Macintosh (Plus) analog board blown c8

[futurebiscuit]

I was hoping someone could give their expert opinion on a fault with a Macintosh Analog board from a Mac Plus I just purchased.

Visually everything looks to be in great condition. The only things I've noticed are the following:

* Capacitor C8 is completely destroyed.
* Corrosion on battery holder (leaked battery?).
* Maybe some slight corrosion on the T1 transformer (see image)?

I was hoping this might be fixed by just replacing the C8 capacitor? but as this component is connected to the T1 flyback transformer I guess it is a symptom of it failing?
From the basic research I've done I gather these transformer are not made anymore, and are an expensive thing to replace?

Any insight on what the problem is would be much appreciated!

 

[fri0701]

At least in my experience, the flyback transformers in these machines are pretty hardy. I wouldn't worry about it until all other avenues of repair have been explored.

For the corrosion on the battery holder, I would take a cotton swab with vinegar and clean off as much as you can. The important thing to do now is to remove any remaining corrosion on the board (and of course, remove the battery!).

I would say the next step to take is to clean up the board around C8 and replace the capacitor. I don't have a Plus analog board out at the moment, but I imagine either the schematics are online or another member here could point you toward a suitable replacement.

Also, I noticed that your board is marked 630-0108. I believe this is the international model, although I could be wrong. If this is the case, be absolutely sure the voltage it runs on is correctly configured for your area (i.e. 120V or 220V). Having this configured incorrectly could damage the board and result in more repair headaches for you down the road.

Good luck!

 

[Brett B.]

MMM crispeh  :lol:

I'm guessing it melted down like that because it failed and was left powered on for an extended period of time.  Replacing the cap will more than likely fix it.  I had an analog board cap blow on an SE and it stayed powered on over a weekend at my office, but it wasn't that bad.

Wouldn't worry too much about the corrosion, clean it up like was mentioned, should be fine.  The Plus I just got has the same crusty stuff on the flyback transformer but it seems to be working just fine.

 

[techknight]

C8 is the victim of a catastrophic failure somewhere. it has seen massive voltage, either that, or it physically shorted internally and blew apart. It can happen both ways. 

But usually they dont do that unless they see way more current/voltage than designed for. So chances are, something else is damaged in the circuit as well. 

It is a high likelyhood the flyback went bad taking out C8 by overvolting it. Its hard to say without extensive troubleshooting. 

The other better possibility is CR9, and CR11 could be shorted. 

Judging how dark brown the glue is on the flyback transformer, I would suspect that first. The darker the glue, the higher the amount of hours on it is. 

 

[futurebiscuit]

Thanks for all the replies!

Yes it’s a International board with 240v configured (I’m in the UK).

I'll try replacing the C8 capacitor and test the adjacent diodes.

I found the following parts list, although some things are missing (has the C8 though). Are there any additional resources with a full list?
https://docs.google.com/spreadsheets/d/132RwzC8HM5ask-BdY_31txErOCwJDSkz099GY2XLpE0/pub#

Anyway I'll order the parts and keep you up to date on the results (probably will be a month till I get time to look at it though).