Tip: Boards that won't boot no matter what, check the clock crystals (See video)

[smiba]

Hey everyone,

I don't think I've ever posted this fix anywhere other then YouTube, but it might help people who just can't get their 68k mac to boot or can't seem to find the fix for their cold booting problems.

A while ago I got a LC III in for repairs but no matter what I tried it did not want to boot, but if you kept it running for long enough it would eventually POST but either crash or stay working for a while.

I did a recap, no fix.
I checked a lot of PCB traces, no fix.
I reseated the ROM, no fix.

After I was soldering around the area at the top, I noticed that if I were to boot it up after I've worked in that spot it would actually power on. Leading to the discovery of pointing a heatgun (At around 50C/122F, give or take) at the 341S0851 chip it would very often allow the device to Chime.

At that point I made the conclusion that the silicon inside this chip has degraded and simply can't get the crystal stable. Tried to replace the crystal for good measure but even though it kinda changed the symptoms it did not resolve the issue.
The reason this chip might die is that its always on, if you have the power plugged in and the switch on it will be powered and running.

Anyways, I present to you the solution: Replacing the crystal with a full oscillator on a PCB.



PCB itself, just a 3.3V LDO, 2 caps and an oscillator:


Also check it out in action:

Video: Heating up the 341S0851 chip: https://www.youtube.com/watch?v=vWI_YMTVW0w
Video: Clock PCB in action: https://www.youtube.com/watch?v=RAzfCv27byU (Excuse the massive gauge wires)

Alright so, how do I know this is the issue I'm facing with my device?

Try to heat up your 341S0851 chip while having the power plugged in, do NOT use temperatures hot enough to desolder or exceed water boiling temprature. 50C or 120F seems to be good enough, if your heat gun can't go that low control the temperature by distance (keep it further away)

You can also try to keep your device running for an hour or so, usually once everything gets warm it will try to post. From testing this required between 10 minutes and multiple hours (and a few rare no shows).

Questions, let me know! If you have a dead 68k mac feel free to try this as well, if it ends up the case I'd love to know!

 

[bibilit]

Don’t remember the part reference but sounds like the Egret chip to me. 

If so, pretty common issue, lot of trouble in the Classic II and Classic Color. 

I remove it, clean underneath, and put it back with clean solder, works fine most of the time. 

Thanks for the hack. 

 

[bibilit]

Got a board working today after an Egret swap, the board was not booting before, or at most was showing a plain screen. 

Fortunately, had a faulty board (Maxel bomb) where the chip can be removed. 

 

[defor]

I've also revived quite a few machines that have failed to massive corrosion issues on that same chip...

would love to get my hands on some NOS replacements...

 

[Bolle]

Did you try to just replace the crystal with a new one?

I had a few SE/30s where the crystal for the sound chip simply went bad which resulted in bad audio.

Heating the area made the audio work again but it was none of the chips but the crystal close to the ASC the was responding to the heat treatment.

 

[smiba]

Did you try to just replace the crystal with a new one?

I had a few SE/30s where the crystal for the sound chip simply went bad which resulted in bad audio.

Heating the area made the audio work again but it was none of the chips but the crystal close to the ASC the was responding to the heat treatment.

I did, the issue wasn't resolved and the crystal showed no odd behaviour in other tests. Important is that the crystal is not at fault a lot of the times with this IC (Egret), but the IC itself is no longer able to drive the crystal.

 

[techknight]

egret chips seem to be a common problem. 

I think they are 68HC705 CPUs? I dont remember. 

With the RTC chip replacement project happening with the SE in the Hacks forum, tangentially might be a good idea to start looking at equivalent egrets using a PIC or AVR. And the egret's ROM being reverse engineered, of course. 

We dont really have a good grasp on how age and heat affect solid-state electronics over decades, as the technology simply isnt old enough. We are just now getting there. Oldest of the oldest being the worst affected, such as 60s/70s Hi-Fi audio, etc.