8500/180 + PCI ATI = static

[jmacz]

I've been slowly working on restoring a Power Macintosh 8500/180. It's fully recapped and I just have a ton of broken plastic to deal with. But noticed some strangeness today and was hoping for any insight before digging into it. Note that this machine came with a Newer 500MHz G3 CPU card installed.

Basically if I boot up using the onboard video, the speaker sounds normal. The only issue are some artifacts as I drag windows around but I saw another post that that might be related to the Newer and that installing the accelerator driver for it should resolve that. But the important thing is the speaker sounds normal.

If I install a PCI based ATI Rage 128 (that the machine came with) into any of the PCI slots AND plug in a monitor, the speaker does play all the sounds it's expected to but I'm hearing a lot of noise. All SCSI accesses I can hear through the speaker (sounds like an old physical spinning hard disk), all mouse movements cause noise, etc. If the ATI video card is installed but not plugged into a monitor, speaker sounds normal. It's only when the card is being used and outputting video that I hear static associated with ADB and SCSI traffic. Different VGA cables, different monitors, doesn't change anything so I don't believe it's the VGA cable itself or the monitor it's connected to. I tested the VGA pins on the ATI card for shorts but didn't find any.

 

[Byrd]

Hi jmacz,

No damage on the card especially around any coils or chokes? Sounds like poor shielding of the card itself; you could consider making up some sort of metal shield potentially. Check also the internal speaker itself which is probably unshielded.

 

[jmacz]

No damage on the card at all, it’s pristine and works well. I will check the shielding and recheck for ground issues.

 

[lauland1]

You should try the card in the different slots to see if there's any difference in the noise. This won't tell you what is going on, but will be a data point. If the noise changes, it makes it more likely that shielding on the card will help (because the components on the card will be closer or further from various parts on the motherboard). But this only makes it more likely, not for sure.

I've got an 8500 (with original caps) and have used a Rage 128 card out of B&W G3 in it, and haven't experienced any noise like you describe.

I hate to say it, but whenever I hear "noise" I first always suspect capacitors, and the fact that your machine has been recapped, and so may have different kinds of caps than the originals, is a red flag. I'd check that the replacements were all correct with the original values, and whomever replaced them didn't use ones that were just "close enough" anywhere. I know without another board in its original state to compare to this is pretty much impossible, though.

If you have any other cards, such as standard USB ones, etc, maybe even just random PCI cards that won't actually function, like a random PC video card, you should try booting with one of them in there. If you get the noise, then the problem would be a general PCI one, and not specifically the Rage 128, so you could eliminate it as a specific variable. This is NOT a good test, but again, another data point.

 

[jmacz]

Thanks for the pointers. I recapped the board myself so I know the capacitors are all in spec and good ones. I did not however recap the power supply - not sure if it could be that or not but going to recap that one next. I can't remember if the Quadra 800 power supply is the same as the one in this 8500. I guess I should check. That one is recapped.

The noise is there and the same regardless of which slot it is in. I found a cheap second board that is working. Once it comes, I'll try that one to help rule out various components.

 

[nathall]

It sounds like ground bounce to me. Do you get it through the headphone/speaker jack as well?

 

[jmacz]

It sounds like ground bounce to me. Do you get it through the headphone/speaker jack as well?

Did not try this... need to.

 

[jmacz]

Headphone jack as the same issue.

I got the second working logic board and the sound follows the ATI rage card. I moved it to the second logic board and same issue. Remove the card, all is good. Also recapped the power supply and that didn't help either. Voltages all look good and strong.

On another note, the screen artifacts came back when using the onboard video. I thought it might be the video ram itself but I swapped out with multiple sets of other sticks and same issue. And this also happens on both logic boards. One is recapped, one is not. I think the artifacts are due to the Newer G3 500MHz cpu card. I don't get the artifacts with the ATI rage card but I get the annoying sound.

I've got a stock 180MHz 604e card on the way to rule in/out the Newer CPU card.

