What chips can a Classic boot without?

[davidg5678]

Hi all,

I am troubleshooting a Macintosh Classic that has severe battery damage (probably a lost cause, but I like a good challenge). The AB, PSU, and CRT are all tested and known good.

This machine displays horizontal stripes at power up, which I understand means the VRAM is not getting initialized. I've confirmed the CPU is receiving a good clock signal. It is not held in /RESET or /HALT. I also confirmed that the address/data lines are all pretty active in this machine. I've patched more dead traces than I can count, and verified proper continuity for every pin on the CPU, VIA, RTC, ROM, SOUND, and VR1.

I'm sure the culprit is going to be a broken trace leading to a rotten VIA connected to a bad IC. The trouble is figuring out which ones and where!

In an effort to troubleshoot this problem, I'd like to start pulling ICs from the PCB. Anything that isn't needed for the Classic to start up can go --I'm okay if this results in a "sad mac" for the time being, my goal is to resolve these horizontal stripes first. I suspect that several chips are probably damaged, and that one of them is probably bringing the whole system down (--if this theory is nonsense, please let me know...)

Does anyone here know which chips aren't needed? Can I get away without some of the capacitors as well? What sort of start up/initialization behavior should I be expecting?

I'd like to confirm the operation of the sound chip as well, but couldn't find documentation on how it is supposed to function. Any pointers?

Any and all advice will be greatly appreciated! Thanks!

 

[cheesestraws]

At risk of telling you something you already know, but before you go about removing chips, have you checked that the power on reset is working? I've seen this symptom when no initial reset is being generated. You can test this by pressing the reset button manually when it's at the stripes, and seeing if that makes any change...

 

[davidg5678]

At risk of telling you something you already know, but before you go about removing chips, have you checked that the power on reset is working? I've seen this symptom when no initial reset is being generated. You can test this by pressing the reset button manually when it's at the stripes, and seeing if that makes any change...

This is far from a silly question! I just checked, and when I short the reset button's pads together, nothing seems to happen at all! Before I go about swapping the sound chip, is there any way to bypass the sound chip and traditional reset circuit entirely? I wonder if I can tap directly into the /RESET pin on the CPU and bring it low externally with a temporary troubleshooting bodge somehow?

 

[cheesestraws]

I wonder if I can tap directly into the /RESET pin on the CPU and bring it low externally with a temporary troubleshooting bodge somehow?

You could give it a go and see if anything happens! (Apologies, very late here and I don't have confidence in my ability to talk sense).

 

[davidg5678]

Update: I temporarily tied /RESET to GND using a wire soldered to the CPU pin on one end, and tapping the other end of the wire against GND (through a 10k resistor in series). This produced no changes. I might try again with the sound chip completely removed (after using this same setup to first pull /RESET high).

It is probably time to swap the sound chip with a different part. Getting the reset functionality to work will be an important part of getting the computer to work, so I will focus on this circuitry for now.

 

[zachlapidus]

Update: I temporarily tied /RESET to GND using a wire soldered to the CPU pin on one end, and tapping the other end of the wire against GND (through a 10k resistor in series). This produced no changes. I might try again with the sound chip completely removed (after using this same setup to first pull /RESET high).

It is probably time to swap the sound chip with a different part. Getting the reset functionality to work will be an important part of getting the computer to work, so I will focus on this circuitry for now.

I'm having a similar problem with the Classic I'm working on now, having no power on reset. Out of curiosity, if you're able to check, could you please tell me what voltage are you getting on pin 7 when you press the reset button? And what voltage across C3 on pin 8 when the reset button is pressed and not pressed?