Mac Classic checked board issue even after recap

[gkmaia]

I have replaced some leaky capacitors on the analog and digital board of this mac.

I did check some of the key voltages on the switching power supply block that seems to be inline with that Bomarc schematic. 

One test I did was to disconnect the +5 and +/-12 lines from the analog board and feed the digital board with clean power from my HP power supply. Computer worked just fine. Definitively something wrong with my analog board.

One thing that I noticed was the positive end of the main bridge rectifier has a massive positive sine. As if the CP1 coupling capacitor was doing nothing to clean it. I did test the rectifier and replaced CP1 and got the same result. 

Any ideas?

 

[bibilit]

Hi, just to understand, are you getting all the right outputs from the AB board ?

So the +5 +12 and -12 are present ?

 

[gkmaia]

They are there but do not align. If I adjust the POT I cannot get them all balanced.

Find the +12 has a bit of a ripple.

I did test Dp11, Dp5, Dp7, Dp4, Dp8, Dp12, Dp2 on a good curve tracer and they are good.  The feedback Mosfet seems ok under basic testing. Haven't got it to the curve tracer.

Only weird behaviour I found on the RP6. It is leaking at around 70v when reverse biased. Not sure if that is a problem as that wind does the 5v rail. Also none of the signal ones leak reversed and they all share the same peak voltage.

At pin 5 of IP1 I am not getting the suggested 100Khz/5vpk square. I am getting the same that goes in pin 6 which is the plot attached. And that is really confusing because IP1 is the feedback controller. If it was faulty I would not be getting feedback therefor no 5,12 volts at the secondary and my SMPW would be completely nonfunctional.

Will the SMPW work and provide secondary voltages is IP1 is faulty?

Here is a video of the system booting when the digital gets external feed. So digital board seems totally fine. 

https://www.youtube.com/watch?v=Rgj3HN0fEqE

 

[bibilit]

So far I have found many leaky diodes on the Classic AB, most were tested ok. 

Dp3 and Dp4 most of the time. 

The Opto CNY17 goes bad also

 

[gkmaia]

Dp3 and Dp4 signal diodes are fine on mine. Reverse biased up to 100vdc and no leak. CNY17 is also good. 

I think my culprit is the signal out of IP1 pin 5. 

Any other ideas?

 

[bibilit]

ok, so i think you can replace IP1, in my box of spares i have new ones and have replaced a few, did you replace the capacitors around DP3 and DP4 ? (CP4 and CP5)

Part number is TDA4605, and yes pin 5 is driving the Mosfet.

I have seen the video and the wobbly image.

Those Classic AB boards are a nightmare, still have myself a few that i have been unable to fix permanently.

POT I cannot get them all balanced.

Are you talking about PP1 ? be careful don't tweak it to much, if the 5 volt is too low and something is wrong with your board, the 12 volt line will go up well above the required voltage.

The classic is happy with a 5 volt line around 4,85 volts , while the 12 volt can be a bit above but not much.

 

[gkmaia]

I ordered some TDAs. 

At this stage as it is impossible to trace where the problem is so removed every semiconductor on the board and will get them tested on a curve tracer. 

If none of the fail I will mode to inductors, then ceramics, then resistors. 

I will put it completely apart till I find what is wrong there. 

 

[techknight]

I ordered some TDAs. 

At this stage as it is impossible to trace where the problem is so removed every semiconductor on the board and will get them tested on a curve tracer. 

If none of the fail I will mode to inductors, then ceramics, then resistors. 

I will put it completely apart till I find what is wrong there. 

I admire the effort. But it sounds like throwing a battleship to something that only needs a handgun. 

 

[gkmaia]

Yes, you are right. But was also good to get to know the board better. Really did not need to do that but I find fun putting a board apart like that.

In the end was the TDA4605.