68kMLA Classic Interface

This is a version of the 68kMLA forums for viewing on your favorite old mac. Visitors on modern platforms may prefer the main site.

Click here to select a new forum.
Macintosh 128k - Analog Board Issue
Posted by: BadGoldEagle on 2016-06-19 10:34:11
Hi

I was working on my 128k, preparing floppies and stuff, when suddenly the screen blacked out. Can't get it back up working even after letting it cool down for an hour.

Apart from the screen, the computer works fine. It can boot and it chimes all right.

I noticed the two filter caps were bulging. Can they be responsible for what's happening here?

That is annoying. I wanted to show it off to some friends tuesday evening. I have to fix it by then.

Tomorrow I'll be travelling back to my parent's place to pick up my Plus. I think I'll just swap ABs for now.
Posted by: fri0701 on 2016-06-19 11:48:30
Although filter caps do frequently go bad and should be replaced, I doubt that's the problem here.

Does the screen flicker back on if you tap the right side of the case? If so, you might have the typical problem of the flyback solder joints cracking. Reflowing the solder joints around the flyback and the connector J1 should fix it. Since you said the computer functions just fine, just with no video, I'd bet this is the case.

-fri0701
Posted by: BadGoldEagle on 2016-06-19 12:02:21
Nope, hitting it won't help at all.

The screen just went black all of a sudden. 
Posted by: BadGoldEagle on 2016-06-20 03:58:21
So I swapped the boards. It didn't do much.

I noticed that if you hit the CRT itself it comes back. 

This analog board had good solder joints last time I checked (2 weeks ago), but now every time I try hitting the side of the cabinet the screen jitters. How odd?

I think I either have a bad CRT or bad solder joints on the yoke connector. What do you think?

What's good about this is that I can still use it. At least for a few days.
Posted by: bibilit on 2016-06-20 07:02:21
Did you kept the Yoke connector from your 128 when replacing the Analog board ?
Posted by: BadGoldEagle on 2016-06-20 07:44:16
Yes I did. I think that's the problem.

I also kept the LB power cable but it can't be causing this kind of trouble.
Posted by: bibilit on 2016-06-20 08:13:52
You can probably replace one, then the other to locate the culprit.

I have a spare of each if required.
Posted by: BadGoldEagle on 2016-06-20 08:41:39
I also kept the neck cable. Forgot to mention that. I think it's interchangeable too.

The CRT's a Samsung. They had a special yoke. The connector is soldered to the yoke. It looks like this:



Maybe we could reflow the solder joints on this thing? (just 4 cables)

Could the yoke itself be toasted?

Now I'm afraid the CRT itself might be dead ?  🙁  I checked the glass tube (near neck connector), I can see the filament all right.
Posted by: unity on 2016-06-20 11:04:45
The yoke connector is the end piece. It pulls off. The other one forward is the deflection coil connection.

If you power it up, regardless of displaying anything, the very end will light up a bit - orange. Check for that. Also if there is any black specs near the center on the front, then the vacuum was blown.
Posted by: BadGoldEagle on 2016-06-20 11:30:02
If you power it up, regardless of displaying anything, the very end will light up a bit - orange. Check for that. 
I can see the orange filament. 

Also if there is any black specs near the center on the front, then the vacuum was blown.
What do you mean, on the display or the glass neck? I didn't really check the neck but the display itself is as sharp as it always was. No black spots on the display when it works.

I noticed that sometimes it starts up all right, and hitting the side or the CRT won't unsettle it at all.
Posted by: bibilit on 2016-06-20 13:15:13
I noticed that if you hit the CRT itself it comes back.
probably bad connection then.
Posted by: BadGoldEagle on 2016-06-22 03:11:56
Interestingly enough, it worked fine when I got there. Just as good as new!

I'm going to put the original AB back inside today, see if it makes a difference.

Odd fact: I can hear the flyback "discharging" when I flip the switch with both boards.

But it doesn't happen when the Plus AB is connected to the Plus??
Posted by: BadGoldEagle on 2016-07-10 05:02:53
So I did put the original AB back in there. Didn't work  🙁

Left it on 'bout a minute, started to smoke... So the AB failed.

I guess the flyback finally gave in... I'll have to replace it. Can't be a bad solder joint, right?

Yoke's fine BTW, since it worked flawlessly with the Plus AB.

I'm going to practice my soldering skills on my newly acquired LCII, then on my Classic II (AB sorta working but is going fail completely really really soon!).

Need to get myself an iron first though before doing all of that!
Posted by: Johnnya101 on 2016-07-10 05:22:27
Ooo. Double hit!

Once one of those CRT related things fail, something else is going to. I guess its better than it failing when your showing your friends! 

"Hey check this out Bill!"

Turns around

CRT blanks out

Did the CRT blank out or change when it started smoking? Its possible its a few capacitors, not the FBT.
Posted by: BadGoldEagle on 2016-07-10 08:28:36
I think this is still the same problem, only worse this time. The CRT actually blanked out all of a sudden. I turned around to pick up some manual and 3 seconds later, nothing on the screen. Thank god it worked when I was showing it off to some friends!

Well I'd better replace everything on that board first then.

I'll go through the usual suspects (C1 etc...), and replace all caps on that board (including those pesky filter caps!!)

I know that when the flyback goes it usually takes other stuff along with it...

The most worrying thing is the BU406 transistor, which is impossible to find now. There is an alternative but it won't look pretty. Hope this guy is all right!

The Mac got hot rather quickly, and was particularly hot just around the flyback. Was cold everywhere else. 

As I'm sure everyone here knows, there are vents on top of the machine, and the left one when facing the computer (AB side) was really hot just in one place, right above where the flyback sits....
1