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Very sick Classic --- could it be a bad VIA chip?

stepleton

Well-known member
I've had a Mac Classic (actually my first-ever computer) that's been ill for a while. Here's a report from a long time ago --- although it's a lot sicker now. I'd love to get it working again (since it was my first-ever computer :) ).

The symptoms today are these:
  • Switch it on after a long time switched off, and you get horizontal black and white bars. No bong.
  • It doesn't do anything at first, but if you wait long enough, the reset button starts "working". Pressing it gets you as far as a partial bong and the dark grey screen you see before a normal boot would take you to the usual 50% grey, a happy Mac, and so on.
  • You can keep pressing the reset button as often as you like, and most of the time, you get a partial bong. The dark grey screen never changes.


The fact that the reset button behaviour only works after a few moments makes me think that there's a chip that's sensitive to heat. I have a thermal camera, but nothing's getting too hot. Still, I try spraying the chips one-by-one with an upside-down air-duster can. For most of the chips it doesn't change anything, but if I chill the VIA chip --- that's UB5 over on the left-hand side of the board --- then the reset button stops working. After the VIA warms up again, then the partial-bong-on-reset behaviour is restored.

(Ruling out some other possibilities: the board has been recapped, there's little damage from the leaky old caps, and the power rails all look good.)

Does this sound familiar to anyone? What do you think --- bad VIA? Does that older post I mentioned seem consistent with that problem? Is there any resource online that describes a kind of theory of operation for the Classic or similar machines (so that I can try to diagnose what's going on)? A block diagram of the Classic logic board would be really helpful as a start.
 

stepleton

Well-known member
Also: is it easy to get your hands on an extender cable for the 14-pin connection between the logic board and the analog board? It would make probing the logic board a lot easier.
 

cheesestraws

Well-known member
For most of the chips it doesn't change anything, but if I chill the VIA chip --- that's UB5 over on the left-hand side of the board --- then the reset button stops working. After the VIA warms up again, then the partial-bong-on-reset behaviour is restored.

That certainly sounds reasonably convincing to me... though because I am lazy I might look at joints around the VIA before the chip itself. I don't suppose exerting pressure on it changes anything?

Is there any resource online that describes a kind of theory of operation for the Classic or similar machines (so that I can try to diagnose what's going on)?

Your best bet here is probably to look in the Guide to the Macintosh Family Hardware, 2nd ed. for the SE (as the Classic is basically a cost-reduced SE). There's a Classic Devnote here: https://bobparadiso.files.wordpress.com/2014/09/mac_classic.pdf which basically says "SE but we rerouted some of the signals to the VIA, not your problem" (helpful, right?)

The schematic is in the bomarc dump that's out there on archive.org, if that helps.

is it easy to get your hands on an extender cable for the 14-pin connection between the logic board and the analog board?

The connectors are just standard Molex Mini-Fit Jr. connectors. I can't remember where I got mine—I think just from someone better at crimping than me—but a number of the usual suspects (RS etc) seem to sell Mini-fit Jr cable assemblies, and they might do extension leads as well. Or if you have a suitable crimping tool then they're pretty easy to knock up with eBay clone connectors and some wire :) .
 

stepleton

Well-known member
That certainly sounds reasonably convincing to me... though because I am lazy I might look at joints around the VIA before the chip itself. I don't suppose exerting pressure on it changes anything?

I did try that --- it doesn't seem to help, unfortunately! I also tried checking the connections between legs and pads (or nearby vias), and they seemed OK.

There's a Classic Devnote here: https://bobparadiso.files.wordpress.com/2014/09/mac_classic.pdf which basically says "SE but we rerouted some of the signals to the VIA, not your problem" (helpful, right?)

The schematic is in the bomarc dump that's out there on archive.org, if that helps.

These are helpful. In particular, it points out that much of the ADB chip routes through the VIA. A theory I'd had in the days of my "Maaaaaaaaaaac Claaaaaaaaaaaaaaaaaaassic" post was that the VIA was receiving spurious interrupts from the ADB chip and reading zeroes from it. Now I'm wondering if the VIA is hallucinating interrupts on its own. I think it may be time to order a new VIA (which apparently you can still do).
 

cheesestraws

Well-known member
I think it may be time to order a new VIA (which apparently you can still do).

With the caveat that I don't know how those machines work on the hardware level very well, I'd tend to agree: responding to temperature change but not physical pressure feels like something has gone awry internally to the chip somehow.
 

LaPorta

Well-known member
I made a 3 foot long extension cable myself. There are suitable connectors on Mouser and the like. What is the exact part number? Lost to memory :(.
 

stepleton

Well-known member
A puzzle: do I buy a replacement VIA or do I try swapping in the chip that I pulled from a very crusty badcap-blasted parts board that a friend gave me? I'm leaning towards "swap" right now, but I'll sleep on it and see how I feel about it tomorrow.
 

volvo242gt

Well-known member
Could try swapping it. Depends on if you want to do twice the amount of work, if the chip from the parts board is bad as well.
 

stepleton

Well-known member
Great news: I think the VIA replacement might have done the trick! The parts-board chip was pretty crusty --- it wasn't far from some caps that had leaked --- but it seems like it's got it where it counts. I've had the computer running for an hour or so now, and there's been no sign of trouble yet.

Looking forward to getting it on @cheesestraws 's wifi localtalk at some point :)


PXL_20220306_162653844.jpg
 

cheesestraws

Well-known member
Glad it's working! WiFi coming very soon ;-)

... also, what is that sitting on? Anything that has an 'APL' switch on the front panel fills me with dread by default ...
 
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