Power Supply takes a while to reach full output voltage

[benanderson89]

The Macintosh Classic I've been working on is now done. All the large electrolytic caps on the analogue board and the caps on the logic board were all replaced. Voltages measure stable and within spec from the floppy disk port. C2 on the CRT neck board was also replaced.

However I've noticed that when the computer is left switched off for a long period of time, the voltage readings are very low, the screen is the wrong shape, it shakes violently and there's a chequerboard pattern. If you wait, the voltage eventually creeps up and it boots (but the memory check takes a while), wait even longer and the screen eventually returns to the correct shape and stops shaking.

Anyone know what could cause such an issue?

 

[Garrett]

I read something on one of the FB groups about a diode (specifically DP6) going bad, which can cause issues you're noticing. It's a MB4 1045 S. Perhaps that diode has gone bad?

 

[benanderson89]

You know, I think I meant to replace that and never got around to it (partly because I couldn't find it since it's one of the small ones). I'll open the case tomorrow and test the diode.

EDIT: Wait, no, the giant one with the heat sink. Why did I think one of the little ones? Yes, there was a lot of gunk around that area of the board and it'll absolutely be that diode I forgot to replace. I'll remind myself to order a part tomorrow.

 

[bibilit]

Never had an issue with MBR1045.

Your issue is more likely related to diodes DP3/DP4 or the Optoisolator.

 

[benanderson89]

Do you know the part numbers for DP3 and DP4? I may as well buy all the parts. I have two new optos in a baggy in the garage so I may as well buy all the diodes and just replace everything in one shot if I have to spend the time setting everything up anyway.

 

[bibilit]

1N4148

 

[benanderson89]

Thank you! I'll get those ordered.