Mac Classic with a wobbly screen after recap

[AndiS]

Hi!

I have a Mac Classic that is working fine but has a wobbly screen. The "wobblyness" is more apparent at the top and can be linked to Harddrive activity. Moving e.g. a white window in front of a dark background does not skew or wobble the screen!

I'm in the process of completely rebuilding/restoring the Classic from two non-working machines, so I'm not unhappy with my current progress, but this has to be resolved of course.

Here is what I've done until now:

• - The obvious cleaning and a bit of Retrobriting
  
• - Recapped the Logic Board - The caps already showed some minor leaking underneath, so it was high time.
  
• - Recapped the Analog Board. - I replaced all the electrolytics, quite a few of them had also started to leak alredy, but no visible damage yet.
  
• - Replaced he Harddrive with a SCSCI2SD which is working fine.
  

The wobbly screen was apparent before the re-cap. So even altough there have been some leaky caps, this was not causing the issue. Wobblyness is linked to drive activity, but since I'm using a SCSI2SD, power demand from the HD should also not be the issue.

Any ideas how to continue? I found some Threads about changing some addidional diodes and/or optocouplers, but I'm a bit lost there at the moment.

BTW: The Analog board is a 240V European model - There is a sticker with "630-0420 240V" on it. I'm aware that there are different revisions, I'm not sure which one this is. - Even more interesting, the back cover on the solder side states that changing the Jumper JP1 changes the configuration from 240V to 120V. I thought that these Logic Boards where non-interchangable?

 

[bibilit]

Classic AB boards are International, so yes you can change voltages.

Concerning your wobbly screen, DP3 and DP4 diodes are usual suspects, as well as the Optocoupler

 

[AndiS]

Classic AB boards are International, so yes you can change voltages.

OK I see. The thing is, I have another board from a 110 V Classic, that looks a bit different (Its number is 630-0525). The flyback transformer looks different and capacitor values are also not the same.

I take it, this is just the other revision I have read about. How can I tell that, and what is the difference between AB revisions? I've read that they are not interchangeable but why.

Regarding the Diodes and the Optocoupler? Is there any information available about which replacement parts to order? I could try to take them from the 110 V AB I have which is in a bad state and probably does not work anymore. But I'd prefer to get new parts if possible. Furthermore - is the Optocoupler the six-legged chip called "QP1" ?

Thanks for any more info!

 

[davidg5678]

Regarding the Diodes and the Optocoupler? Is there any information available about which replacement parts to order? I could try to take them from the 110 V AB I have which is in a bad state and probably does not work anymore. But I'd prefer to get new parts if possible. Furthermore - is the Optocoupler the six-legged chip called "QP1" ?

The Diodes and Optocoupler just happen to be parts that commonly fail. Replacements are cheap enough that it is probably worth swapping them all out with new ones, even if the existing components are still mostly working correctly.

DP3 and DP4 are called 1N4148 Diodes. These make it so that electrical current can only flow in one direction. If you type this part number into an electronics website such as Digi-key, suitable replacements are about 10 cents. https://www.digikey.com/en/products/detail/on-semiconductor/1N4148/458603

The Optocoupler is designed to transmit a signal across two different sections of the board while keeping them electrically isolated. It is basically an LED and Photosensor inside a piece of plastic. I think the LED loses brightness over time, and it eventually stops being bright enough to work. The replacement part is called CNY17G. https://www.digikey.com/en/products/detail/vishay-semiconductor-opto-division/CNY17G-3/4072867

The Optocoupler is the 6 legged QP1 component on the board labeled CNY17G.

 

[AndiS]

Thanks @davidg5678 for confirming the placement of the Optocoupler and also Links to the part numbers!

I know enough electronics to know what diodes and optocouplers do, but I'm not confident enough to be sure with what to replace them with if I don't have an exact part number. Now I can order the parts and replace them. Any idea how screen wobbling can be connected to harddrive (SCSI2SD) activity tough?

 

[Byte Knight]

I had a Classic with a wobbly screen and after I turned up the voltage (I forget where the adjustment is on the Analog Board) it worked perfect.

 

[AndiS]

Any more Information about how much you turned it up? I have not yet measured the 5 and 12 Voltage rails but I will do that when I have read up about how to do that - hopefully tomorow. Somewhere I read that one has to be very, very careful to not fry anything permanently when increasing the voltage. So I'm a bit afraid of doing so honestly.

I will also test the diodes after removing them, they look good, but you never know. I have no idea about how to check an Optocoupler tough. But all are cheap enough to just change them. I'll probably have to get a 100-pack of diodes to get a better price/diode due to shipping costs

 

[davidg5678]

I also read that adjusting the voltages can be really risky if done incorrectly. (It was something about a potentiometer labeled PP1 and the 5V and 12V rails going out of sync.)  I think replacing every obvious component first, and seeing where that gets you is probably a good idea. I just repaired my Classic's analog board, and I was struck by just how badly all of the capacitors leaked everywhere. There was greenish-brown sticky goop covering lots of components, and so I spent a ton of time cleaning it up. I didn't really test out my Classic's analog board before recapping it, because I was nervous about applying power to the leaking capacitors. Because of this, I am not sure whether or not the shaking screen problems you are experiencing are a unique problem, or just require a few more replacement parts to repair. I just replaced all of the capacitors, the DP3 and DP4 diodes, and the Optocoupler in one go. Thankfully, it seems to have worked so far. I think that there is also a controller chip labeled TDA4605 that could go bad. I did not need to replace this part, but I have read that some people have had success doing so. Unfortunately, it does not seem to still be in production, so you would have to buy it off of eBay as new old stock. Hopefully, this will not be necessary.

