Commodore CMD 2040/3040 Drive Spin-up on Power Down

Background

I recently finished restoring two non-working Commodore CMB 3040 Machines. After a while, I finally got them both in working state and able to read disks, but one of the drives still had one small oddity. When I turned the main power switch off, the disk drive would slightly spin up for a couple seconds. I decided to dig into it a bit to see what was causing this.

Analysis

After poking around, I narrowed down the issue to the main logic board. The main board controls the drive via connector P2, and I found that the control lines tell the drive to spin up at power down. There is likely enough power in the large capacitor for the drive to do so for a short bit.

Pin 7 & 3 of connector P2 jump high during power down. Let’s take a look at what’s going on on the input and output of the NOT gate UE6.

Yellow is input and blue is output. It seems that when unit is powered down, the input slowly goes down as the capacitor discharges. Once the input voltage drops to about 2.2v, the IC tries to drive the output high. The output jumps to a bit over 1v as that’s what still remains in the capacitor.

The 1v input into UJ6 shouldn’t be enough to trigger the NOT gate, but it does. Let’s take a closer look.

Blue is the UJ6 input (same as above), and yellow is the output. Notice this DM7406N chip has a 5v input and 12v output. When the input reaches about 1v, the output drops to 0v, and the drive spins when UJ6 output (yellow) is low. According to the datasheet, LOW level input max is 0.8v and HIGH level input min is 2v. For inputs between 0.8v~2.0v, output can be unexpected, hence the chip does seem to be in spec.

Solution

I wouldn’t be surprised if this drive behaved like this from the factory. I’ve decided to replace the UJ6 National Semiconductor DM7406N chip, as it’s a bit odd for it to flip the output to be low when input is only at 1v. The replacement IC which I ordered and arrived was a TI SM7406N, which is the same brand IC in my other CMB 3040 drive which doesn’t have this issue. After popping it in, my drive now powers down quietly without spinning up the drive.

Different chip manufactures may have slightly different behaviors in the 0.8v~2.0v ranges. This issue will depend on the combination of the IC chip brands used in UE6 and UJ6. If your drive experiences this, switching to a TI SM7406N might just resolve it.