Mac II powers on but does not chime

[bear]

Here is the answer to the riddle:

The trace between the + side of C14 and pin 10 of UI14 (the Sony sound chip closer to the MC68020) had come open. I repaired it and the II lives. Or at least, gives me death chimes, which is expected since I have no RAM in it while it's on the bench. (The next step will be to start buttoning it all back up and see how far we get with that.)

Sadly the results came out of a bunch of guess-and-check hunch-work, rather than any concrete info. Trying to guess how the reset circuit worked, I found one (but only one) of the Sony sound chips connected to it, but didn't know why. Later, when looking for a clock line I could get at with a logic analyzer (to see if the clock was even working) I discovered that that same Sony sound chip (but again only that one) was connected to it (and didn't know why this either).

Guessing that it must have something to do with controlling the clock during an external reset and possibly also bringing the RESET line high at the appropriate time (after the right number of clock cycles), I observed on my one good II that the clock stops for as long as you hold the reset switch in. Checking the bad II, I found the clock stopping and starting itself at intervals, and that if I pushed the reset switch the clock would stop and never start again. So I began to think that that Sony chip might not be a bad place to start looking.

There was no change in behavior after I replaced it (UI14). Even though neither CLOCK nor RESET are connected to the other Sony chip (UI16), the two Sony chips are connected to each other, so it seemed reasonable that a fault with UI16 could cause UI14 to malfunction with a corresponding glitch on RESET. When I cut UI16 off the board to replace it, I found some corrosion under it (collateral damage from leaked electrolyte, I'm sure) that hadn't come clean in the wash (in any of the washing). I got the idea to ohm out each of the three traces that run through there; sure enough, the above mentioned trace was open-circuit. Replaced UI16, bridged the trace... life.

Sweet.

I have enough spare Sony chips to do four more boards if it came to that, but I think maybe it won't be necessary for the next one.

 

[bbraun]

Nice! I'm glad you got it working, and thanks for sharing. It sure helped me.

 

[techknight]

Well, we need to get a repair tips thread started, and this placed in it. So when people run into the same issue, they know where to find it.

 

[CelGen]

Ah yes, the elusive North Pacific Gremlin.

 

[bear]

Here are the final results of the restoration effort.

 

[uniserver]

so again what were all your issues and what did you preform in-order to fix them? thanks

 

[bear]

Myriad.

To start, the batteries were dead. They were replaced.

All of the 47µF SMD electrolytics were replaced.

The trace between R3 and C6 was repaired.

All of the axial electrolytics were (unnecessarily) replaced.

The Sony sound chips were (unnecessarily) replaced (and socketed), though doing so enabled the discovery and repair of the following fault.

The trace between C14+ and UI14 pin 10 was repaired.

Four Quantum Q280s failed bench testing, three with unrecoverable format errors and the fourth with a catastrophic head crash, before finding a fifth that (miraculously) still works. These are not likely to be electronics problems, so I'm not confident that the prognosis for repair is good. I'm kind of glad that the Explorer configuration sticker on the back indicated the machine shipped with an 80 MB drive, not least because 80 MB is only barely big enough for the Lisp load bands, software, and necessary paging files, but also because my situation with Q250s is even more dire. I think out of something like 15 drives, I have two that still work.

Failed EtherTalk card (no life on the AUI port) put on the repair pile for future attention, and swapped for another that still works.

Floppy drives both needed head cleaning; one needed more than any other drive I've encountered, ever. But in both cases, head cleaning was all that was required apart from the inevitable lubrication of the eject mechanism.

The machine was acquired as an empty shell, with only a PSU and mainboard. Most of it was built from parts in my spares inventory. The microExplorer processor and Lisp software came from a trade with a gentleman in Norway. A 47µF SMT tantalum blew off the microExplorer memory board and was replaced even though the board still functioned normally without it.

The original 13" AppleColor High Resolution Display developed an HV arc the night before the demo. I haven't even taken the cover off of that one yet (thus, the comically huge widescreen LCD running 640x480 in the photos).

I think that's it.

 

[IPalindromeI]

I don't know Lisp, but the concept of the Lisp machines always interested me.

 

[CelGen]

Is this the same Mac II you were struggling with before or is it another one this time?

 

[bear]

Same one. The project was always to restore this microExplorer.

I have another II that needed attention, but its problems were much more straightforward and were solved relatively quickly.

Now if I could just solve the apparent address/data line problem that's preventing this IIx from working... I haven't really seriously tackled it yet though. Just kind of half-assed it so far. Been working on a couple other basket cases instead (like this one, which I finally had full success with yesterday, after about six months of working on it: http://www.typewritten.org/Projects/Intergraph/h0jhvb.html).