Need help with a Mac IIx no chime, no video

[pezter22]

I have been working on a Mac IIx motherboard that will not boot. I have repaired the startup/ shutdown circuit and it will turn on by pressing the power button. However, I do not get a boot chime nor video. There was slight battery corrosion and slight capacitor corrosion near C9. Nothing major and that repaired my startup/ shutdown circuit.

I had thought that it may be a ROM issue because I have booted without a ROM stick on my other Mac IIx and I would get a similar result of no chime, no video. I have changed the ROM socket and I am getting voltage to the pins. I wish there was a bomarc schematic for the Mac IIx so I could follow traces for each pin.

My Flir camera shows activity. I also see heat from the CPU. I have another Mac IIx and the heat pattern is similar except for the area around Q1. I have replaced it and it looks like there is activity, but still cold.

I am stumped with this one. Any help or guidance would be appreciated.

My image a Flir image of my working Mac IIx, middle is Flir image of the board I am working on with the red circle and last is image of the board.

 

[joshc]

Have you tested the speaker?
Any chime if you plug headphones in?

 

[pezter22]

Have you tested the speaker?
Any chime if you plug headphones in?

Yes. I have attached an external speaker and do not get a chime at boot. I do get a crackling sound as I plug in the speaker, so there is sound being produced. When I also check the sound chips for voltage, I get the similar crackling sound from the internal speaker when I touch one of the pins on the sound IC chips.

I also placed known bad RAM on the board to at least try to get a death chime, but I get a similar result. Which means no POST. My Flir images show at least there is life and the CPU gets warm, so I know I am close.

Luckily I have a working Mac IIx and I guess the only way to check what is wrong is to match voltage readings. I am guessing a break in a trace with the ROM because of the close proximity of C9 and C12 that leaked a bit. The traces were either repaired (C9) or looked okay. I have been looking at the bomarc schematics for the Mac II, because of their similarities, but there still is a difference with the boards.

 

[joshc]

Does seem like a ROM related issue yeah. Usually I'd suggest wiggling the ROM SIMM about but you have a new ROM socket so that should be less of an issue.

 

[pezter22]

There appears to be an issue with my ROM. I checked voltages on my good working Mac IIx and the non booting one. I had steady voltages on the working one and many of the ROM pins on the non working unit were jumping. Not too much, just a couple of tenths of volts - like 1.8v to 2.2v. They just weren't steady. This is why I wish there was a bomarc schematic of this board, so that I could follow the traces to the IC that is producing the jumpy voltages. I guess the Mac II is the closest I can get to it.

I've attached the voltage readings from the two boards. They definitely are off.

 

[pezter22]

I think I am going to check the IC RP5 which is near the two PRAM batteries. I did have some corrosion near there from the batteries. RP IC chips lead to the ROM socket according to the bomarc Mac II schematic. (image is of another Mac IIx board of mine).

 

[pezter22]

Still no luck. I changed two of the Resistor Packs RP5 and RP6 and still no boot. I'm placing this board on the shelf to take a break.