Mac Classic II black screen and low voltages?

[Huttles]

Hello,

I have recently bought my first compact Mac, a Classic II.

I have recapped the board as the old caps had indeed blown and while some cap placements dont look pretty (I am new to electronics and old Macs in general) I have checked continuity and all 13 caps look ok. I did have to bodge 3 of them as the pads lifted so yeah, not pretty but they have continuity.

I left the analouge board alone, for now, as it looks pretty clean with no residue on the board, just dust. The seller said it powered on but had the Zebra bars (listing had photos of this).

But now after the recap, the machine wont power on, I can hear/see the fan spinning but it just has a black screen with no sound. I proceeded to check the voltages using the floppy 19pin port on the back and I get the following:
+12v: 11.19v
+5v: 4.33v
-12v: -11.36v

They all seem a tad too low, I have watched a video on how to alter the voltages so I can do that but I just want to make sure with others who know more than me as the voltage adjustment is in a bit of a dangerous space

Any help would be very much appriciated

I also have a SE/30 coming and so I bought the Classic II to practice on (also because I got it for cheap)

Cheers

 

[cheesestraws]

Welcome to the forum!

The Classic / Classic II analogue board is... of questionable quality, shall we say. You are likely to need to replace the caps on it even if you can't see any leakage, because it gets quite warm and so things can dry out. I would personally suggest doing that before trying to fiddle with adjusting voltages.

Did you verify whether it produced zebra bars when you got it (rather than when the seller sent it...)?

 

[bibilit]

The Classic is happy with 4,85 volts in the 5 volt rail, below at best zebra bars with the shaky screen.

Analog boards are all bad until caps have been replaced (and not only, because those boards are cheaply made)

 

[Huttles]

Ah ok well luckily I anticipated that the caps on the analouge board may be bad and have ordered some new ones. Ill have a crack at that before I go about fiddlying with the voltages. Thankfully the analouge board are all through hole so it should be easier than the logic board.

On another note, when it comes to discharging the CRT, my father gave me a old discharge pen he used to use when working on forktrucks back in the 90s. It has two prongs on either end and a LED, on the back it does say 4.5 - 450v AC/DC. Would that suffice to discharge the CRT?

Thanks for the replies

 

[Huttles]

Welcome to the forum!

The Classic / Classic II analogue board is... of questionable quality, shall we say. You are likely to need to replace the caps on it even if you can't see any leakage, because it gets quite warm and so things can dry out. I would personally suggest doing that before trying to fiddle with adjusting voltages.

Did you verify whether it produced zebra bars when you got it (rather than when the seller sent it...)?

I did not, which now I realise was stupid and I should of checked as soon as it arrived...... but oh well, ill have a go at recapping the analouge board. Cheers

 

[larwe]

Ah ok well luckily I anticipated that the caps on the analouge board may be bad and have ordered some new ones. Ill have a crack at that before I go about fiddlying with the voltages. Thankfully the analouge board are all through hole so it should be easier than the logic board.

On another note, when it comes to discharging the CRT, my father gave me a old discharge pen he used to use when working on forktrucks back in the 90s. It has two prongs on either end and a LED, on the back it does say 4.5 - 450v AC/DC. Would that suffice to discharge the CRT?

Thanks for the replies

CRT voltage is in kV You don't need a special tool for discharging them, just a screwdriver and an alligator clip wire. Adding a series resistor (1M is good) will make the discharge less exciting, and is generally considered a Good Idea.

 

[wthww]

will make the discharge less exciting

Thanks for the chuckle @larwe !