Mac II powers on but does not chime
Posted 21 July 2013 - 03:19 AM
I have been working on bringing an old Macintosh II back to life. I'm not new to the process, but this one is stumping me.
[*:b50k8lel]replaced both batteries
[*:b50k8lel]replaced all the 47uF SMT caps
[*:b50k8lel]replaced all the radial electrolytics for good measure
[*:b50k8lel]repaired a failed trace in the power circuit (somehow I've had more than a few IIs and IIxes with this fault)
[*:b50k8lel]cleaned the board with flux cleaner, then again with 99% isopropyl
[*:b50k8lel]tested the assorted PSU rails
Symptoms are, power switch works to turn the machine on and off, ADB works to turn the machine on. No bong, no crash sound. Even if I remove the memory, I don't get a crash bong. RESET seems to be held low, so the CPU never comes out of reset.
I'm not sure if this is meaningful, but watching the RESET line relative to logic ground, I see 0V at power off, then it slowly climbs to 0.175V when the machine is switched on.
Clues, other tests to run? Thanks!
Posted 21 July 2013 - 04:55 AM
Pre-Mac: ///, //e x2, IIgs
Other: iPhone 4s 32GB, Dell D830
Posted 21 July 2013 - 07:43 AM
does it look like at any time there was a battery leak?
but as you said, it sounds like an even lower level issue then that.
when you did the smt/radial lytics was there any sign of leakage?
Posted 21 July 2013 - 07:14 PM
Battery voltage is going where it needs to go because the machine switches on. That circuit seems well understood. It's what happens next (the reset circuit) that doesn't seem to be documented, as far as I can tell.
Posted 21 July 2013 - 07:35 PM
And what you are saying is its staying triggered, almost like someone is holding the reset button in and not letting go.
sounds to me like a solder bridge short, or some other kind of short some where.
how did this work before you performed the re-cap? Or is this something you freshly acquired?
maybe look under the PCB for a little piece of solder or solder ball jammed between some through hole pins?
Posted 21 July 2013 - 08:46 PM
This II is new to me, so I'm not sure what its history is. I received it in non-operating condition.
I did bring out my other old II (which had only needed batteries and the power circuit trace repair before, but since being in storage needed its SMT caps replaced and another power circuit trace repaired before it would live again) used it to swap the CPU and HMMU, and verified that those chips are both good.
Somehow one of the SONY chips in the audio circuit is also tied to RESET (no idea why), and there's a Japanese SE/30 repair manual that suggests a problem with this chip in the SE/30 can cause a similar problem to the one I'm seeing (though this chip is apparently packaged differently in the SE/30, so it's not clear how their workaround applies to the II)... I might try looking that direction if no one has any better ideas in the meanwhile.
Posted 23 July 2013 - 12:33 AM
Bad ROM chips, perchance? I recently switched to an fx board in my II. IIRC, James1095 here on the forums may still have my old board, which did chime and display the blinking ? when powered up. Maybe send him a PM and work out arrangements to grab said board from him.
I had forgotten to install several capacitors, including those on the power rails to the ROMs. It happened because I ran out of caps in the middle of the job and had removed all of the like values at once. That board should be in good working order now but I have not bothered to test it.
Posted 23 July 2013 - 05:26 AM
Wanted: Anything Nubus, Video capture and editing hardware and software for 68k mac. Original software including A/UX.
Looking for 9150, Mac II, Color classic, 840av motherboard, and Nubus cards.
Posted 23 July 2013 - 07:20 PM
Posted 24 July 2013 - 01:11 AM
My first repair simply put a long piece of bus wire between the appropriate sides of R3 and C6. As you noticed, they are on different sides of the board, so it is ugly. I put some effort into this most recent repair, though, and found a couple of vias to attach the bus wire to. This puts the bus wire all on the back side, which is much tidier. (though it's still a fairly long piece, since it needs to go pretty much from one end of the board to the other)
Posted 30 July 2013 - 05:59 AM
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.
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.
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