SE/30 (SE) Analog Board Damage

[goncalo]

I noticed this burned area on the Analog Board.

Is this common?



Everything else on the AB is fine and dandy, as is the Sony power supply that sits underneath.

I guess I'll have no problem replacing this Resistor (R29, 2W, 12Ohms 5%) and Linear Voltage Regulator (U3, 7905 -5V 1A) – even though some vias in the bottom of the board where also severely burned.

I also noticed the fan wiring has a different colour. The ones I saw elsewhere are yellow/black and this one is red/black – was it swapped?

Analog Board model identifier:

Macintosh SE Analog

820-0206-B 630-0147-C

© 1986, 87 Apple Computer

 

[apm]

R29 and U3 collectively generate the -5V supply to the logic board from the -12V rail. If something shorted -5V to ground, the regulator could supply about 1A which would overload R29 and turn it into a big heating element -- about the result that you see there.

After removing the parts, check that there's no short from P4 pin 6 to ground or to P4 pin 7. Then check the logic board for a short on the same signal (P4 pin 6, the -5V rail). There's nothing else on the analog board that uses that output AFAIK so oddly enough it was probably a logic board fault.

 

[goncalo]

Hello APM,

Thanks for your reply.

I'll be looking more thoroughly for eventual shorts to ground around that area and in the wiring and Logic Board.

Hope it’s repairable :-/

By the way, I’m looking at these components for replacement:

• MC7905CTG (Linear Voltage Regulators 5V 1A Negative)
• OY120KE (Ceramic Composition Resistors 2watt 12ohm 10% Surge)

Any corrections/suggestions?

Thanks!

 

[goncalo]

I'm a visual guy..

Here are some photos of the affected area.

Macintosh SE/30 Analog Board - Bottom Scan


 
Macintosh SE/30 Analog Board - Top

 
Macintosh SE/30 Analog Board burnt via

 
Macintosh SE/30 Analog Board burnt resistor

 

[apm]

Those parts should work fine. It's even possible that the 7905 on the board still works -- they are designed for current limiting without damage -- but it's a cheap replacement so why take the chance? Definitely replace the resistor.

The easiest way to check for shorts is to attach the logic board to the analog board (but don't power it up!) and then test for continuity between P4 pin 6 and pretty much anything else on that connector.