The patch in my Commdoore 1541 was official!

Hardware

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:

Photo of the motherboard of the 1541, showing the two patches

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:

Diagram showing patch R57 with a resisto, a cut trace, and another patch R58. These are referenced above, and in the manual

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.

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