Mac Classic with a wobbly screen after recap

[dragonpunchu]

I have done this already. But do you now you get higher voltages if you turn PP1 counterclockwise and lower voltages if you turn it in a clockwise direction - just like it is described in the Repair Notes?

I believe it's the other way around but yes. I had this already but my 12v would go up a lot and my 5v would just nudge a bit.

After this second clean both 12v and 5v were high. I turned pp1 down a bit and both 12v and 5v were in spec, now everything seems to be ok.

 

[techknight]

Yeah, that has been my first guess too. But it really feels like peaking a tuned circuit. I could easily tell the maximum by just listening to the RPMs of the fan.

Is there any information about the PP1 potentiometer? Its range etc? It would be easy to replace and test of course.

Or is there any kind of contact cleaner that could be useful in such a situation? I don't want to make it worse by spraying something on the part, that does not help, but if anyone has already tried this successfully.

I have done this already. But do you now you get higher voltages if you turn PP1 counterclockwise and lower voltages if you turn it in a clockwise direction - just like it is described in the Repair Notes?

Yea, but its not a tuned circuit, at least not the feedback portion of it. It isnt supposed to do that. You can try DeOxit, otherwise you can replace the pot. you would need to match up the specs, and footprint to something modern on Mouser or Digikey. 

 

[AndiS]

OK thank you. I will take the pot out and see if I can find anything about its values and replace or clean it.

The story will continue after that ;-)

 

[AndiS]

So applying a generous amount of contact cleaner fixed the problem. The potentiometer seems o work as it should now!

This almost finishes this Classic's restauration. The only thing left to do, is to finally figure out how to make the SDcard extension cord working for the SCSI2SD (as in this thread)

By the way, I now have a working Apple SC40 Harddrive that was internal to this Classic. I would give it away for shipping or trade with some other retro Mac stuff. Anyone interested? Preferrably in Europe but this is no strict requirement. In any case, I won't throw it out, just put it into storage until it dies a quiet death due to old age ;-)

 

[monx]

I just ran in to this same problem with the same conclusion over the weekend. I just recapped a Mac Classic analog board, I had not powered it on before recapping, and had the "wobblyness" problem as  well, also more apparent at the top of the screen.

I found schematics and started checking voltages. The 5v and 12v rails were low. The 5v is the only one monitored via optocoupler feedback, I'd expect it to be nearly exactly 5v, it was 4.6-4.7v. I wasn't expecting this to be related to the monitor problem, but the monitor's voltages would be affected, any variation on 5v would affect the rest. So I checked the resistors and connections around the 5v feedback circuitry, everything looked good. I then decided to try adjusting PP1 with only an old hard drive connected to provide a load (mb disconnected) and the voltage jumped around from 4.5 to 5.1v randomly - likely bad or "scratchy" pot. I removed it, check it, sure enough, the resistance jumped all over - it's a 220 ohm potentiometer, resistance jumped to several k then several M as it was adjusted. Cleaning didn't work in my case. I substituted another one of similar value and was then able to set 5v correctly and the wobbly problem went away.

I would replace that potentiometer and not rely on cleaning it alone, you'll probably have trouble again.

Mouser and Digikey have almost the exact replacement as part number Amphenol Piher PT10LV10-221A2020-S. This appears to be almost the same one, but the leads are formed differently. According to the datasheet, the "V05" version seems to be the exact replacement. The leads on this one can probably be bent and may need to be extended. I'll be ordering a couple soon to try. Any suitable 220 potentiometer would probably be ok, but it should be one like this that can be adjusted form the bottom as from the top would be difficult and potentially unsafe.

I too am using an SCSI2SD. It's easy to set up and i used this method to load the OS on the drive, it made it quick and easy to get os 7.5.5 installed - ran in to some trouble with extensions, but that was easily solved.

 

[AndiS]

So its a 220 Ohm pot - thank you for the information. I'll order a similar looking one, and if need be, I can always extend the leads and solder it on "free-floating" or fixed with hot glue.

