Mac Classic II - CRT went black, but why?

[Superdos]

About two years ago I liberated a Classic II from a yardsale for $10, which included a mouse and keyboard (now currently lost, using an old late 80s mouse at the moment.) It had a strange problem where it would not boot, but would show stretched boxes and such everywhere. I put it aside and went about my business. back at the end of July, I picked up a Mac SE that won't boot... but since it's just a silly SE, I decided to open up the Classic II once again and work on it some. some fine-toothed cleaning later, and the addition of a new jumper to designate 512k ROM status, the thing quickly chimed and came to life! and, after popping in a 500MB Apple-branded SCSI drive I saved from someone's trashpile, I had a Classic II with 10MB of RAM (courtesy my Quadra 700) and MacOS 7.6.

Needless to say I'm very happy about this, but the problem I face is that after I got it working, I adjusted some of the geometry pots in the back to try and center the image and fix the brightness/contrast. and it went dark.

I went and read up about this on the internet and it points to some diodes or soething in weird places that I can't even see on the analog board itself (the guide was made for a Mac SE, etc. so the analog board type and model are not the same at all.)

I was hoping I could reach out to other (or previous) Classic II owners that have come across and/or solved this problem somehow. Could it be something related to the pots themselves that I did wrong, and can this be fixed with any ease? xx(

 

[Superdos]

UPDATE!

Opened it up again this morning.

Wow.

QL2 and IP2 on the analog board with the heatsinks were BURNING HOT. so I'm going to need to replace those, I guess. However, I'm not sure where I can pick up the parts. would these little guys on the end of these heatsinks be locally available at a radioshack or something? or do I have to order them online?

 

[phreakout]

Chances are, Radio $hack does not carry those parts in store or on their web site. You may have to check with Digi-Key or Mouser. I don't have a listing as to the part numbers for both of those at the moment. Does anybody else know?

73s de Phreakout. :rambo:

 

[techknight]

need to find out WHY they are red hot.

 

[Superdos]

need to find out WHY they are red hot.

that's something I know not how to do.

I have a simple multimeter with a 8v inside and literally no knowledge of how to use it.

Later on tonight if I'm feeling brave I'll get in there again and see if I can't get the numbers off the things... and if they're not dead due to heat damage when the actual culprit is found, if this turns out not to be the case, I'm giving them a fresh coating of thermal compound.

 

[techknight]

youll need to read up on how to troubleshoot and repair electronic circuits, and how to use a multimeter to test components, what to expect, otherwise your better off just replacing the board and calling it done.

The components are red hot because they are shorted, or something along the same circuit path has shorted, you can find these with a multimeter, but you must know how....

 

[GnatGoSplat]

What are QL2 and IP2? I'm guessing one is a transistor? Use the diode check and resistance functions of your multimeter to see if any two pins are shorted. A short would be 0-ohms in resistance mode, 0V in diode check mode.

Total guess, but if these are semiconductor parts getting hot, it's possible the parts themselves aren't bad. Shorted semiconductors often blow up, releasing their magic smoke, and then blow the fuse. If they get hot without blowing up and the fuse doesn't blow, there is usually some other component failure causing them to not fully switch on and off. The excess resistance of just partially switching makes them get hot. After enough time, they could go into thermal runaway and short/blow up/smoke. It's not unusual for bad caps to cause transistors to overheat/fry. Check to see if any of the electrolytics are leaky or swollen on top.

 

[techknight]

in most cases i would agree with you there, But ive seen failed shorted transistors get hot again, it all depends on the design of the circuit.

I think there are schematics on the internet out there for these analog boards? I dont have it....

I could take a look at it and compare against it with the blown components, and further be able to help you.

If it gets too bad, ship it my way and ill fix it.