Macintosh Classic II - voltage values

[leonardcarvalho]

Hi all!
I have a Macintosh Classic II that wouldn't give any sign. I did the recap of the logic and analog board, now it starts and stays on the floppy disk with the interrogation, but it doesn't finish the boot.

When evaluating the voltage on the floopy rear port the multimeter returns that there is no voltage, any hint of what it might be?

 

[A24A]

Hello,

When evaluating the voltage on the floopy rear port the multimeter returns that there is no voltage, any hint of what it might be?

That does not appear to be possible, since you can start the computer, and something can be seen on the display. Normally, you should be able to read approximately +5 V, +12 V and -12 V via the DB-19 floppy drive port. Have you checked that the multimeter is connected to the correct pins?

now it starts and stays on the floppy disk with the interrogation, but it doesn't finish the boot.

Please clarify. A flashing question mark indicates that the computer cannot detect a valid system. Have you tried inserting a known-good boot floppy disk with a Macintosh system compatible with the Classic II? System version?

Macintosh Classic II: Technical Specifications - Apple Support

support.apple.com

 

[leonardcarvalho]

goodnight! I ended up measuring directly on the pins coming from the logic board, the values are high, 6.8, 13 and -13, but no measurements on the rear door.

I tried to power up the system with a bluescsi db 25 and there was no power signal, I replaced it with a 50-pin one, but I had to supply 5v externally, as there was no power signal either.

Even using bluescsi, the operating system did not go up

 

[A24A]

According to Larry Pina's book Mac Classic & SE Repair and Upgrade Secrets (ISBN 1-56609-022-9), 1993, page 162, good results are 4.85 to 5.15 (+5 V), 11.9 to 12.80 (+12 V) and -13.00 to -11.30 (-12 V).

So, especially the +6.8 V reading is very high. You should try to find the cause for this before carrying out further tests.

Of interest at the rear DB-19 floppy drive port are pin 5 (-12 V), pin 6 (+5 V) and pin 8 (+12 V) measured against ground (nut on the connector). You may need a thinner probe or paper clip to make contact.

In order to rule out other devices, any chance of finding/making an appropriate startup floppy for use directly in the internal floppy drive? This could (for example) be a disk tools disk for this computer, or you could use a Network Access Disk 7.5.

http://download.info.apple.com/Apple_Support_Area/Apple_Software_Updates/English-North_American/Macintosh/Utilities/Network_Access_Disk_7.5.sea.bin

 

[bibilit]

Yes your 5 volt rail is far too high (are you sure ?) when the 5 volt rail goes high, the 12 volt rail goes up also, and yours is not.

Probabaly two issues here, as i don't see why the floppy drive is not working.