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Recapped Macintosh Classic with checkered screen

Fred1212

Well-known member
I got it from Digi-Key their part number is BYW76TAPGICT-ND It's a Diode Avalanche 600v 3A and its near QP2 just a small blob with wires
 

goerz

Well-known member
Thank you for the detailed information. In other news, this morning I switched it on for the first time in two weeks, and it worked! I don't know what happened: I turned it on several times during the day, and it always worked. The voltage is rock solid at 5.071V.
 

alexGS

Well-known member
I recapped one of these recently and found that the QP2 scr and DP13 600v diode as well optocoupler needed replacing
DP13 on the boards I have is beside the heatsink for the 12v voltage regulator, seems to be a 1N4001 according to the Bomarc schematic.
QP2 is a MOSFET that’s some distance away. I wonder if you meant DP12? - that’s a BYD34J in the schematic.

I’m chasing the same problem as everyone else. Have replaced caps, DP3/DP4, optocoupler, and still have low voltage (4.3V)/wobbly screen
 

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xeiter

Active member
Thank you for the detailed information. In other news, this morning I switched it on for the first time in two weeks, and it worked! I don't know what happened: I turned it on several times during the day, and it always worked. The voltage is rock solid at 5.071V.

How is your Classic now? I am in the exactly the same situation as you: fully recapped analog board together with all the other components your replaced (except PP1) and getting a 4.7v checked board with wobbly screen.

Voltage slowly rises and after about 10-15 mins when it reaches about 5.05v it boots. The voltage reaches 5.09v and the flacuates around that a little (5.06-5.09v). The screen still wobbles though. After that it boots straight away if I turn it off and on again.

I have not tested the cold start again - will do tomorrow morning.
 
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goerz

Well-known member
How is your Classic now? I am in the exactly the same situation as you: fully recapped analog board together with all the other components your replaced (except PP1) and getting a 4.7v checked board with wobbly screen.

Voltage slowly rises and after about 10-15 mins when it reaches about 5.05v it boots. The voltage reaches 5.09v and the flacuates around that a little (5.06-5.09v). The screen still wobbles though. After that it boots straight away if I turn it off and on again.

I have not tested the cold start again - will do tomorrow morning.
As I wrote, I don't know what caused the problem, and I don't know why it got away! After a couple of weeks of testing, I sold that particular Classic (I have another working one in my collection). Now I'm working on another one with logic board issues, but voltages are not a problem this time.
 

alexGS

Well-known member
Yes. The two I was trying to fix are still exactly like that. Slowly-climbing voltage, screen still wobbling even when over 5v. I don’t get it.

In the split second when there is still a picture after turning off the power, it doesn’t wobble. It is also not quite the same wobble as shown on the Mac Plus in the other thread - the wobble I have is both vertical and horizontal.
 
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xeiter

Active member
When using a hairdrier on the analog board, I can instantly bring the voltage to 5.1v and the mac boots well and no wobble. I did see a similar post here. Currently looking into why it could be.

The colder the weather is the lower the initial voltage is. I tested it today by leaving my Classic in a half open garage overnight. It was +7C in Sydney, Australia where I am at night and the Classic gave me 4.2v in the morning.

When the weather is warmer (when I check Classic during the day afetr it's warmed up) it usually starts from around 4.7v.
 
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xeiter

Active member
Experimented with freeze spray to try to find the fault cause of cold start with low voltage....

After freezing all the areas of the analog board while having the mac running found that freezing this area resulted in an instant voltage drop down to 4.6-4.7v and the mac going back to how it starts cold: wobbly screen and checked board.

Once I heat the area up (with hairdrier), it goes back to normal. Was able to consitently recreate the issue by spraying this area. Spraying other areas didn't cause to drop the voltage - I was not very thorough with other areas though since I found this one.

So I guess I will be re-soldering those high voltage cables and other things around them tonight.

1683703058337.png
 

kupouzar

Active member
Experimented with freeze spray to try to find the fault cause of cold start with low voltage....

