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.I recapped one of these recently and found that the QP2 scr and DP13 600v diode as well optocoupler needed replacing
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.
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.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.
Re-soldering anything will most likely not help. The OP1 optocoupler is almost certainly at fault. You'll have to replace it.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.
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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.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.
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.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.
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.I had already replaced the CNYxxxx QP1 before replacing the TDA