Help diagnosing a Mac Plus

[reallyrandy]

I've been trying to resurrect this Mac Plus for weeks now. At first I thought it was the PSU because it was so mucked up. After cleaning it up, reflowing the solder joints and measuring the floppy port volts, it seems OK. Here's the next issue:
When I hit the power button I get a beep (all good), the screen comes on and displays the blinking question mark (more on this in a sec). I put in a known working 6.0.8 floppy and I get the Happy Mac face. The floppy goes through its seek routine for about 15 to 20 seconds and then stops. Nothing happens after that, the happy Mac stays on the screen.

I tried using my FloppyEMU to boot but the EMU screen comes on and stays blank, no menu ever shows although it does light up.

The voltages on the floppy port are a little low, about
-11.37 on pin 5. (should be -12)
+4.69 on pin 6. (should be +5)
+11.29 on pin 7. (should be +12)
+11.29 on pin 8.(should be +12)

Also, the screen, although turned all the way bright is only lit up immediately surrounding the question mark or the happy Mac face while the rest of the screen is black. See attached image.

Can anyone think of what this might be? The behavior is the same with both PSUs I have tried.

The image shows the few surrounding pixels lit up like the entire screen should be:
The second shot is the Logic Board.

 

[Stephen]

A few thoughts, this is not medical or legal advice and should not be considered investment guidance:

1. You are missing the checkered background on your happy mac and it makes me think there's a a RAM issue.
2. Try with no RAM or 1 bank of RAM. You will need to adjust your resistor. You can swap matching pairs in RAM Bank 1 during this test.
3. This could also be a ROM issue but I think that's much less likely.
4. Adjust your R45 pot on the analog board so that the board produces 12v, then measure your 5v line to make sure it's not overvoltage.
5. Adjust the CRT voltage pot on the analog board. I forget the identifier. I normally adjust this so max "brightness" is just a little to bright for comfort and "minimum" is barely visible. That way you have a reasonable level of control over the brightness as light conditions vary. Other people may have better (or contrary) advise on this point.

 

[Patrick]

everything that Stephen said.
also... i know that Mac plus have issues with 3 chip ram. you have 2 chip ram. wonder if they are causing an issue?

(its just a wild guess. something would at least try)

 

[Daniel Hansen]

Is that known good / compatible RAM?

(EDIT: LOL - didn't see everyone else's responses...)

 

[reallyrandy]

I'll do some experimenting. I hadn't even thought of that.

2 of those chips say "DO NOT USE IN MAC SE".

I'll have to go through all these and see if something works:

 

[reallyrandy]

I put four 1MB chips in and I got a much better screen and mouse cursor. A little crooked but I'll fix that later. Double checked the 256k resister and it's clipped.

Now, the known good floppy drive and boot disk are just ejecting like something is wrong with the disk. this disc will boot a MacII though so I know it's good. I tried an original 7.0.1 tools disc and it ejected too.( I tried a red striped floppy cable and that didn't go well!) This floppy and drive would start to boot before and then stall, I'm assuming due to the RAM. But the drive read the floppy and gave me a happy Mac face. Now it just ejects.

I tried the FloppyEMU again and got a white screen on the EMU. I also tried an external SCSI2SD and failed.

Any thoughts?

 

[reallyrandy]

Spoke too soon! I kept swapping out RAM and got 4 chips that worked. It boots off the floppy disk now!

Now I have to figure why my FloppyEMU still has a white screen.

Maybe it's the voltage coming out of the floppy port? it's about a half a volt low but not too bad, right?

 

[Stephen]

Spoke too soon! I kept swapping out RAM and got 4 chips that worked. It boots off the floppy disk now!

Now I have to figure why my FloppyEMU still has a white screen.

Maybe it's the voltage coming out of the floppy port? it's about a half a volt low but not too bad, right?

Just my $0.02 but I think 0.5v is a bit too low. I still think you should visit the voltage adjustment pot on R45 (or R51? I forget, google is good). You want your 12v line to be closer to 12v without your 5v line exceeding 5v.

I'm not shocked that your floppy drive doesn't work great but it is more surprising that FloppyEmu isn't playing nice. That being said I don't have experience with the FloppyEmu, I'm a BlueSCSI kind of guy. Hopefully it's not a problem with the SWIM.

 

[reallyrandy]

Looking up how to adjust the voltage now. I fired up a working SE I have and the FloppyEMU is showing white screen there too. I think the EMU screen came unseated. It's kind of a known problem.

I'll take your advice and turn the voltage up a tad, I'll let you all know how it goes.

Thank you everyone for your help so far.

 

[reallyrandy]

Thank you all, I now have a working (second) Mac Plus.

I adjusted the voltage 11.98v and 4.99v in the floppy port, adjusted the screen cutoff, now I just need to figure out how to move the screen over a smidge so it's centered.

 

[Stephen]

Ah, a platinum plus. I only have a few but they are beautiful when the case is in good condition like yours. Congrats!

 

[Daniel Hansen]

I just need to figure out how to move the screen over a smidge so it's centered.

There are two purity rings on the CRT neck you can adjust, that 'sweep' the display so you can center it...

Larry Pina's Repair Secrets discusses the process starting on page 51 (page 68 of this PDF): https://vintageapple.org/macbooks/pdf/Macintosh_Repair_&_Upgrade_Secrets_1990.pdf

You can use his test pattern generator app to guide all kinds of adjustments, including centering: https://macintoshgarden.org/apps/larry-pinas-test-pattern-generator

And @JDW demonstrates some video adjustments here:

Note: the focus / brightness pots are in a different location on the Plus AB than on the SE/30 AB shown in this video... though you may know that already

 

[JDW]

And @JDW demonstrates some video adjustments here..
Note: the focus / brightness pots are in a different location on the Plus AB than on the SE/30 AB shown in this video... though you may know that already

This video shows display adjustments for the 128/512K/Plus (and earlier in that same video you can see board variants too):