After freezing all the areas of the analog board while having the mac running found that freezing this area resulted in an instant voltage drop down to 4.6-4.7v and the mac going back to how it starts cold: wobbly screen and checked board.

Once I heat the area up (with hairdrier), it goes back to normal. Was able to consitently recreate the issue by spraying this area. Spraying other areas didn't cause to drop the voltage - I was not very thorough with other areas though since I found this one.

So I guess I will be re-soldering those high voltage cables and other things around them tonight.

View attachment 56346
Re-soldering anything will most likely not help. The OP1 optocoupler is almost certainly at fault. You'll have to replace it.
 

kupouzar

Active member
The OP1 (TDA4605) ? Yea I am still waiting on the delivery of it so I can replace that too.
Exactly, the optocoupler is an important (if not the main) part of the voltage stabilization mechanism in a switching power supply and the LEDs which are inside of it can go bad with time. If the voltages aren't correct after replacing the OP1, you can adjust all of them using the PP1 trimmer. Be careful though, it is quite sensitive. And also don't use a metal tool.
 

xeiter

Active member
Exactly, the optocoupler is an important (if not the main) part of the voltage stabilization mechanism in a switching power supply and the LEDs which are inside of it can go bad with time. If the voltages aren't correct after replacing the OP1, you can adjust all of them using the PP1 trimmer. Be careful though, it is quite sensitive. And also don't use a metal tool.

Thank you - I will will update on progress
 

xeiter

Active member
Update: ok so I received the OP1 (TDA4605-3) and PP1 in the mail as well as the large capacitor (220uf/400v) and installed them into my Mac Classic.

Also, re-flowed those cables I mentioned in my previous post.

And......... exactly the same behaviour: cold start voltage is low 4.12v now with the new PP1's default setting (was turned all the way down). I player with PP1 and was able to get it to around 4.5v.... Then waited for the Classic to warm up, the voltage gradually went up and the Classic eventaully booted no problems.

If I forcing the Classic to warm up with the hairdrier - it boots up instantly.

Something else, when cold in my Mac Classic, causes the voltage to be low.... back to the drawing board.
 
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kupouzar

Active member
Update: ok so I received the OP1 (TDA4605-3) and PP1 in the mail as well as the large capacitor (220uf/400v) and installed them into my Mac Classic.

Also, re-flowed those cables I mentioned in my previous post.

And......... exactly the same behaviour: cold start voltage is low 4.12v now with the new PP1's default setting (was turned all the way down). I player with PP1 and was able to get it to around 4.5v.... Then waited for the Classic to warm up, the voltage gradually went up and the Classic eventaully booted no problems.

If I forcing the Classic to warm up with the hairdrier - it boots up instantly.

Something else, when cold in my Mac Classic, causes the voltage to be low.... back to the drawing board.
Actually the TDA4605 is not the optocoupler, that's just a power supply controller. It can go bad but this fault isn't that common. The component that I meant is the 6-pin OP1 IC, labeled CNYxxxx. I've replaced mine with a CNY75GB which is compatible and quite easy to obtain. Sorry for the misunderstanding, I didn't remember the codes. See the green part in the picture.

EDIT: you can buy it on Aliexpress
EDIT2: I'm also not exactly sure whether the label says OP1 or QP1 as it's a bit hard to read
EDIT3: So it is in fact QP1 :censored:
mac_classic_analog.jpg

Screenshot 2023-05-15 at 20.40.37.png
 
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kupouzar

Active member
I had already replaced the CNYxxxx QP1 :( before replacing the TDA
What number did you exactly use? Are you 100% sure the replacement part isn't from new old stock? Anyway, if it works after heating up that area, it must be some of the semiconductors. I'd try replacing the transistors/diodes as well.
 

xeiter

Active member
Update: the Classic was in the garage all day turned off and it's gotten quite warm today (+22C). When I tried to switch it on - it just booted ok straight away.
 
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