Mac Classic II (Gen 2) - No CRT?

[Uloga]

Hey All,

Got my hands on a Mac Classic II (G2) for €20, good deal no?

So the fact is that the motherboard itself is in decent shape, I have cleaned it out and recapped everything before I even tried to turn it on (learned that the hard way on previous repairs)

So after all this, I flick the switch and hear the harddrive spinning and "reading" and the CRT lights up really really briefly (room was dark, otherwise I wouldn't have noticed).

I turned it off and turned it back on to see if there's anything happening, but the screen stays dark.

My thought => Analog board, so after reading multiple posts about people having luck with recapping that, I decided to measure the voltages on the low-tension site, 12V was 11,3, 5V was 4.9 so all was a bit off.

I'm now at the point that I have recapped the analog board and still I get no screen (I also don't hear any "charge" sound like when you turn on a CRT screen), only the harddrive spins. I have also tried with uncoupling floppy and harddisk from the A/B but nothing helps. Also tried the brightness button (I flicked the 2 black ones on the back of the Mac)

Anyone has any idea what to check/swap?

 

[LaPorta]

I would suspect a busted flyback transformer. Those can be a bit pricy to come by these days. Someone else may have a bit more info for you. You could also check the solder joints of the flyback to make sure they are all in good shape.

 

[Uloga]

Ugh if that's the case, that's the 2nd one I have with this issue :-( 

Never bothered to start looking for one here in EU.

 

[68krazy]

There is a lot that goes wrong with these analog boards, particularly two diodes, an IC, and some of the caps.  I’d look at those before jumping to flyback.

 

[dochilli]

11.3 V is too low! Should be 11.85V or more.

You could try to adjust the voltages with pp1. Sometimes the poti has some contact problems. But you have to look for the voltages for some minutes. they should be constant. If not, you have another problem on the board. If you get 4.85 V the classic will chime.

A lot of things can be wrong with Classic analog boards (the two Diodes DP3 and DP4 (1N4148) are almost bad everytime, TDA4605, CNY17G, IRFBC40).

TDA4605: Have you measured the voltage at pin 6 of TDA4605? Should be 12V. Voltage is AC!

 

[bibilit]

Probably not related to a low voltage, your 12 volt rail is too low, but the screen should kick on. 

With that kind of voltage, you should see a checkerboard pattern or black and white one. 

Your issue is strange, both the 5 and 12 volt rails are linked together, so adjusting PP1 is a bad idea in general when one voltage is fine (and your 5 volt is ok) because cranking up the 12 volt will also crank up the 5 volt one well above a suitable range. 

You can have a try anyway for a short test, if the adjustment doesn’t work, your problem is related to one or several known components ( two diodes, the TDA chip or the Opto)

I posted a lot on those, still have some issues, those AB are a nightmare.