Quadra 950 No Chime, No video

[MacLovin_0315]

Power LED comes on and there is a click from the speaker. CPU quickly gets quite hot. No POST, no video.

What should I try next?

Here is what I have tried/observed so far:

• Tried multiple RAM configurations with tested good SIMMS following Apple's Memory guidance
• There is a tested good PRAM battery in the logic board
• Tried both with the original PSU (which is at least providing good voltages) and an ATX PSU adapted following Geekdot's guide
• CPU is XC68040RC33E which the seller claimed was pulled from a working system.
  • 8 pins fell off the XC68040RC33E CPU that was in the machine. No visible corrosion; possibly I damaged it while removing it...?
• Scoped CPU pins following attached pinout diagram
  • No waveform on TK, BCLK or PCLK. There is a sine wave coming out of the nearby 66.67 MHz oscillator.
    • I am using a Zoyi ZT-703S oscilloscope, which I believe has sufficient bandwidth so I should see a 33 MHz square wave on one of the clock pins
  • VCC is +5V
  • RST is +5V
• Reset and interrupt buttons are flaky, but they are not sticking closed
• At the logic board power connector, with no PSU plugged in, there is low resistance (11-12 ohm) between +5v and ground, but maybe that is normal...?

Logic board photos below and attached. Appears to be in good condition. There was minimal corrosion here and there which I mostly cleaned up.

 

[Unknown_K]

You do have the CPU heatsink, correct? You can't run a full 68040 without a heatsink.

How did you remove the CPU? I have a black pry tool that came with a CPU upgrade and removing the Q950 CPU using it was pretty easy with no damage or broken pins.

 

[MacLovin_0315]

You do have the CPU heatsink, correct? You can't run a full 68040 without a heatsink.

How did you remove the CPU? I have a black pry tool that came with a CPU upgrade and removing the Q950 CPU using it was pretty easy with no damage or broken pins.

Yes, I do have the CPU heatsink! It is not installed since I figured it would not get very hot being on for a few seconds, and would not prevent it from POSTing. However, it does get surprisingly hot.

I removed the original CPU with a small screwdriver. I probably managed to damage it that way. If anyone can recommend an appropriate tool for a 68040, I would definitely get it.

 

[Unknown_K]

Mine came with a Newer Technologies 68040/50 CPU upgrade, but it looks like this intel one:
You want one that will fit in between the pins of a Motorola 68040

 

[imactheknife]

My q660av doesnt have a heatsink and no little connectors on side to hold one on. Maybe only needed for anything 33mhz and faster?

i put a full 68040 in my lc 575 with heatsink. Also the lc 475 has no heatsink either

 

[MacLovin_0315]

I installed the CPU heatsink with some thermal paste. Still no chime/no video.

I took some thermal images after about 5 mins with power on. No obviously overheating components. The CPU temp of 46 C is well below the max operating temp of 110 C in the data sheet.

Voltage regulator U89 (Motorola MC 7805CT) is a bit toasty at 70 C, but well below the max operating temp of 125 C in the data sheet.


I also decided to double-check, expand, and share my multimeter/oscilloscope readings.

• VCC: 5.007 V
• RSTI (active = low): 5.009 V
• RSTO (active = low): 5.008 V
• BCLK:

• PCLK:

I do not know why I found no waveforms on CLK pins when I checked before, but now there are. Perhaps I had a bad ground or probed the wrong pins.

Unfortunately, she's still not POSTing :-(

 

[MacLovin_0315]

Quick update: I did some more reading. Fortunately, the wonderful 68040 has 4 pins on it that can report the processor status: PST3-PST0. Here is what I found:

• PST3: 0V
• PST2: 5V
• PST1: 0V
• PST0: 5V

If we interpret these as logic low (0V) and logic high (5V), we get binary 0101 (hex $5). According to the datasheet, this error code indicates "Halted state (Double Bus Fault)."

Address Lines
I also checked several address lines, specifically A1, A3, A4, and all were seemingly stuck high (5V).

ROMs
I decided then to investigate the ROMs (U23 and U50) to see if the CPU would try to access them. I found some recreated schematics over on TheRealBolle's GitHub. This is the first time I have been able to find any schematics for the Quadra 950!

At both ROMs, there is a dead short to ground on D10.

Has anyone else seen anything similar? Any advice on how to tackle the "Double Bus Fault" or remedies for the dead short at D10 would be greatly appreciated!