 

[jmacz]

Just as a note there for my future self, I did take a Quick Look at ground loop issues and determined that my LCD test monitor is plugged into one electrical circuit and my logic board PSU is plugged into another, and obviously there's a VGA cable connecting the two via the ATI card. I had plugged in the monitor into an available outlet which is on another circuit. But however, putting them on the same circuit didn't fix it. Same noise at the same level.

I did watch the voltage lines, ADB signal line, video output, speaker wires on a scope to see if I saw any weirdness when I move the cursor (when the sound is very evident and matches the cursor movement) but the signals were steady. No shakes/jitters whatsoever. I'm curious if it's EMI. I did try putting an insulated piece of metal between the video card and the CPU/speakers but that didn't provide any noticeable change unfortunately.

 

[jmacz]

No idea what's going on.

Tried two different motherboards, two different PCI video cards, two different CPU cards, still the same issue. The only things I haven't tried replacing are the power supply and the speaker, as I don't have access to another. I don't think it's the speaker as static reproduces via the headphone jack as well. The issue reproduces every time and even with only the motherboard, vga/pci card, power supply, cpu card, and speaker. Happens with/without the case. Tried adding additional ferrite beads, didn't help. The power supply is fully recapped and also swapped out the safety capacitors too. I don't have this issue on any other computer, just this one.

But the machine is running fine... just the buzz is annoying. With the case on it's fairly muted but it's still there.

I guess I'll try with another PSU sometime in the future if I get access to one.

 

[nathall]

But that means you have to open it up again

Still sounds like a grounding issue to me. When all else ruled out, PS sounds likely. Or something that’s a part of the ground path to Earth.

 

[jmacz]

Yeah, has to be... but didn't spot it during my last pass... will have to open it all up again... freaks me out each time as who knows what next piece of plastic will break.

 

[lauland1]

Very strange. If it's a grounding issue, you wouldn't think it would just happen to you. The case has metal in it that I'm guessing is shielding to temp down things inside interfering externally...does it physically connect to the metal of the power supply or ground to the motherboard in any way? Could yours not be "touching" or something like that?

I've got a good working 7600 (very similar but not same motherboard as 8500) and several pci video cards I can try in it. I'll turn the volume way up and/or listen through headphones to see if I notice any noise. It may be "just one of those things" that similar machines have and many people don't notice or mention.

It has a sata, usb card, and g3 upgrade, so that will contribute to the electromagnetic environment and be a good test. Again, different system from yours, different (weaker?) power supply and case, but should be decent test.

Will let you know what I hear, especially if it is different with different cards.

 

[jmacz]

I'm guessing right now it's a grounding issue inside the PSU as that's the only constant. The case metal shielding is in contact with the motherboard and the PSU so I don't think it's that. The case shielding I would think would reduce interference outside of the computer but I'm having an issue with the speaker that's obviously inside. Until I am able to test with another PSU, my hunch is it's something with it. I will need to open everything back up at some point and take another look at it.

 

[lauland1]

My thought was if the shielding weren't connected to the power supply, it could possibly resonate at a slightly different frequency (electrically?)...or something...my electronics knowledge is self taught.

I've got 5 cards, a pair of rage 128's, a pair of low end radeons, and an ix3d. I believe I've previous tested them all as working in the 7600.

The radeons have different colored boards, but otherwise seem identical as far as parts. Chip is under heatsink, so unable to tell if the same.

The rage 128's are identical, but one has a heatsink on its chip.

The ix3d is a beast unto itself.

My 7600 is under a bunch of other equipment, but I'll unbury so I can try the cards out, with headphones and see what I hear (ha).

Even if the problem is in your particular power supply, it will be interesting to see (hear) if the cards make any difference.

 

[lauland1]

This was good to do because it looks like one of my radeons got damaged at some point and shows vertical lines through everything!

No difference detected as far as sound goes for cards vs no card.

Heard crackling at a few points while loading extensions and changing volume, but otherwise no noise detected at all.

For what its worth, the Rage 128 had the best picture. Radeon display was a bit fuzzy.