Rubenerd

Last year I talked about fixing my second-hand Commodore 1541 floppy disk drive. It turns on, is detected by whichever Commodore computer I plug it into, and all the mechanical components have been cleaned and lightly lubricated. But the stepping motor that controls the movement of the read/write head doesn’t move.

One thing I noted at the time:

A couple of traces had been bypassed with patches on both sides; I’m not sure if this was done at the factory or someone else tried to fix this drive one before.

Here’s a photo showing what I meant. Note the long patch in the lower left, near the two large capacitors, and the smaller patch below it:

I’ve since found another of Commodore’s 1540/1541 service manuals, and page 29 spells out the exact upgrade:

PCB Assembly #1540048 (Schematic 1540049) requires an upgrade to the reset circuit. The upgrade consists of:

1) COMPONENT CHANGES:
R25 Change To 6.8K ohm, 1/4 W, +/-5%
R57 Add 220 ohm, 1/4 W, +/-5%
R58 Add 2K ohm, 1/4 W, +/-5%

2) CUT circuit trace near UA1 (See diagram)

3) INSTALL JUMPER WIRES:
BETWEEN: UA1 pin 8 and UB1 pin 5 UA1 pin 9 and CR4 Anode
UB1 pin 6 and UD3 pin 5 on BACK of board

4) CUT circuit trace between CR7 Anode and UD3 pin 5 on BACK of board.

Here’s the diagram from step 2, showing the exact fix:

So whatever is wrong with the stepper motor on this drive, this is now less likely to be the problem. Commodore.ca has a copy of a Commodore service cheat sheet that suggests the issue may be with the 325572 or the 6522.

Replacements for these are much cheaper than I thought; and Gamedude in Australia even sells a modern replacement for the 325572. I’m tempted to try swapping these out and see if it improves things, given a scope isn’t in the budget right now.