Classic II strange problem

[marcelv]

am trying to repair a Classic II

- replaced the capacitors on the mainboard and the analog board

- removed U14 , cleaned underneath and replaced

- removed egret , cleaned underneath and replaced

- reflowed some connections

problem is when I turn on the Classic II, the attached screen appears, after turning it off and on a few times the classic II will start normally.

mainboard with another analog board same problem, analog board with other mainboard works fine.

So problem must be on the mainboard.

Any suggestions will be appreciated.

 

[techknight]

check your voltages they may be slightly too low. The other thing that can do that is a floating reset line. 

 

[marcelv]

checked the voltages at the hd power connector, 4.96V and 11.97V

Boosted the voltages a bit to 5.08V and 12.20V

problem seems to stay about the same.

For the reset line, how do I check this?

 

[nvdeynde]

What happens if you press the reset switch a few times ?

Is there a Pram battery on the logic board ?
I know in 9/10 cases it's not necessary to have one installed for the Mac to start. However I have a Classic II board with a similar issue. Without Pram battery, I get a garbled screen and with a battery it works every time.
 

The voltages: are they measured immediately upon startup when the computer is completely cooled down ? Otherwise if they start low and go up within a minute, it's probably the optocoupler that's going.

That can also give issues like this so that initial the board has not enough voltage to power on correctly. 

 

[marcelv]

Pressing the reset switch does not help.

Tested the board yesterday again. tuned the voltage back to 12V and 5V (as close as I could get) installed a PRAM battery (empty don't have a charged one)

Measured the voltages, go up to the 12v / 5v in seconds and stay stable (cool boot).

Was measuring arond and suddenly the mac booted....

After reset still booting.

This morning tried again (without the pram battery) and got the garbled screen again. After waiting 30 seconds the mac did a reset (on its own) and started working.

The problem is slightly changed that I get the garbled screen on cold boot and after 30 seconds it reboots and starts ok.

Anyone suggestions/

 

[nvdeynde]

Replace the optocoupler on the analog board. It's likely fading away as your +5 and +12v lines should be up in 1 second or less, not coming up slowly. If the initial voltage is too low to boot then you can get the garbled screen or other symptoms. If the voltage adjustment screw is already set over half way, then you are already giving it more voltage than the standard setting. I would replace the optocoupler, it's an 1$ component.

Another possibility could be a resistor, diode, cap out of specs. 

Does it make any difference is you disconnect the hard drive and FDD as the HDD draws a lot of power on spin-up ? 

 

[marcelv]

Will replace the opto, have a number of them in stock.

disconnecting the HDD is same result

Does not explain that the board wih another analog board has the same problem. And another mainboard with both anolog boards is working ok..

 

[bibilit]

Your problem is on the Logic Board side, not Analog one.

I will check around the Egret again, maybe a trace is wonky, had a similar issue:

 

[nvdeynde]

Yes the problem is on the logic board. I missed the line where you wrote that you had swapped the board in another machine.

It can be a faulty capacitor or resistor/diode out of specs... 

Since it does boot spontaneously after a while I personally don't think it's a broken trace as then I would probably not boot without issues after a while. 

Do you have an ESR tester to check the capacitors ? 

Can you post a high resolution picture of the front and back of the board ?