Macintosh Classic won't start

[patatino]

Hello everyone,

My beloved Mac Classic used to work until about 10 years ago. It then started to fail booting, with a gray screen. I never looked into it (due to lack of time), but recently decided to try reviving it.

I have started by cleaning and recapping the analog board, and cleaning the logic board (I did not want to recap it yet unless really necessary, as I am not overly confident in my SMC soldering/desoldering skills).

Unfortunately, the Mac just won't do anything when switching power on. No sound, black screen, fan and hard drive are not spinning. I do no see any 5 or 12V on the HDD connector, so I suspect something is wrong with the first stages of the power supply.

I did not test it before recapping, so I do not know if I damaged anything that was all right...

I have tested a few components mentionned in the threads here, such as the RP2 surgistor, or diodes, but could not find any obvious failure.

Could anyone advise on what the next steps for isolating the fault would be?

Thanks !

 

[cadenya]

Have you tested the fuse? Funnily I have a Mac Classic analog board that does the exact same thing. Just died out of nowhere one day. I also recapped it and haven't gotten it working.

 

[dochilli]

Check the solder joints of the power connector. I had one classic where they were broken and the AB did not get 240 V.

 

[patatino]

Thanks to both for your replies. The fuse and the solder joints of the power connector are fine.

 

[imactheknife]

Thanks to both for your replies. The fuse and the solder joints of the power connector are fine.

Hi there, weclome to classic hell lol.. these boards can be tricky.

Fine, meaning you reflowed them with solder iron? Or look good but didnt reflow them? Sometimes they look fine but flux and a soldering iron reflows them for better contact. The board looks like it had lots of cap leakage down near the speaker. I would reflow all the wire connections around the speaker as well.

 

[patatino]

Hi there, weclome to classic hell lol.. these boards can be tricky.

Thanks, indeed

Fine, meaning you reflowed them with solder iron? Or look good but didnt reflow them? Sometimes they look fine but flux and a soldering iron reflows them for better contact. The board looks like it had lots of cap leakage down near the speaker. I would reflow all the wire connections around the speaker as well.

I did not reflow them, but I tested the continuity from pin to pin with my multimeter. I guess it will not hurt to do it, but I am not convinced that there is no other issue unfortunately.

 

[bibilit]

There are so many threads about those boards…

Unfortunately, using a multimeter is not enough, two diodes are known for being leaky (so testing ok with a multimeter but faulty most of the time) those are DP3 and DP4.

I will start with those, cheap enough to don’t try.

 

[patatino]

ok, just ordered those parts to replace next (after having gone through a number of threads discussing similar issues):

DP3, DP4 diodes - 1N4148
QP1 Optocoupler - CNY17G-3
DP11 5.6V zener diode - BZX79B5V6
IP1 Chip - TDA4605
QP2 MOSFET - IRFBC40

 

[patatino]

I am making progress! I replaced all components mentioned above and checked connections.
The Mac will now power on. However I get no bong, and the display remains gray and shaky:




I measured low voltage at 4.8 V / 11.97 V.

Am I looking now at issues with the logic board or could it still be the analog board?

 

[patatino]

... and now that I think about it, that is probably the issue I was having 10 years ago when I stopped using it. This seems to be a case of "simasimac"

 

[marcelv]

probably combination
analog board I still see capacitor fluid residu from the old capacitors, please clean that

you can use pp1 to adjust the 5v/12v voltage , take care, high voltages around!!!
adjust the 5V a bit higher, but also measure 12V so that it is not too high

logic board probably needs new caps

 

[patatino]

I gave another try today and decided to leave the mac runnning a little bit longer. After a while, the zebra stripes disappeared, the HDD spun to life, and the computer booted ! The screen remained very wobbly for some time (maybe 5-10'), but eventually all was almost normal. The sound did not work at all though.

It was very nice to find my documents and applications as I left them so long ago!

I suspect that the next steps are indeed giving a little boost to the 5V and recapping the logic board.

I will study how to increase the voltage safely. I didn't like the idea of playing with the AB with power on...

