Macintosh Plus - Weird Startup Problem

[mchineboy]

I recently acquired a Macintosh Plus that I knew would need repairs. I have already rescued a 512k with success.

At the start, it was clear that the Mac was unhappy. The symptoms as I received it are as follows:

* The startup beep is a low frequency buzzing sound. The length of the beep is about the same as a normal chime.
* The screen is filled with this pattern. It "blinks" and rolls vertically.

What has been done so far:

* Complete recap of the analog and digital boards. Using the kit that console5 sells. Each capacitor is done individually and polarity is carefully confirmed.
* Reflowed the solder on the various harness connectors.
* Swapped RAM around.
* Reseated the ROMs with Deoxit on the sockets.
* The voltages at the floppy port are within spec. 5.01vdc and 12.11vdc respectively. The -12 matches the +12.

Because of the unusual layout, I have not tried to test reset circuits. Nor I have I tried to scope data and address lines.

Observations:

The sockets for the ROMs are single wipe. Any thoughts on replacing these with upgraded double wipe sockets?
The battery compartment had minor corrosion, but it didn't appear to have leaked on the analog board. There was some minor corrosion which I was able to clean up and inspected the traces closely.
A few of the analog board capacitors were indeed leaky, but it hadn't spread beyond the immediate footprint of the capacitors themselves.

 

[mchineboy]

I went ahead and reflowed all of the joints on the analog board with fresh solder. Same symptoms as before, no change.

 

[RetroTheory]

Have you checked the RAM is correct and matches the cut resistors config on the board ? Sometimes resistors are cut and then RAM is downgraded which can lead to odd behavior

 

[mchineboy]

Have you checked the RAM is correct and matches the cut resistors config on the board ? Sometimes resistors are cut and then RAM is downgraded which can lead to odd behavior

It's got 2 1mb 30 pin SIMs in it. Unfortunately I don't have any other 30 pin sims other than what's in my Iici.

Edit: The resistor is clipped according to the documentation and I've confirmed that its not making contact.

 

[RetroTheory]

Hmm, how many chips on the SIMMs ? if its 2 or 3, its kinda hit or miss if they work. 8 or 9 is more common to work, having said that I have 2 chip 30pin SIMMs in my Plus. https://support.apple.com/en-asia/112183

 

[mchineboy]

Hmm, how many chips on the SIMMs ? if its 2 or 3, its kinda hit or miss if they work. 8 or 9 is more common to work, having said that I have 2 chip 30pin SIMMs in my Plus. https://support.apple.com/en-asia/112183

The SIMs I have are the 8 chip 1mb. Only two so I'm putting them in the lower bank.

 

[Builde68]

That distorted sound reveals something isn't being read correctly. If the chime is the right length but the wrong frequency or texture, it suggests the CPU is executing the startup code, but the data being fed to the sound buffer is corrupted.

Replacing those original single-wipe sockets with high-quality turned-pin or double-wipe versions is a highly recommended insurance step. It rules out mechanical failure before you start chasing phantom logic bugs. Also, if you have access to an EPROM programmer, it is worth verifying the checksums of those ROM chips. Internal bit-rot is rare, but it isn't impossible.

Since you are using two 1MB SIMMs in the lower bank, you'll want to ensure the RAM SIZE resistors are strapped correctly for a 2MB configuration. If the board is still strapped for 1MB (with the 256K bit resistor installed), the memory controller glue logic will miscalculate the addressing.