Mac II powers on but does not chime

[bear]

Folks;

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.

• replaced both batteries
  
• replaced all the 47uF SMT caps
  
• replaced all the radial electrolytics for good measure
  
• repaired a failed trace in the power circuit (somehow I've had more than a few IIs and IIxes with this fault)
  
• cleaned the board with flux cleaner, then again with 99% isopropyl
  
• 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!

 

[volvo242gt]

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.

-J

 

[bear]

Well, perhaps. But the ROM chips play no part that I can tell in the reset circuit. Reset has to come high before the CPU can execute any code out of the ROMs.

 

[uniserver]

are you sure batt voltage is going to where it needs to go?

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?

 

[bear]

Yes, of course they had leaked (not the batteries I mean). That is why I cleaned the board twice. This is the part I am new to, and may not have gotten the art of yet. I understand others have brought boards back to life by cleaning them and wouldn't mind some advice from those who have.

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.

 

[uniserver]

So the reset circuit should only be triggered when an IC or the reset button enables it.

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?

 

[bear]

There's a little more to a reset circuit than that. RESET is held low during power up; it's the RESET circuit that is responsible for bringing it high at the right time. For example, I had a SonicBlaster that would cause a IIgs to not boot, and it was because a 74LS series chip had failed in such a way that it was holding RESET low. Fortunately it was an easy replacement. It's not an expansion card in this case, since they are all removed for now. It has to be something on the main board, either another failed track, or a failed component.

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.

 

[uniserver]

yeah, maybe some ESD hit the sound chip?

 

[James1095]

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.

 

[CelGen]

If you were closer I would let you loan my Snooper card. It would at least tell you at bus level if voltages and clock were there.

 

[bear]

Hm, clock. That's an interesting thought; one of the leaky caps is near the crystals, and they are sensitive to stray capacitance (like from leaked electrolyte). A failed clock could certainly produce this failure mode.

 

[CelGen]

If I'm aware the on/off circuit doesn't need the system clock. With a dead crystal you could be turning the lights on but nobody is home.

 

[bear]

Your understanding of the soft power circuit matches mine. The reset circuit would need the clock, though, as I understand it counts clock pulses to accomplish the correct delay between power-on and starting the CPU.

 

[Unknown_K]

If the ROMs are on a SIMM remove it clean it and reinstall. I seen issues like that on a IIx where the ROM SIMM needed cleaning and reseating.

 

[CelGen]

Mac II had the ROMs socketed.

Have you by chance also thrown the board through the dishwasher to try and blast whatever you couldn't scrub out with a toothbrush?

 

[bbraun]

I'm curious, which trace did you fix? I wasn't aware of a common problem with a trace on II's and IIx's and it would be helpful to know.

 

[bear]

Somehow a trace involved in the connection between R3 and C6 comes open, which prevents the power switch from turning on the machine. I'm not sure about "common", but it has happened on both my IIs and my IIx. I'm not sure how it happens, or exactly where on the board it's going bad, but it's definitely going bad, and it's definitely the same part of the circuit each time.

 

[bbraun]

Thanks for sharing that. Both my II and IIx had the same problem. C6<->R3 also goes to R18 on the II, so putting a wire between R3 and R18 is convenient with both on the bottom side of the board. On the IIx, it's R19 instead of R18.

 

[bear]

ISTR that when I traced it out, R3 was still connected to R18, but neither were connected to C6.

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)