Apple IIc horizontal lines on boot

[AndyDiags]

I'm troubleshooting this Apple IIc which shows horizontal lines on boot with no sound:

It got into this state because a capacitor on the floppy drive went bad and shorted the +12v to Ground. By the time I found the culprit, the +12v did some damage on the main board, and need help troubleshooting what's damaged.

First thing I checked was the RAM. It seems that 4 of the ram chips bad. Probing D(in) pin 2, 4 of the 16 ram chips seem to be pulled low. Thermal check shows that the four RAM chip are running at a higher temperature than the others, so I've ordered replacements for these.

However, I don't think that's the only issue here. I get the same video output with and without the CPU/ROM. Looking at thermal, the CPU and ROM give off no heat. I've tested the ROM separately and confirmed it works. My guess is that the CPU drives reading of the ROM so since CPU has no activity, neither with the ROM. I checked and CPU gets 5V in. The lines BE (Bus Enable), RDY (Ready), and RESB (Reset) are all high as expected. My guess is either the CPU is bad or maybe oscillator is bad. I tried to check the clock input at PHI2 and not sure if it looks good, I thought voltage would be higher (image below). I don't have a good reference to see what I should expect, my oscilloscope is very cheap so I don't fully trust it. (DS0138mini). I’ve ordered one of these CPUs in case it is bad.

The power supply +12, -12, and +5 look good, so no worries there.

I’ll keep the thread updated on the progress, as it will take a while for chips to arrive. If anyone has any other suggestions please let me know.

 

[AndyDiags]

Small update.

I received a 2nd working IIc. First interesting finding is that when I turn on the working IIc, the CPU and ROM stay cold! The CPU being this cool is expected it seems, as no instructions are being fed. I've also swapped the CPUs and both work fine.

When I remove the CPU on the working machine and turn it on, the UI output is pretty similar.

This leads me to believe that the RAM is the only item at fault here. Still waiting for the replacement RAM and sockets to arrive.

 

[retr01]

This will be fun to see how it turns out! Rooting for you, @AndyDiags!

 

[AndyDiags]

I've been waiting for over a month for the replacement ram I purchased on eBay to arrive, but it hasn't yet.

However my new DRAM tester I recently purchased arrived. I decided to pop the ram which I expected to be bad, but to my surprise the RAM ICs are good!

This would mean the problem lays elsewhere. I first marked all the ICs which are directly connected to the ram by referencing the schematics, and there are several!

I previously checked the socketed ICs, so I started with the 74LS374 and 74LS245 logic ICs since they can be tested with a TL866II programmer, and they passed.

I then pulled the SER1, MAP and finally the IWM chip. Once I pulled the IWM, the Din/Dout on the for RAM ICs finally returned to normal.

Connecting the IIc without a IWM chip, I got the following interesting video. Much better than the previous horizontal lines:

The culprit ended up being the IWM chip, which makes sense as it's one of only a few ICs that is connected to the 12v rail where the short originally occurred.

I had a spare Mac Plus logic board laying around, I pulled the IWM chip (344-0041-B) from that board and placed it in this IIc machine.
The IIc originally had a 344-0041-A, but I assume the Rev B is fully backwards compatible.

The IIc is now back up and running now with the replacement IWC chip

 

[retr01]

Yay!

So, the IIc boots up normally? Have you run the Apple Service diagnostics on the IIc to ensure full functionality?