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Need help troubleshooting a dead Mac 512k

T

timester

macrumors newbie
Original poster
Dec 19, 2011
4
0
Hi all,

I have an old Mac 512k that I pulled out of storage, and it has an odd problem when it boots. When I flip the switch, I hear no "startup chime", but I do see a perfect-looking "happy mac" screen, and the floppy drive starts going. However, it isn't actually trying to boot the disk. It just starts the motor spinning, but the read/write head never moves. The mac will sit in this "happy mac" screen forever.. it never gives up, and won't display the flashing "?" (like it does if it can't find a boot disk). It just hangs at this just-before-reading-from-disk phase.

Things I have already done, with no change in behavior:
* Removed and cleaned the drive (incl. read/write head) and even swapped it with another working 400k drive.
* Fed it numerous working 400k boot floppies
* Tried booting with the floppy drive disconnected (same results)
* Removed ROM chips, cleaned contacts & put back
* Cleaned the logic board, checked all the solder joints.
* Checked the solder joints on the analog board, esp. the speaker.

I suspect this is a logic board issue (maybe ROM or RAM chips?), but I'm not 100% sure because of the "no chime" thing. I sort of thought that you couldn't get a happy mac if there was no chime?

Any ideas what could be causing this?

I can take pictures of components if it will help.

Any suggestions are greatly appreciated!!

Thanks a bunch,
Tim
 
If it's a 110V only analog board, check CR22 and L7 which is -12V

IIRC, the only things that use -12V is the floppy drive, vertical drive and the speaker amp circuit. The height of the picture would be non-existent if it had failed on the analog board, but it's worth checking. Also, on the motherboard, a -5v regulator uses the -12v supply, which will affect the serial ports.

-12V should appear on the 11 pin connector that goes from the analog board to the mother board on Pin 8 (using the square pad pin as pin 1, the gap(key) as pin 2 and so on). Pin 8's cable is usually red. Check that -12V shows up on the motherboard.

You can even check for -12v on Pin 5 of the external floppy D-Sub connector.

If the -12V is low, try disconnecting the floppy drive and external floppy if you have one connected. Don't adjust the voltage if the +12v and +5v are ok. +5v should usually be somewhere from 4.85-4.15V
 
Checked the voltages

Thanks for the help!

I think it's 110v only. It says this on the outside, under the power connector: 120VAC 50-60Hz 1Amp 60W

I checked the voltages at CR22 diode and L7, as well as at the external floppy connector, and it was -12.6v at all 3 spots. I also checked the other voltages at the external floppy connector and had +11.9 and +4.95 volts.

I thought that -12v might be a bit high, so I lowered the voltage a bit (until I had -12.1v) but that didn't make a difference. (Should the -12v and the +12v be the same?) So I put the voltage back to where it was originally.

I also tried removing the internal floppy and the result was the same.. no beep, and a never-ending happy mac.

So I guess it's not voltage related?

Any other ideas?

Thanks again,
Tim
 
Hmmmm....

until I can think of anything else, try plugging headphones into the speaker jack and see if you get sound from the headphones. Just a thought... :(

Is the picture on the screen steady? ie, not waving or shuddering?

And, are you using an external floppy drive or Apple HD20 Hard drive?
 
Hi MacTech68,

No sound in headphone either. No external drives hooked up.

Well I verified that it's the logic board (swapped with another). Here's a couple pics of screen, which looks great to me (hard to get a pic without a scanline in it):

512k_a


512k_b


After looking carefully at the solder joints, I see that some of the joints on pins from the 68000 CPU look tarnished/discolored. I touched them up with a soldering iron, but it didn't make a difference. I did swap the ROM chips (no difference), but I don't really want to start unsoldering parts from the other board.

So unless somebody has some insight, I guess I'll be shopping for a new board...

Thanks,
Tim
 
If the logic board doesn't work then you might as well try soaking it overnight in a tub of warm soapy water. After one day, use a soft brush on the chips, then change the water to soap free water and let it soak another 24 hours. Then take it out and let it sit for a few days to dry. I tried this after reading several suggestions to do it. Unfortunately it didn't solve my problem either, but it has worked for quite a few people before.

