More SE Problems

[DL757]

Hi! You might remember me from the last post I made, where:

1. I accidentally called a resistor a capacitor

2. I thought that a resistor looked exploded, when in fact it was totally normal

Now that I've stopped being a total moron, I'm still having issues. After a few days of emotional distress from it immediately exploding, I've taken to trying to fix it again.

I totally disassembled the machine AND the power supply. A thorough visual inspection, the analog board as well as the logic board are totally intact visually. Same with the power supply.

However, upon starting it back up, the machine goes through it's normal boot process, but there's never any video. It chimes and the HDD sounds as if it's being read. The filament in the CRT does visually get hot, so there's voltage going to it. The screen just stays black.

My brain tells me that the analog board has gone bad in some fashion that can't be visually detected. My wallet is praying that it's not as such, since a new board would cost about as much as I paid for the machine.

Anybody who is actually knowledgeable on the subject have an opinion?

 

[unity]

Brightness dial.

 

[DL757]

Tried it in both extreme positions and somewhere I assumed was the middle. No dice.

 

[IlikeTech]

Check the crt socket connection? And the video connector?

 

[DL757]

The small board at the end of the CRT I removed and reseated multiple times, still no dice.

I'm not sure what you mean by the "video connector". I reseated both the yoke board cable and the CRT power cable (I assume? The beefy cable coming off the CRT).

 

[IlikeTech]

Ok. Can you hear the crt come on?

 

[IlikeTech]

The Dead Mac Scrolls says to replace resistor R22 on the analog board. It is a 470K 1/2 5% resistor. The page number is 150 in the book.

 

[DL757]

Ok. Can you hear the crt come on?

Well, I'm not entirely sure what I'm listening for.

I hear these things:

- The fan (constantly)

- The hard drive, if I have it connected (constantly)

- -beep-

- Later on, there's a slight click that sounds like it comes from the speaker. Usually happens around when the "Welcome to Macintosh" goes away and the Mac OS splash itself loads. (Of course, that was when I could see the damn thing.)

That's the lot of it. What should I be listening for?

 

[IlikeTech]

A slight hum when you turn the computer on. Also, try replacing above resistor.

 

[DL757]

There is a very low and soft hum coming from the CRT area when I power it on and shove my finger in the fan to stop it, yes.

 

[IlikeTech]

Hmm. I don't know, I think that the resistor is bad. It is the only part with those symptoms listed in the Dead Mac Scrolls. Do you have a soldering iron?

 

[DL757]

Could the flyback have gone bad?

 

[IlikeTech]

I don't know. I think it would do something different if the flyback was bad.

 

[DL757]

Hmph. It's times like these I actually knew how to solder.

 

[IlikeTech]

Yeah. Look up some guides. I think that resistor is the culprit.

 

[techknight]

just because the filament is lit in the CRT, doesnt mean there is any high voltage. the CRT filament is powered off of the 12V sweep voltage going to the CRT neck video board. 

When you turn the machine on, hear a chime, youll hear a "static" sound as the CRT charges. if you dont get that, you dont have any high voltage. 

You cant really test for that unless you have a HV probe. You could stick your hand by it, but i dont recommend that for obvious reasons. 

 

[DL757]

just because the filament is lit in the CRT, doesnt mean there is any high voltage. the CRT filament is powered off of the 12V sweep voltage going to the CRT neck video board. 

When you turn the machine on, hear a chime, youll hear a "static" sound as the CRT charges. if you dont get that, you dont have any high voltage. 

You cant really test for that unless you have a HV probe. You could stick your hand by it, but i dont recommend that for obvious reasons. 

Hmm, yeah, there's no noise like I'd usually hear from a CRT, just a low hum that might not actually be coming from the CRT, upon further investigation.

So while there's power to everything else, the HV part on the analog board is screwed?

 

[techknight]

Well there is a component failure somewhere preventing it from building HV. Could be a resistor, could be a transistor, could be the flyback, could be anything. 

Electronics troubleshooting will likely need to be done here. You could shotgun it with what a book tells you and hope it works. But gosh, I dunno... 

Since the flyback secondary winding provides sweep voltage to the vertical amplifier, if you dont have HV, you wont hear any "pulsing" coming from the yoke either. 

 

[IlikeTech]

SEems like it.(Sorry.)