Macintosh Plus - Power Issues

[Thebobfather1992]

Hello everyone,

I recently acquired a Macintosh Plus (PAL) on eBay. The seller mentioned that the computer has a faulty ROM and only has 512K of RAM instead of the original 1MB.

When it arrived, I decided to open the Macintosh Plus after watching several videos about common issues, such as cracked Rifa caps and cracked solder joints around the connectors. I addressed these problems by replacing all the capacitors, including the Rifa caps on the power board and logic board, and reflowing any questionable connections.

I posted a few photos on Facebook, and someone pointed out that my Plus has a 400K drive instead of an 800K drive. A few speculated that this computer might have been upgraded by Apple, with the original owner opting to leave the drive, but we are not entirely sure.

Now, back to the problem. I have tried to power on the Macintosh after the recap, but I am not getting any sound or picture from the CRT. With the help of some fantastic members from the RMC Discord, they suggested that I check the voltages on the power supply. I verified that the large capacitors are receiving around 300 volts, and the power switch is both receiving power and showing continuity.

All the capacitors I've used are high-quality ones from Nichicon and Rubycon, and I've double-checked my work to ensure that they are all properly installed and soldered.

Any suggestions would be greatly appreciated. Thank you!

Jake

 

[wskjinfen]

The questions you raised are summarized as follows:
1Disk: It is not recommended to use floppy disks and drives anymore. They have been manufactured for decades. The instability caused by aging will be aggravated during use, and it is difficult to find replacement parts. It is recommended to use electronic devices such as FloppyEM and Bluescsi instead.
2CRT monitor has no sound or image: Most of the time it is a problem with the analog board.
It is recommended to swap the logic board and the analog board with another working mac to locate the fault. If there is no other mac, locate the fault by the following means:
The sign of the logic board working is the power-on sound, and the sign of the analog board working is +12V +5V.
① Make sure the speaker coil and the connecting wire are not broken, otherwise there will be no sound
② Then make sure the connecting wires between the analog board and the logic board are in good condition (some batches of connecting wires have fuses inside, use an ohmmeter to test the continuity), the connecting wire breakage will cause the switch power to flip-flip-flip-flip and there will be no sound or image
③ Measure the +5V +12V voltage on the back of the analog board when the logic board is connected (with load). If the voltage is abnormal, there is a high probability that there is a problem with the analog board. If the voltage is normal but there is still no sound, there is a problem with the logic board



Locate the fault first, then communicate

 

[JDW]

Welcome to TinkerDifferent, Jake.

I sure hope you got a good price on that Mac Plus, because no one in their right mind should be selling that machine at "going rates" without a normaly 1MB amount of RAM. Your story is the first I've hard of a Mac Plus selling with insufficient RAM. The Plus starts with 1MB, and can go up to 4MB. What you have now is basically a 512Ke with SCSI. I get quite disappointed in sellers when they do things like that, mainly because if I had a Plus for sale, I would NOT sell it until I had acquired at least 1MB of RAM for it. And honestly, I'd probably just buy 4 SIMMs and upgrade the thing to the full 4MB just to be nice.

The 800K drive will function fine even on a 128K Mac, and I know because I've tested that. HFS or the lack thereof only is an issue on Macs older than yours with the older 64K ROMs which don't support HFS. But your Plus has the 128K ROMs, so using the 800K drive is a non-issue if yours is properly cleaned and lubed. It would not prevent the "bong" at power-on, nor would it prevent your CRT from displaying a picture. Well, not unless something is shorted. But you need only disconnect the floppy drive's ribbon cable entirely and then do a POWER-ON to confirm.

You really need to check voltages at the external floppy drive connector:

SOURCE

There's also negative voltage too you can check, which is shown in my marked up diagram below (and no, it doesn't matter that the diagram below is for a 512K):



The fastest and safest way to measure is with a Mac-O-Meter. Unfortuantely, CayMac is shutdown until Sept. because the owner, Will Jacobs, needs to care for an ill family member. So if you can't wait to get yourself a Mac-O-Meter, you would need to use a multi-meter, and take great care not to short anything by accident.

If your voltages are normal, it could be a dead component on the Analog board, and there are two great references for troubleshooting that:

https://vintageapple.org/macbooks/pdf/The_Dead_Mac_Scrolls_1992.pdf

https://vintageapple.org/macbooks/pdf/Macintosh_Repair_&_Upgrade_Secrets_1990.pdf