PDA

View Full Version : Temporary fix for vintage mac "non-boot" due to faulty caps



tezza
January 9th, 2012, 02:43 PM
Hi,

Most people here would know the "leaky caps issue" in many of the late 80's/early 90s Macs. Almost three years ago when I was quite new at this I bought back my Classic II back to life by washing leaked electrolyte off the mainboard in the dishwasher. Ok, maybe that was a little extreme. I've since tried a gentler method by just swabbing the mainboard with isopropyl alcohol and Q-tips. It seems to work just as well.

Anyway, I wrote up the experience (http://www.classic-computers.org.nz/blog/2012-01-09-classic-ii-cottonbud-solution.htm) for those who have one of these old Macs, but neither the skill or equipment to tackle the cap replacement. It's still only a temporary easy/quick fix but hey...I'm still using my original "dishwashed" Classic II and it's been almost three years already!

Tez

olePigeon
January 10th, 2012, 11:49 AM
The dishwasher method worked well for me, at least for a little while. After about a month I started to get the dreaded "squeal" out of the sound on my LC. After I drummed up the courage, only a recap was able to fix my ailing LC. As you mentioned, though, great for temporary fixes (and for some, apparently permanent.)

tezza
January 10th, 2012, 11:56 AM
Yes, I had one Classic II with the "falling bomb" sound. I found the dishwasher made little difference to that particular one. For whatever cap was causing the problem it wasn't stray current on the board that was the issue. It had just lost too much material I think.

Did you find a recap straighforward? It looks difficult but I do have an unused board which will be of no consequence if I **** up. Any tips on undertaking this delicate work?

Tez

olePigeon
January 11th, 2012, 09:42 AM
Prior to soldering my LC motherboard, I'd never soldered before in my life. I jumped onto YouTube for soldering advice videos. The recapping was pretty straight forward, but I ran into some issues. On 2 other LC boards, I kept accidentally ripping the little metal pad off the board when I was trying to desolder. What I had done was melt the solder, then lift once side of the capacitor up so it wouldn't stick to the board again, then try and melt the solder on the other side. What ended up happening is that the little pad the capacitor was stuck to just bent and broke off. I couldn't get solder to stick to that area again, so I had to chuck the board. :(

What I eventually did was use two soldering irons. I would heat both sides of the capacitor simultaneously for about 10 to 20 seconds, then very gently nudge it. Eventually the solder on both sides would melt and the capacitor would just slide off ezpz. I would then use some solder wick to clean the pads of extra solder, then use isopropyl alcohol to clean the board. This worked out best for me. I used two soldering irons because I could never get the solder wick to pick up the solder from the pads enough to actually remove anything.

To put new capacitors on, I use solder paste. It's like the name implies. About the thickness of frosting. It has flux and little solder beads suspended in it. I just dab a bit of that goop on each pad, then sit the capacitor in it. I lay pair of tweezers or bulldogs on top of the capacitor just for a little bit a weight and keep it steady, then just touch the paste with the iron. All the flux evaporates and the solder just melts into it. It's the best stuff. Even with my shaky hands, it's easy.

In all the videos I watch, the capacitor just sort of floats into place as you melt the solder, but the people in the video always had a heat gun and melted both sides at once. Since I was doing one side at a time, I had to hold it in place with the bulldogs or it'd go all wonky and wouldn't line up on the pads.

Also, if you're replacing the capacitors, I suggest tantalum capacitors where applicable. They won't leak and they'll help future proof your repair job. The other thing I learned is about the voltage rating on capacitors. As long as it's higher than what is required, you should be OK. I spent many a hours at the parts store looking for just the right capacitor with the exact same voltage rating as the original. Then the guy there just handed me a bunch with higher voltage. I didn't know that it was just the tolerance rating, not the amount of volts going through. :P Just shows how much of a noob I am. Actually, getting caps that are rated higher than the originals wouldn't hurt and might help protect your repair job should something else short or something.

Al Kossow
January 11th, 2012, 12:57 PM
I suggest tantalum capacitors where applicable.

The problem with tantalum caps is they can explode and burn a hole in the PCB.

Make ABSOLUTELY sure the polarity is correct, since a reversed tantalum WILL explode.

olePigeon
January 11th, 2012, 01:23 PM
I didn't know that. I guess I've been lucky enough to not put one on backwards.

twolazy
April 20th, 2012, 11:30 PM
I was considering doing a recap job on my Mac Classic. Seems pretty forward. My machine still works and looks like new, so kind of hesitant to replace them, but I most likely should as preventive maintenance. If you guys want, I can document my steps, pretty sure a Classic II is extremely similar inside.

On a side note, I have 4-5 machines here that could use recaps. One machine is my tandy 2810hd. Starts for a second then poof nothing. Wondering if I should document the repairs on them at all. 99% of the time I don't , but as systems age might be valuable information to others. : ponders :