th is a 1. This is indicating that in the first 8 chips of the lower bank of Ram that there is a fault, specifically chip 7 in this case.

Now it's just a case of working out which actual chip number 7 is and of course changing it. So please excuse the rather crappy text diagram below, as I'm far too lazy at this moment in time to do a graphic representation of the bits (that will follow a little later on.)

This is how the actual chips are positioned in the motherboard as you look at it from the front.

Lower bank (second 16 bits) Upper bank (first 16 bits)
bit chip bit chip bit chip bit chip
1 U3 9 U4 1 U5 9 U6
2 U10 10 U11 2 U12 10 U13
3 U18 11 U19 3 U20 11 U21
4 U22 12 U23 4 U24 12 U25
5 U27 13 U28 5 U29 13 U30
6 U34 14 U35 6 U36 14 U37
7 U39 15 U40 7 U41 15 U42
8 U44 16 U45 8 U46 16 U47
If it ain't broke, test it to Destruction.

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PhilC
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Re: Fixing 32 chip Atari RAM problems

Post by PhilC » Mon Jun 17, 2019 5:41 pm

Photo of the actual chips on the board, and in my case bit 7 or U39 was faulty.
20190617_174032.jpg
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If it ain't broke, test it to Destruction.

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frank.lukas
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Re: Fixing 32 chip Atari RAM problems

Post by frank.lukas » Mon Jun 17, 2019 7:17 pm

D9 was faulty ...

The Ramchip conected to D9 on the Shifter.


I1 Ram/data line error 0000000000000000 0000001000000000 000008

I1 Ram/data line error 0000000000000000 0(D15)000001000000000(D0) 000008

on the right side it is D0, at the left it is D15

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PhilC
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Re: Fixing 32 chip Atari RAM problems

Post by PhilC » Mon Jun 17, 2019 7:29 pm

Or D9 as you say. I was trying to relate it directly to actual chips. I know this will be different on some boards but once you know where and how to look :D
If it ain't broke, test it to Destruction.

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