Here's a video of someone doing it:

http://www.youtube.com/watch?v=CgD4r7RMLCg

I used plain bottled jugs of water and some liquid dishwashing soap. My board would still boot up after doing this but it still had the original checkerboard pattern problem that led me to try the wash.
 
timester said:
Hi MacTech68,

No sound in headphone either. No external drives hooked up.

Well I verified that it's the logic board (swapped with another)...
...unless somebody has some insight, I guess I'll be shopping for a new board...

Thanks,
Tim
Click to expand...

Make sure you try a different floppy drive cable. There are about 3 different ones. 400K drives NEED pin 20 and 9 (spindle speed and -12V respectively), whereas some models of 800K drives can get confused by these two being connected. I'd forgotten about this. :(

Other than that, I'm going out on a limb here, but check Q3 & Q2 on the logic board. This is the speaker driver circuit. Q3 is a 2N4401 and Q2 is a 2N4403. Both are adjacent to the power/video connector.

Also check Pin 8 of the LF353 is +12v and pin 4 of the LF353 is -12V. After that, you're kinda on your own.

When it does power on, with a known good mouse, can you get the mouse pointer to move on the screen? I can't recall if the pointer can be moved without a system being booted. Obviously, you can test for this. If the failed board doesn't pass this test (compared to your good board) then it's likely to be a PAL chip and you can only get one from another logic board.

As always, check all resistors and diodes. Use your good board to compare readings in circuit if they don't tally to the values marked.

Of course, it could be a myriad of other things but these are probably the simplest.
 
Well Tim, if you do decide to try for another logicboard I do have a Mac512K logic board here. It was working when pulled but has been on the shelf a couple years. Let me know.
 
More info

Thanks guys,

I was surprised to learn that if I disconnect the floppy drive, the mouse cursor doesn't appear on the screen. Is that normal? But when it does appear, I can't move it with an attached mouse (that does work). It's stuck in the top left corner.

I tried another 800k floppy drive (that I previously verified works), and the drive continuously tried to eject the floppy, even after it was ejected? It did this with either the cable from the 800k drive, or the orig one from the 512k.

I'm not quite how to test the transistors, but voltages are correct on LF353.

Here's a pic of the board... https://www.asuswebstorage.com/navi...=JMZ1K7nTWECq4Hk08nrE71+KJaOY40eJzOgYsaBKP0w=

You notice the extra wired on component? Is this part of the 800k floppy support?

I might crosss-check resistors, caps, diodes from the good board, but I also might just give up and get a new board...

Thanks,
Tim
PS. Typing on tablet sucks!

----------

Hi Jekyl,

I might take you up on your offer... how much do you want for the board?

Thanks,
Tim
 
timester said:
Thanks guys,

I was surprised to learn that if I disconnect the floppy drive, the mouse cursor doesn't appear on the screen. Is that normal? But when it does appear, I can't move it with an attached mouse (that does work). It's stuck in the top left corner.
Click to expand...

I think it's normal. If the floppy isn't detected POST fails and the system is caught in a loop looking for it. It's also possible that the cursor won't move until a system is loaded but I don't recall the detail.

I tried another 800k floppy drive (that I previously verified works), and the drive continuously tried to eject the floppy, even after it was ejected? It did this with either the cable from the 800k drive, or the orig one from the 512k.
Click to expand...

There's a couple of reasons this happens. See my earlier post regarding the cable. It can also be a failure of the cam to engage the "eject cycle completed" switch, and there are two types of this mechanism. Bit difficult to explain in a forum post.

You notice the extra wired on component? Is this part of the 800k floppy support?
Click to expand...

Nope. That is the 74F253 modification to get the 128K motherboard to address 512K of RAM. The 16 x 4164 RAM chips will have been replaced with 16 x 41256 chips to complete the upgrade.

I notice your board lacks a factory mod on the 6522 (a resistor pack is missing). I'm not sure what this mod achieved. It means that your board is a very early 128K motherboard that has been upgraded by a third-party to 512K. See the yellow part soldered to the large chip just above the barcode label in this pic.

Your board is circa week 4 of 1984. The one I've linked to is about week 35 of 1984.
 
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