For the recapping, I am still on the fence about having someone do it (and who that would be?), or doing it myself. I never worked with surface mounted components and I am concerned I would do more damage than good. I will try practicing and evaluate if I stand a chance.

 

[joshc]

You very much want to recap the logic board before doing any more diagnosis/testing, it will likely clear up a lot of issues.

Practice SMD soldering on a board you don't care about - could be anything with SMD components like an old DVD player or something, something not worth much.

 

[Kouzui]

You very much want to recap the logic board before doing any more diagnosis/testing, it will likely clear up a lot of issues.

This, and I would get the rest of that capacitor gunk off the analog board. Honestly, I'd put some flux on the AB contacts and make sure they all look nice. They're looking a little splattery in your pic.

 

[bibilit]

For the 5 volt rail, you are good to go with as little as 4,85 volts…so you are close. I agree, clean your soldering job, probably that will bring you close enough.

The Classic is by far the easiest Logic Board to work on. Few capacitors and easy access. You can start cleaning again the logic board, use an old brush, the closer you will be from the capacitors area, the better.

 

[patatino]

After a few tiny adjustments of PP1 I now have 12.3V / 5.12V / -12.16V. The Mac boots straight away and the screen is not as wobbly anymore. Good!

Next step is then recapping the logic board, which will hopefully prevent future issues and fix the sound.

 

[alexGS]

It’s amazing that this exact same problem crops up again and again - I’m trying to deal with a Classic analog board doing exactly this at the moment. Blowing air on the optocoupler (an idea I got from someone else) produces low voltage (4.5V) and a wobbly screen, then it quickly recovers (5.0V).

Of course I’ve replaced the optocoupler several times, the 200-ohm trimpot, the 4148 diodes, 5.6V Zener diode, the power supply controller TDA4605, the IRBCF40 main MOSFET, various small ceramic caps, all the electrolytic caps, and I’m even working my way through some of the resistors such as the 560-ohm in series with the optocoupler LED.

When the optocoupler is first replaced (with a new CNY17G-3), it makes an improvement (becomes not temperature-sensitive), but reverts back after a few restarts.

It must be a minuscule shift in temperature from blowing air through a straw, yet it’s enough to change the voltage when directed at the optocoupler.

Although I have three Classics exhibiting this behaviour, I have one that does not - even if the optocoupler is replaced with one that is temperature-sensitive in the other four boards.

Let us know whether yours turns out to be temperature-sensitive?

 

[patatino]

Hello, here's an update.

The logic board was cleaned, recapped and repaired by a very kind forumer who contacted me to offer to do what I was surely unable to myself. Many thanks to him!

Unfortunately, I was probably too eager to try my mac and should have - as suggested - cleaned the analog board more thoroughly. While it had worked a couple of times before, it does not any more.

After switching the mac on yesterday, I got the simasimac screen again. I then switched it off and on and heard a bang. I assume some component of the AB failed, but I can't visually tell which one. The fuse is fine.

I cleaned the board as well as I could without removing the components, and tried again. I still have the horizontal stripes, and now voltage is around 10.7 / 4.3 / - 11V, which is obviously way too low. I presume this is due to the failure mentioned earlier. Would anyone have an idea what it could be? Diode somewhere, a cap, MOSFET...? There are several threads mentioning low voltage issues but I couldn't identify what the usual suspect should be.

I did not try to adjust PP1, since voltage was ok before. I don't want to take the risk to fry the LB and/or the HD.

 

[alexGS]

So you heard a bang, but you’ve still got some voltage produced? Check the two 0.1uF capacitors that are wired across the AC input - they are near the metal frame. They often split/explode. If anything else has gone bang, it seems impossible that it would turn on at all.

To fix the low voltage, you could try replacing the optocoupler (the six-pin IC in the middle), CNY17G-3. Often seems to have a temporary effect and as I stated above, often temperature-sensitive for some reason that I don’t understand

 

[patatino]

Well, bang is maybe a bit overly dramatic, but there was definitely the kind of short popping noise that you don't want to hear in electronics.

I'll have a look at those capacitors, thanks.

The optocoupler is new. I still have some spares, I could see if swapping it for another new one changes anything