⚡︎ Mac Plus Analog board outputting only 4v instead of 5v [SOLVED]

[AndyDiags]

Hi all,

I have a Mac Plus analog board 630-0102 (non-international) which I just can't seem to get to work properly

Symptoms

- No bong
- Logic board has steady 4.07v output on the 5v rail via floppy port
- Logic board has -9.94v on the -12v rail via floppy port
- Adjusting the voltage pot on analog board has no affect on logic board voltage
- Tested with 3 different confirmed working logic boards, screen will either show jail bars, checkerboard, or fuzz depending on which logic board I connect (screenshots attached)

History

- Received this using in this state exact, don't know history
- Previous owner somewhat poorly recapped the unit, probably in an attempt to repair the board
- Seems previous owner thought issue was on logic board, as the 2 rom chips were reversed, and the voltage regulator on logic board was removed

Attempted Fixes So Far:

- I fully this recapped the analog board along side of 4 other Mac Plus analog boards. The 4 others work fine after recap, this one had no change in behavior after recap.
- I checked every resistor on board twice, and compared values to a working analog board, all match
- Checked all diodes twice with multimeter (reverse and forward bias) and compared values to working analog board, all matched
- Checked all transistors twice and compared values to working analog board, all matched
- No physical dry joints visible, but I still re-soldered the cable connectors as well as any suspicious looking joint
- Flyback (Rev.A) looks in good condition. I compared all the joint resistor values to a working one while unplugged and values match.
- I tried to check the remaining components to the best of my ability using the voltmeter and comparing the values to a working analog board, and I don't see any differences.
- Analog bard was cleaned with as much as possible with 99.9% IPA.
- I verified the resistance value on the pots do change when I spin the pot.


I'm out of ideas at this point. If anyone has any suggestions on which component could have failed it would be greatly appreciated! As mentioned above, I have couple working boards to swap out components to help root cause this issue.

Here are the screens that can come up while analog board gets the 4v:

Image of analog board also attached. Thank you!!

 

[AndyDiags]

Root cause found! Bad U3 chip!!
I pulled up the schematic for the board, and it showed the +12v is directly regulated, and the rest are slaved to the +12v. I checked and the +12v was reading +9.7v, so I figured it was not being regulated correctly.

I already checked all the components in the above except for U1 or U3, so I figured it's either one of these, or a bad joint. I replace both and board worked! From there I verified board works when U1 was reinserted, but failed when U3 was reinserted.

U3 is an 4N35 optoisolator. Hopefully I can use a generic replacement like this one

It's interesting that with the 4v input, CRT was displaying different screens depending on which logic board was connected, I would've expected the same to be displayed for all 3.
Here's the guy responsible for all the trouble

 

[pfuentes69]

Nice. I had the same issue in my Plus and the optocoupler was also the culprit.

 

[Daniel Hansen]

Nice. I had the same issue in my Plus and the optocoupler was also the culprit.

Same - had a couple of these go. Nice that the part is easy to source and inexpensive

U3 is an 4N35 optoisolator. Hopefully I can use a generic replacement like this one

Digikey has tons in stock: https://www.digikey.ca/en/products/...HIUhAZm0sQMeQIAmXKWAAMctp0ggQhdgE9WuLjbYKLpAA

 

[AndyDiags]

U3 strikes again!
I re-capped an international Mac Plus analog board. It booted just fine. The 5v rail was a bit low at 4.78v, but when I went to adjust the pot nothing happened. After cleaning and testing the pot, I was still getting the same results. I then realized it was the U3 probably again!

After replacing the U3 I was able to adjust the voltage again. This time it was black GE branded 4N35 IC that went bad.

I tried to desolder the legs, but to my surprise, they had tiny sockets! Be careful, your IC may be socketed too.

 

[Daniel Hansen]

Another one! Yes - I've never seen the sockets on NA boards, but I've heard of them being on International boards. Certainly makes troubleshooting / fixing easier.

 

[pfuentes69]

Mine had that weird socketing too, and I also didn’t notice and desolder (and lost)