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Four macs experiencing the same thing..... this tells me it is likely not a random fault like a crack somewhere but more of a faulty component issue. Were all four macs affected by bad capacitor leakage or are some very clean and dint need heavy cleaning?

I didnt replace DP5 and DP7 (dont have the new ones - will buy them). With the optocoupler, I soldered at 260c and each pin for 1s - 1.5s. I usually solder at 300c. I didnt blow on DP6 but I did replace it during my initial recapping. CP24 has already been replaced - I forgot to mark it. I...

Could not make myself wait to get the socket until tomorow and resoldered a new CNY17 (did it quickly and carefully) - same issue. Took off DP5 and DP7 diods (near all the caps) and cleaned the electrolyte that came out of the leaked caps under. When initially recapping, I missed this. My...

I am starting to suspect the optocoupler (CNY17) - it is in the middle of the air blowing square :). Perhaps I could have damaged it (twice) when soldering it in? I will get a socket and put a new CNY17 onto it.

How did you go with it?

Update: replaced CP21 and CP40 .... same thing. Cold start - low voltage 😭 What I did afterwards is: take two paper straws and insterted one inside the other to make it long and started blowing on components one by one while the Classic was on. What I found is that it was VERY sensitive to...

Looks like my local electronics store has the ceramic caps I need... dont need to wait for it to me posted. I will also try and get the CP40 too while I am there. As I have already changed the QP2 = IRFBC40 .... this will hopefilly be it (fingers crossed)

Yeah I did adjust PP1 but that didnt fix the cold start issue. Correct about the ticked components - they have all been replaced as well as most caps and My board is 820-0395-A (early) so CP21 is 6.8nF then. I have also found this thread (same issue as mine) but fix was in a totally different...

I changed my attention to CP21 (B 682K 08). I noticed there is a very small crack next to one of the legs and when I was using freezing spray on it - the voltage would drop instantly. I do not know if this will lead anywhere but after I took it off the board and used freeze spray on it its...

Update: received DP14 and DP11 in the mail and changed them. First DP14, then DP11. Unfortunately, exactly the same behaviour - low 5V (4.5v) and checked board + wobbly screen. Once the Mac warms up - voltage goes up and the mac boots up properly. Wobble also goes away after mac boots up and...

Thank you for following my updates and giving very useful suggestions. Fingers crossed for the DP14 fix. I will definitely post back once I swap it out.

Update: changed QP3 and QP5 - same behaviour from Mac Classic. After that, I played with temperature of DP14 which is on the same path as QP3 and QP5 and rapidly cooling down of DP14 seemed to cause instant drop of voltage from 5.1v to 4.5v. I was trying to target DP14 with my freezing spray...

yeah I have already replaced these. Thank you for the schematics - I will try to make sense of it .... I am very new to radio electronics. Been going on common sense and logic so far without any fundamental knowledge of components etc.

The restarts could be happening from the attempts of the Classic to boot and due to low voltage failing to do so. It is repeats and repeats due to voltage being too loo and flactuating around that value.

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.

I actually replaced the CNY twice: 1) CNY17-3 - ebay 2) CNY17G-3 - https://www.digikey.com.au/en/products/detail/vishay-semiconductor-opto-division/CNY17G-3/4072867 Same result for both - currently I have 2) installed. Thank you for the tip, I will start on finding transistors/diodes to replace.

I had already replaced the CNYxxxx QP1 :( before replacing the TDA

Did you end up fixing it?

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...

Thank you - I will will update on progress