Mac Classic Troubles

[cheesestraws]

The typical approach to "wobbly video" is to check for cold / weak solder joints

Given that we're also having variable voltage with draw on the PSU, I'm more inclined to suspect that.

I have tested the outputs on the FDD connector. The results are as follows;

Firstly, those are significantly too low.

Secondly, if it's struggling to provide that voltage (which we can tell because the voltage dips when more current is drawn), adjusting potentiometers will do precisely nothing meaningful, because it won't magically allow the power supply to push more juice.

You probably need someone who actually knows about switch-mode power supplies to fix this, unfortunately. Or knows more than I do at any rate.

 

[paulwilco]

So I've made some progress thins morning! I tried adjusting the potentiometer again and got it to exactly 12v with the screen still wobbling. After a couple of minutes there was a beep and it booted to the question mark icon! After another few minutes the screen stopped wobbling. So I'm now effectively back at square one! One interesting thing is that when I initially set the potentiometer to 12v it started climbing the longer it was switched on, I kept adjusting it to bring it back down and now it's sitting at a solid 11.95v.

I'm going to switch it off and leave it for a bit to see what happens from a cold startup now the voltage appears good.

 

[joshc]

Are you testing this with the original caps on the analog board? Sorry, I've lost track a bit of what has been done so far other than adjusting PP1.

 

[paulwilco]

Hi Josh,

This is with all replacement caps.

 

[joshc]

Ah OK, good! Sounds like where you've got to is a good place? Worth testing it for a bit and specifically under load (reading from disk, opening/using apps) to see if the voltages remain stable.

 

[paulwilco]

Yeah definitely, it doesn't currently have a hard drive in it, I need to collect it today. Once I have it in hopefully it will read it reliably and the remaining issue will only be a wobbly screen for a couple of minutes.

 

[mg.man]

The behaviour also sounds similar to the post Iinked - it might be worth replacing that optocoupler? It also sounds like PP1 could have been clogged up with cap goo. You can remove / clean it with IPA, but make sure it's thoroughly dried before reinstalling. While out you can also test it with a multimeter.

 

[paulwilco]

Yeah I read about the optocoupler, I have ordered the diodes that are suggested on that thread. If I get it to a state where all that is wrong is that it takes a few minutes to boot without screen wobble I'm not sure if it's worth changing more components incase it triggers more problems!

 

[cheesestraws]

After another few minutes the screen stopped wobbling

OK, now this sounds thermal. Does it gradually stop? If so, it sounds like something is warming up and making a better contact as it does so. @mg.man may have been right and I wrong - might be worth looking at the solder joints in the PSU carefully to see if there are any cracked.

 

[mg.man]

@mg.man may have been right and I wrong

Oh, I would be so hard on yourself... I am *no* expert on PSUs, but you're right, it does sound thermal? Also, have a look at that thread I posted a few back... the behaviour sounds very similar. And I have had situations where I went over all the joints and still had wobbles... I have not, however, dragged out any of my "wobbly ones" to try replacing the optocoupler... it is on my list tho....

 

[paulwilco]

Ok, another update! I have put the hard drive in and it spins up and accesses without a problem (so far). So hopefully the cap change fixed the hard drive issue. Still have to wait a few minutes from cold for it to stop wobbling. I might try reflowing some solder joints around the power supply, I'll probably give that a go tomorrow.

 

[paulwilco]

One more update, i have been starting the computer up and testing it over the last couple of days. Basically it starts, wobbles for a bit then boots and all is well. I think i may leave it alone and see how it goes. Just one more question, after it boots it shows it has about 800kb left of the 2mb installed memory, most of the software i have tried requires at least 1000k free. Is there anything i can do to free up memory or should i source some memory to upgrade?

 

[imactheknife]

Do you have expansion memory card?

 

[paulwilco]

Yes I do

 

[cheesestraws]

Oh, I would be so hard on yourself... I am *no* expert on PSUs, but you're right, it does sound thermal? Also, have a look at that thread I posted a few back... the behaviour sounds very similar. And I have had situations where I went over all the joints and still had wobbles... I have not, however, dragged out any of my "wobbly ones" to try replacing the optocoupler... it is on my list tho....

I rarely mind being wrong finding out one is wrong is far better than being wrong and not knowing it!

@paulwilco what version of the System are you running? Extra RAM is a big quality of life improvement on those machines. I've got a couple of spare Classic expansions with SIMMs already on—you can have one for cost of postage, I don't need multiple spares of those. If you're interested DM me . [edit: I see you've got the carrier board already; offer still stands if you'd like a spare / some more SIMMs)

 

[paulwilco]

It's version 7.1, I would really appreciate the ram upgrade, I'll dm you.

 

[paulwilco]

Hi everyone,

Well i thought i had the Mac in a stable condition after all your great advice but having gone back to it after about 10 days its back to the constant reset on boot issue. I have checked the voltages while it is doing this and its reading (at the drive port) 4.75 and 12.3v. If i leave it long enough it will eventually stop the resetting about boot. Here is link to a video i took of it; https://youtube.com/shorts/2MoLOCVee0Q?feature=share

Im thinking it may be a capacitator with the power supply. I replaced the caps as per the re-cap-a-mac guide so it didnt include all of them o the analogue board.

Any more advice would be gratefully appreciated!

 

[alexGS]

Hi everyone,

Well i thought i had the Mac in a stable condition after all your great advice but having gone back to it after about 10 days its back to the constant reset on boot issue. I have checked the voltages while it is doing this and its reading (at the drive port) 4.75 and 12.3v. If i leave it long enough it will eventually stop the resetting about boot. Here is link to a video i took of it; https://youtube.com/shorts/2MoLOCVee0Q?feature=share

Im thinking it may be a capacitator with the power supply. I replaced the caps as per the re-cap-a-mac guide so it didnt include all of them o the analogue board.

Any more advice would be gratefully appreciated!

I have the same problem myself.
Low 5V, wobbling screen, have now replaced most analog board caps (a few more than in the guide, I did the brown one up by the flyback transformer, since all the brown caps elsewhere had leaked) and I’ve replaced the optocoupler and the diodes, and the small electro caps nearby, all with no improvement.

I would love an idea of what to check next, please. Especially if you’ve had any further success over the last month or so

-Alex

 

[paulwilco]

Hi Alex,

I haven't switched it on for about a month so I'm no further ahead! Sounds like you have changed most things! Do you have the resetting problem too?

 

[dochilli]

4.75 V is too low. Did you clean PP1? Did you take it out and measure the Ohms?