I think it might also be helpful to reflow all of the solder joints on the back of this analog board, just in case there is a crack or intermittent connection somewhere. There's no guarantee that this will fix anything, but it will at least rule out another possible culprit.

My hunch is that this is a problem with the Optocoupler chip causing the voltages to be too low/dip. I vaguely remember reading about a similar problem at some point, though I definitely could be wrong. Other people here know way more than me about fixing analog boards. Either way, pretty much every thread I have read about Classic analog boards reccomends replacing the diodes and optocoupler, so this is a good place to start.

 

[AndiS]

Thanks again @davidg5678 - I will do as you recommended. First replace all suspicious parts and only then try to turn up the voltage.

The AB I recapped was not looking too bad and was working (albeit wobbly) before. The caps were leaking just a bit from their underside and the goo was only visible after removing them. So this was not a big issue. The second (110V) AB I have, looks much worse. I also plan to remove the caps to prevent further damage to other components that might be valuable in the future. But I don't expect this to work ever again.

I'd still like to hear more ideas and more information about the different AB versions I have, so if anyone wants to chime in - the replacement parts will not arrive for a few days and I could still profit from more information!

Thank you all!

 

[Byte Knight]

Any more Information about how much you turned it up? I have not yet measured the 5 and 12 Voltage rails but I will do that when I have read up about how to do that - hopefully tomorow. Somewhere I read that one has to be very, very careful to not fry anything permanently when increasing the voltage. So I'm a bit afraid of doing so honestly.

It was a while ago, but the voltages were a little on the low side, so I just turned the pot up a smidge while the computer was on and it instantly fixed it.  I had already recapped both the logic board and the analog board without any improvement in the wobble.  Someone suggested reflowing the solder on the analog board's monitor wires but that didn't help either.  Then I read somewhere that it could be due to low voltage and bingo!

 

[bibilit]

Dp3 and Dp4 are often "leaky" so tested ok with a multimeter but bad in the end, given the cost of such items, i replaced those as a matter of fact.

The Opto and TDA replaced only if both diodes are not solving the issue.

 

[AndiS]

I have now replaced the Diodes and the Optocoupler and it did not help. The screen is still wobbly on Disk (SCSI2SD) Activity. The Picture now seems to be a bit narrower on the top too, I did not see this before.

Two things are notable:

1. I could not get a CNY17G-3, at least not quickly. So after reading through Datasheets I decided to try a "plain" CNY17-3 which is easily available from several producers. Good news - it works just as "fine" as before. According to the manufacturer, this is the replacement for the CNY17G-3 which is no longer made.
   
2. I measured the Voltages at the Floppy connector, and they are a bit low. I got 4,69 instead of 5V, 11,26 instead of 12 and -11,92 instead of -12.
   

My next step would be to increase the voltage a bit just as it was proposed above.

Any ideas what else I could try?

Any specific tips about how to safely increase he voltage?

Any suggestion what the problem really could be?

Tanks!

AndiS

 

[bibilit]

The 5 volt rail is best around 4,85 and 12 volt one can be low. 

Try using PP1 for a test, if you don’t see any improvement, you still have an issue elsewhere. 

 

[AndiS]

Success! - but more Questions

First off all - adjusting the voltage fixed the wobbly screen issue - Hooray!

But this did not work like I expected. Reading through the Classic Mac Repair Notes, I found that moving PP1 counter-clockwise should increase the voltage - although it does not specify that for a "Mac Classic".

What I experienced feels more like tuning a circuit. A bit to the left, the voltage was dropping, a bit to the right - same. I could even tune in the maximum just by listening to the fan speed! Is this expected?

In any case, the maximal voltages I could reach were 11.91 and 4,93 Volt on the 12 and 5 V line respectively.

What is the matter? Bad potentiometer? What could I replace this with if necessary?

 

[bibilit]

Hi, still an issue i think, but anyway as said before 4.85 volts is good enough for the 5 volt rail and the Classic is happy with as much as 11.8 for the 12 volt rail.

The important voltage rail is the 5 volt one, 

try to reflow here and there, you will probably be able to get better readings.

 

[AndiS]

Yes, but what about the fact that I had to "tune in" the maximum voltage?

It is not like described in the Classic Mac Repair Notes. Turning  PP1 to the left decreased the voltage, as did turning it to the right. There was a sharp maximum in the middle with the voltages written above.

For me this feels exactly like tuning a circuit to resonance. Is there a difference between the Classic AB and the earlier ABs described in the Repair notes?

 

[techknight]

Could be a bad potentiometer, developing dead spots. 

 

[dragonpunchu]

I have seem to have fixed my voltage issues by cleaning the AB again with massive ammounts of IPA. I now get good voltages if I adjust PP1.

 

[AndiS]

Could be a bad potentiometer, developing dead spots. 

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 seem to have fixed my voltage issues by cleaning the AB again with massive ammounts of IPA. I now get good voltages if I adjust PP1.

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?