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LC III - odd problems

T

thekhakinator

6502
Hi gang. So I've been working with an LC III that's absolutely driving me up the wall.

When I got it from eBay, I plugged it all in and it did nothing. Black screen. No sound. Left it for 10 minutes then suddenly it gave the startup sound and booted! I browsed around System 7 on the machine, then tried a floppy disk... as soon as I put the disk in the machine went into a dead crash. Image frozen on the screen, no response. I tried to reboot. Chimes of death. No video.

I left it for two weeks. Switched it back on - booted, after sitting there doing nothing for ten minutes again! Then again, same problem with the floppy disk - crashed the machine. Reboot - chimes of death.

So I go to work. Install a new PRAM battery holder and fresh battery as the old one had started to corrode. Not heaps of damage to the board but all the pads the top side of the board look crusty. Gave it a good wash. Replaced the HDD and electrolytics and floppy drive. 

So now what happens is, I can switch the machine on and boot System 7. I can do some stuff, run some programs, whatever for maybe up to 20 minutes. But eventually after a bit of time, it'll crash - just stop dead, frozen screen. So I'll switch it off and on again - and it'll give the startup noise and boot and get to the screen where it's telling you how to shut down properly but it won't draw properly. It'll beep and that dialog will be half drawn and there'll be a video glitch and it'll just stop dead. If I keep trying to reboot it'll generally give the chimes of death eventually. If I leave it for a few hours, it'll boot properly again... until it crashes after 20 minutes or so again.

I apologise for the long explanation but it's my hope that a detailed description of my troubleshooting and symptoms might lead to a clear answer. Any tips are most welcome. 

 
Yes first of all, LC III had bad caps on the PSU side (probably 3 or 4 of them) meaning you will probably have trouble booting.

Then the caps should be changed on the logic board as well, and the floppy drive cleaned.

 
Yes first change the caps out, then possibly the HDD is on its way out as I also had a Power Macintosh 7100/66 that came with dual 230 mb Quantum HDD's. They both worked initially, then, the main HDD started to give me errors. It caused issues as you describe where the machine was working then froze up, a reset would cure it but temporarily. Replacing the Hard Drive cured the issues. My machines have all been re capped.

 
Thanks all. I recapped the logic board and most of the PSU which made the machine a fair bit more stable. Still crashing fairly often when ejecting floppies - reckon I gotta go back into the TDK power supply and swap out the last few 1000uF caps I didn't have the parts to replace.

 
yup make sure to get the goo off the PCB of the Psu.. that residual goo is conductive.   also can we see pictures of your cap job?

 
I'll throw some up a bit later, sure. It's not super pretty, I used standard through hole electrolytics and tantalums. I noticed mine had that backwards silkscreening on the -5V filter cap. I've thrown the PSU on the scope and it's fairly stable now, beyond the usual noise expected on the 12V line when the HDD spins up.

Interestingly though, I have determined the source of continuing problems - the large "VLSI" chip in the centre of the board. Is this the SWIM floppy controller, or something else? I can generally run for a while before the machine crashes, and reboots fail with the Chimes of Death. Cooling this chip then allows the machine to boot again successfully. I'm hoping it's a poorly adhered pad or something similar moving due to thermal expansion, and not a bad chip - it looks beyond my current skillset to replace and there's obviously no ready source for that chip.

 

 
Pictures of my LC III recap job. 

Through judicious use of freeze spray I've pinpointed the problem to the large "VLSI" chip in the centre of the board - labelled 343S1065 - apparently it's the "Senora" chip, responsible for memory, I/O, audio and video. I don't have a replacement so I'm just gonna try reflowing it when I can get my hands on a hot air tool. 

 
Well done, but if i can argue, those caps are pretty bad (Lelon) if you want something that will last i will go for a better brand or tants.

 
bibilit said:
Well done, but if i can argue, those caps are pretty bad (Lelon) if you want something that will last i will go for a better brand or tants.
Click to expand...
I absolutely can't argue with you there. If I can get the machine to working, I'll go back and swap those out. I just wanna see if I can get it going, otherwise I won't bother spending the time and money to get some decent caps in. Being out in Australia you gotta mail order if you want anything decent, component wise. :(

 
Well, I reflowed the VLSI chip to no avail. Machine still crashes when it gets hot. I'll keep my eye out for one but I have a feeling this LC III is a parts machine only now. Disappointing. 

 
weird, did you try removing the fpu?  Maybe some pins are crossed or corroded.  I know from experience I cut fpus off of bad blow up boards to put in color classic and LC's and if those pins are not striaght it can get crazy.  Just a thought.  Also I could not read the speed on the fpu, if its 16mhz and the board is 25mhz or better that may be the heat issue and a heat sink on it may help.  

Its not been upped to 33 mhz has it? if so that could be overheating the cpu also and it would need a heat sink.

Just suggestions to look at.  Did not see any of this mentioned.

 
you know, with as many mac logic boards that I have repaired, I think I only have ever seen 1 bad GLU. and that was due to pin corrosion/rot. 

 
Macdrone said:
weird, did you try removing the fpu?  Maybe some pins are crossed or corroded.  I know from experience I cut fpus off of bad blow up boards to put in color classic and LC's and if those pins are not striaght it can get crazy.  Just a thought.  Also I could not read the speed on the fpu, if its 16mhz and the board is 25mhz or better that may be the heat issue and a heat sink on it may help.  

Its not been upped to 33 mhz has it? if so that could be overheating the cpu also and it would need a heat sink.

Just suggestions to look at.  Did not see any of this mentioned.
Click to expand...
Yeah, tried it with the FPU out. No improvement. I've put the FPU in my other LC III. Fairly confident it hasn't been upped to 33MHz. How would one check that? I'm not familiar with the mod.

techknight said:
you know, with as many mac logic boards that I have repaired, I think I only have ever seen 1 bad GLU. and that was due to pin corrosion/rot. 
Click to expand...
Yeah, that was my leading theory - I expected it to have some sort of pin-related problem rather then being inherently dead. Unfortunately I'm not particularly skilled in reflowing big QFPs. My attempts thus far haven't really netted me any success.

 
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