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Recapped Macintosh Classic with checkered screen

claanu

Well-known member
4.8v is still low. I don't think summer heat is the issue either, it gets toasty in that case regardless of the weather. This stuff is almost always PP1 in my experience, but you've replaced it. What's 12V at with the new pot?

Have you tried cleaning and adjusting PP2? Contact cleaner and a few turns back and forth can do wonders.
 

goerz

Active member
Do you have a source or parts # for all those please?? :)
It's definitely a problem with my parts list: I just recapped another Classic board this week with the same parts, and I get the very same problem. I have a working board I recapped in August 2021, so I compared the August 2021 parts list with the June 2022 list. There are some differences, due to components availability. Now I am going to swap the different components one by one from the working board to one of the two that do not work. I hope to find the problem this way. The parts list can be found here:


Items that differ between the two lists are highlighted in red. (Descriptions were mostly downloaded from my order history on Mouser Italy, so it's in Italian, but you should be able to understand the technical data anyway).
 

kupouzar

Member
It's definitely a problem with my parts list: I just recapped another Classic board this week with the same parts, and I get the very same problem. I have a working board I recapped in August 2021, so I compared the August 2021 parts list with the June 2022 list. There are some differences, due to components availability. Now I am going to swap the different components one by one from the working board to one of the two that do not work. I hope to find the problem this way. The parts list can be found here:


Items that differ between the two lists are highlighted in red. (Descriptions were mostly downloaded from my order history on Mouser Italy, so it's in Italian, but you should be able to understand the technical data anyway).
Hey, have you solved it yet? I have the exact same problem. Which part was causing it?
 

goerz

Active member
Hey, have you solved it yet? I have the exact same problem. Which part was causing it?
Unfortunately, I haven't solved the problem yet. My parts list seems to be fine because the second analog board is working well now after replacing the TDA4605 chip (I hadn't it replaced initially because it's expensive and difficult to source for me). So the part causing the problem must be one I haven't replaced, but I don't know which one.
 

kupouzar

Member
Unfortunately, I haven't solved the problem yet. My parts list seems to be fine because the second analog board is working well now after replacing the TDA4605 chip (I hadn't it replaced initially because it's expensive and difficult to source for me). So the part causing the problem must be one I haven't replaced, but I don't know which one.
Did you try powering it without the grounding point of the outlet? I know it's dangerous but I found out that if I do it, the voltages ramp up way faster and it's more stable. I have 120V AC between ground and neutral though. So there must be a short somewhere...
 

goerz

Active member
Did you try powering it without the grounding point of the outlet? I know it's dangerous but I found out that if I do it, the voltages ramp up way faster and it's more stable. I have 120V AC between ground and neutral though. So there must be a short somewhere...
I'm not sure I want to try that :) My board is a 220V version by the way.
 

kupouzar

Member
I'm not sure I want to try that :) My board is a 220V version by the way.
Mine is also 220V. But I think the multimeter got the measurement wrong as it might not necessarily be at 50Hz. Anyway, if you try it, just be sure not to touch any metal parts. I've found this tip in this thread where it was "solved" using a space heater at first.
 

stepleton

Well-known member
it might not necessarily be at 50Hz

Utilities work pretty hard to keep line frequency constant: if you lose more than half a hertz or so, you are having a Very Bad Day at the Power Company and generators will start to trip and go offline. This can have dramatic consequences. (ETA: I remembered wrong, that failure had little to do with synchronisation, but it's a great story and worth a watch anyway!)

One cool thing is that you can visit websites to see the line frequency live. Line frequency synchronisation covers enormous areas, so this frequency number is for most of continental Europe. As OP is "From Italy" per their profile, this value very close to 50 Hz should be what you see when you measure --- and it should be the same value for the Czech Republic. Here's the situation in the UK, meanwhile.

Also interesting: logs of exact mains frequency has sometimes been used for forensic purposes.

220/230/240V at a frequency other than 50 Hz is not unusual but not all that common as these things go.
 
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kupouzar

Member
Utilities work pretty hard to keep line frequency constant: if you lose more than half a hertz or so, you are having a Very Bad Day at the Power Company and generators will start to trip and go offline. This can have dramatic consequences. (ETA: I remembered wrong, that failure had little to do with synchronisation, but it's a great story and worth a watch anyway!)

One cool thing is that you can visit websites to see the line frequency live. Line frequency synchronisation covers enormous areas, so this frequency number is for most of continental Europe. As OP is "From Italy" per their profile, this value very close to 50 Hz should be what you see when you measure --- and it should be the same value for the Czech Republic. Here's the situation in the UK, meanwhile.

Also interesting: logs of exact mains frequency has sometimes been used for forensic purposes.

220/230/240V at a frequency other than 50 Hz is not unusual but not all that common as these things go.
My mains frequency is totally fine. I just thought maybe the frequency of the short is different because my CRT is wobbling like jelly and the frequency of that is definitely not 50Hz as it is not flickering that fast and the wobble is clearly visible. I don't own a scope though and didn't measure it. But I've seen someone else who had a similar problem did and measured 10Hz.
There's must be some bad component somewhere. The analog board has been recapped and completely cleaned using IPA. I also don't think it is the optocoupler or the TDA chip as the situation doesn't change when they're heated. I guess I'll try the air can method someday.

EDIT: it was this thread. The wobble looks exactly the same.
 
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kupouzar

Member
I'm not sure I want to try that :) My board is a 220V version by the way.
So I've got an update! This problem is apparently caused by the optoisolator and/or diodes DP3/DP4. I've just replaced these parts and my voltage is now perfectly stable and cold boot finally works. Btw, I used CNY75GB, not CNY17.
So in order to eliminate issues with wobbly screen and cold boot not working, just clean the voltage pot with IPA, set it to the minimal value, replace CNY75GB, DP3, DP4. Then turn the mac on and turn the pot clockwise till the voltage is at 5.0V. Also check that the 12V rail is OK (around 12.7V without HDD connected). If this doesn't fix the wobbly screen, replace the big filter cap/s as well.
 

goerz

Active member
I have already replaced the optoisolator and diodes DP3 and DP4, to no avail. I also replaced the voltage regulator. So the plot thickens...
 

goerz

Active member
I recapped one of these recently and found that the QP2 scr and DP13 600v diode as well optocoupler needed replacing
I've already replaced QP2, I'll check DP13 as soon as I'll resume this project. Could you help me identify the correct diode to order as a replacement? Thanks.
 
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