Please help me resurrect a dead 512k

[tattar8]

I'm currently trying to resurrect a dead 512k that until this morning gave a good chime and displayed a Sad Mac with vertical stripes.  However, after replacing one of the LS244 buffers, the chime is now distorted, as are the visible parts of the screen.  I put back the original LS244, but there's now no change at all.  All the RAM has been replaced, as have both LS166s. Does anyone have any ideas?

 

[tattar8]

Update:  I cleaned the board with alcohol and the distortion disappeared.  I then put the replacement chip back in and the distortion was back, but this time cleaning the board did not help.  Is there something weird about the buffers on this board that prevents me from just replacing them with modern TI SN74LS244N chips?  Or could it be because I only replaced one, rather than both?

 

[Dog Cow]

A clear chime + Sad Mac could be either a failed ROM checksum test, or a failed memory test. But the fact that you hear the chime at all means that enough logic is working that the CPU is able to execute the instructions from ROM to make that chime and then later on show a Sad Mac on the screen.

I suspect a fault in address lines, or something in that route between the CPU and the DRAMs, and the Sad Mac is a result of failing the memory tests. I bet if you take a look at the boot beep subroutine in ROM you'll find that it uses some area of RAM, at least the stack, and that's what's causing the distortion.

 

[tattar8]

Turns out the distortion was because one of my jumped traces (from messing up one of the RAM chip replacements) wasn't soldered very well.  As for the Sad Mac and stripes, it was because the CAS0 line between the TSM and RP3 was broken somewhere.  I had tested continuity from each chip to the resistor pack, but not from the resistor pack to the TSM.

 

[Dog Cow]

Nice. Thanks for the followup.