Mac SE Analog Board Troubleshooting

[eraser]

Hi all.

I recently recapped a Mac SE's analog board and am having some trouble with it. When I turn the Mac there is no bong and the CRT lights up with a garbled display. After a few seconds the CRT will begin to dim. I tried adjusting the brightness knob at this time to see what would happen and I get a rolling/jittering image.

Usually my recapping endeavors go really well so having things go wrong on such a simple board is really disappointing. I've double-checked all the cap orientations and everything seems fine. While I was recapping I also freshened up all the cold solder joints I saw.

What should I be checking?

Thanks!

 

[LaPorta]

First: did the machine work prior to this?

 

[eraser]

First: did the machine work prior to this?

It had been in an attic for years and I wanted to rework it before trying to turn it on. I've learned painful lessons from trying to turn on vintage equipment that has sat idle for years. () I suspect that it would not have worked prior to recapping.

 

[LaPorta]

Did you also recap the PSU?

 

[eraser]

Did you also recap the PSU?

Yes. The PSU seems to be working correctly (voltages look good). I also swapped another confirmed working SE PSU on the board and got the same result. So it looks like the fault is somewhere on the analog board itself.

 

[LaPorta]

You also tried the logic board of this machine with the known-good analog/PSU? Just trying to rule anything out.

 

[eraser]

You also tried the logic board of this machine with the known-good analog/PSU? Just trying to rule anything out.

Not yet. I will try that next.

 

[eraser]

You also tried the logic board of this machine with the known-good analog/PSU? Just trying to rule anything out.

Just tested ... logic board works in a known-good SE case. Analog board problem confirmed.

 

[LaPorta]

Ok, yes indeed. I'd verify that each and every cap was the correct rating. Also, is there any chance you have a solder bridge somewhere between components you just replaced?

 

[eraser]

Ok, yes indeed. I'd verify that each and every cap was the correct rating. Also, is there any chance you have a solder bridge somewhere between components you just replaced?

Yes. I double checked every cap ... orientation is correct, specs are correct. There are no solder bridges. I also checked continuity for all of them along the traces and every one of them tests good.

I am going through Larry Pina's Dead Mac Scrolls now to see if I can identify any commonly failed components.

 

[LaPorta]

Also: while the machine is on, check the voltages at the floppy port. You can find that in Pina's book as well.

 

[bibilit]

Don’t remember a 100 % but one of the caps is not standard (not polarized) IIRC

 

[eraser]

Don’t remember a 100 % but one of the caps is not standard (not polarized) IIRC

Yes, C15 ... the 3.9uF cap at the top of the board. I replaced it with an appropriate film cap.

 

[eraser]

UPDATE.

I found a working SE analog board to use as a comparison. I tested continuity and resistance with a multimeter on all the components on the board and found some distinctly conflicting values near the flyback shield.

CR7 - In circuit on a healthy board I get 4.26MΩ going from red to black (direction of the diode). I get 7.37MΩin reverse.
On the faulty board I get 15.70MΩ going red to black; In reverse it's highly unstable. It jumps wildly from 10MΩ to 20MΩ then goes to open circuit.

I also found that resistance across the other two rectifiers in that circuit (CR4 and CR5) are also wildly different in the healthy board vs the failed board. Since they are all in the same circuit I am not sure if a rectifier itself is to blame or if it's a nearby component. This does narrow down the location of the problem and it's consistent with the observations on the CRT.

(CR7 and CR4 are both RGP01-12 fast switching rectifiers and are no longer manufactured although there may be a modern equivalent.)