Macintosh 128K Voltage Setting

[sunapple]

Last week I opened up my Macintosh 128K to solve the issue of the screen going black after booting, leaving only the mouse pointer visible.

I found that the two dials for screen brightness and height (or width, the one next to it) had some corrosion in them and turning them would solve the issue. The solution didn't last long though, so I sprayed some contact spray in there to make sure it makes good contact.

Then I saw two other dials, decided to spray them as well. I figured I could just rotate them and calibrate the screen afterwards.

This is when I found out about the voltage setting. I made a mistake turning that knob without remembering how to set it back.

I found out how you can measure the 12V and 5V voltages from the disk drive port on the back from a PDF file on the internet.

It just leaves me to this: how do you set the dial before measuring the first time? I don't have a starting reference. Could the minimum setting (to the right from the cardboard side) be save and then go from there, or should I put it in the middle?

Hoping not to make a bigger mess than I already have

sunapple

(@MacTech68 )

 

[MacTech68]

Turning the pot fully counter-clockwise (viewed from the solder side) should get the voltage as low as possible.

You can then SLOWLY turn clockwise whilst observing the 5 volt rail. 5.00 is good but you need to compare the 12v rail too (see the pdf you got for further info).

Use an insulated screwdriver. Some models of adjustment pots may allow the center adjust head to be shorted to the left or right pins of the component with a large screwdriver. Other models of adjustment pot are insulated - it's pot-luck (pardon the pun) as to which you have.

Good luck!

 

[sunapple]

Thanks for your respons. I'll put it on the lowest setting and work from there. I understand that the voltages might slightly alter when the system warms up, but I'll just wait and see what happens. It's going to be interesting, that's for sure.

The project will have to wait a few days before it starts, but I'll post the results here for future reference. Thanks again!

 

[sunapple]

It worked perfectly.

Put it on the lowest setting (equivalent to about 10V on the 7th/8th pin) and work from there. The first time the computer didn't boot, but after setting the 12V right and rebooting it comes right up. I checked the 5V setting afterwards and it seemed to be alright as well. There's a pretty big margin so putting it in the middle of it works fine.

Then I set the display brightness, focus and height back to the right settings and now everything works like it should. Easy as pie. Now on to fixing my Apple ][ PSU...

Supporting documents:
1. http://tim.id.au/laptops/apple/legacy/macintosh_128k.512k.pdf
2. https://68kmla.org/files/classicmac2.pdf

 

[MacTech68]

Excellent news - good to see another historical artifact resurrected.

 

[mxrider88]

Hi everyone, sorry to open an old post.
I have a 128k and I get error 0F0004 on screen when I try to boot from the floppy disk.
according to the first link posted above, it is a voltage issue.
I am just not sure where I have to measure the voltage. It is supposed to be 5 volt but when I measure the 6th pin of the serial port of the floppy at the back of the Mac I get 12 volt.

Am I missing out something? I don’t want to do adjustments without being 100% sure of what I am doing and damage it..

Many thanks!

 

[MacTech68]

Here is a diagram taken from the classicmac2 PDF

 

[mxrider88]

I was counting the other way around ?

Thanks!
But I assume if I have 12 volts on the 7th pin I should have 5 on the 6th, it’s only one adjustment knob, right?

 

[MacTech68]

I was counting the other way around ?

Thanks!
But I assume if I have 12 volts on the 7th pin I should have 5 on the 6th, it’s only one adjustment knob, right?

Correct.