But what do yo mean by adjusting from the bottom? In my case I had to reach through and around the CRT and all the cables using a long plastic stick. That's dangerous enough for me although I tried to be careful.

 

[techknight]

i dunno, I suppose YMMV. ive cleaned pots before and have never had trouble. 

 

[monx]

On mine, there’s a hole under the pot on the pcb through which the adjustment tool/screw driver can turn it. This allows safe(er) adjustment so you don’t have to reach across the board like that. If the white protective plastic is in place, it covers the bottom of the pcb and blocks it.

Photo shows arrow pointing to access hole.  Also shows temporary pot - 1k with 330 across it to bring it close to 220. Normally that’s not there!

I’ve cleaned many scratchy volume controls with dexoit only to have the problem return later, so replacing is recommended since it’s failure could potentially damage much more expensive parts in the computer.

 

[HiTTeR]

I know this is an old thread but since I faced the "wobbly screen" thingy (after recapping everything) and busted my hoochie to find who the culprit was, I would like to share it with you.

The symptom was:
Power on -> wobbly screen, no beep.

At first I powered off right away.
Then powered on again.
Then off. Then on.
At SOME point, after a dozen of on/offs, it stabilized, gave me a beep and saw the arrow cursor.
At first I got happy.
But next day, when I first powered it on, it was again wobbling, no beep.
Like.... Capacitors had been discharged or something.
I repeated the above process and the result was the same.
After a dozen (or more) on/offs, it would stabilize and boot.

Next day, again the same.

At some point while trying to read the schematics, I said "#*(*#@( it, leave it on, while I'm searching the internet".

Lo and behold, after quite some time (I think it was 3 or 4 minutes of wobbling, maybe more), I heard a BEEP!

.... So, right now, the case is starting to build up and we have a few steady symptoms.

When the screen was steady (after the above efforts) I measured the 5 and 12v rails and they were 4,8 and 11,6.
I played a little with PP1 and could raise them. But the result was always the same.
On the next day, it would take again 3-4 or more minutes to "warm up" and stabilize.

I removed and measured all the diods. They were fine.

I started cussing again, so since I had read here about the optocoupler, I decided to change it (see attached image, I've put it on a stand...)

That did it.
When I turned it on, it was perfect!

So mind your optocouplers!


P.s. don't ask me why -prior to changing the optocoupler- after a while it would stabilize. I have absolutely no friggin idea.

 

[AndiS]

I know this is an old thread but since I faced the "wobbly screen" thingy (after recapping everything) and busted my hoochie to find who the culprit was, I would like to share it with you.

Ha and there I thought I posted the resolution already, but I didn't

In my case the issue was fixed by changing the PP1 potentiometer.

Also beware that I never had any boot issues - The machine beeped and booted just fine every time and the wobblyness was influnced by harddrive activity. This info might help some people in the future towards what to check first - Optocoupler or PP1. But in the end I did change both anyway.

 

[HiTTeR]

Since I can't edit the post, let me give a part of the solution that I forgot to mention.

I said above:

Quote: "When the screen was steady (after the above efforts) I measured the 5 and 12v rails and they were 4,8 and 11,6."

What I forgot to mention is that at some point I decided to measure those two rails on the "first" power up (after 24+ hours, when it would have the "capacitor discharge effect" as I wrote), and I saw that they were 3,9 and 10,5 respectively.

That gave me the idea that the voltage was not steady, but it was actually building up with time (as I said, after a while, the rails would come to "reasonable" power levels) but I could not figure out the "why?!" for the life of me.

Until the optocoupler was changed.


Playing with PP1 would not do much in my case, because when the voltages were low in the beginning (3,6 and 10,5) it would raise them slightly without solving the problem. Then, when the voltage would be steady, the "raised" PP1 would give 5,1 and 12,3 but it would be useless in my case, because after the "24 hours effect", the rails would drop again.

So what I'm trying to say is that PP1 should be adjusted when you already have steady voltages on those rails, and just trim the value to a just voltage.