I recently had a chance to look at a malfunctioning Commodore 3040 floppy drive.
Smoke coming out of drive
There is a real chance that yours too might emit magic smoke when you turn it on for the first time in decades. So before turning it on, be prepared to turn it off quickly, and then watch it like a hawk. Reacting quickly may prevent shorted tantalum capacitors from bursting. Check the schematics and BOM to see what kind of capacitor to use. (You’ll find all that at https://www.zimmers.net/anonftp/pub/cbm/schematics/drives/old/index.html)
Cleaning
Clean the heads. And if the drive looks really dirty on the inside (the case isn’t exactly dust-proof), do some general cleaning perhaps. It also makes it nicer to work on it!
Maybe move the head position manually a couple times, just to make sure the mechanism is not stuck.
It would probably be best to unplug all connectors and clean them. I did that for most. Before that, the drive would often just stop responding to commands.
PEBKAC #1
Being more accustomed to the Commodore 64’s 1541, I assumed you could just do LOAD “$”,8. Well, you can’t with early versions of BASIC and/or the DOS. You first have to make the drive read in the BAM. This is on the 18th track, first sector. And the drive currently doesn’t even know what track it’s on, so you must initialize it. So assuming that you want to talk to “drive 0”, (the 3040 has two drives, 0 and 1) the first command you execute (in BASIC’s normal interactive mode) is:
OPEN 1,8,15,”I0″
And then:
CLOSE 1
(For drive 1 it’s OPEN 1,8,15,”I1″)
If you get an error light, which I did, you probably won’t succeed in getting a directory listing either. There’s an incantation you can use to get the drive’s error message, but it’ll likely just be “READ ERROR”:
10 OPEN 1,8,15
20 INPUT#1,A,B$,C,D
30 PRINT A;B$;C;D
40 CLOSE 1
Fixing the drive speed
The PET drives are belt-driven. The speed is controlled by a potentiometer on a small board attached to the drive. The belt is on the back side, and there is an indicator that helps determine if the drive is running at the right speed. This indicator works if you have lighting that goes on and off at 50 Hz (or 100 Hz or some other multiple) or 60 Hz (or 120, etc.). In the age of LED lighting, that will likely not be the case. If you want to know what frequency your light oscillates at, just use an infrared LED (a red or green LED will likely be fine too, or even other colors) and attach its pins to your oscilloscope probe’s GND and signal (it’s a floating device, so it doesn’t matter which way you connect it). If you hold the LED close to the light source, you should see an oscillating voltage and your oscilloscope will show it’s frequency.
Or… just attach an LED to a microcontroller and turn it on and off at the correct frequency.
Anyway, my drive speed was out of whack but in my case the potentiometer was just dirty perhaps. Moving it left and right a little bit fixed the issue.
Running drive diagnostics
Having logic diagnostics on a floppy isn’t very useful if the floppy drive doesn’t work, so let’s convert it to a wav file that we can play through the datassette drive with a 3.5mm to tape converter inside. Grab cbm_technician_disk.d80 from the aforementioned site, and use VICE’s c1541 program to run this:
/path/to/vice/c1541 -attach cbm_technician_disk.d80 8 -read “logic diagnostic” logic_diagnostic.prg
Maybe VICE can do it too, but I use a program called prg2wav (https://wav-prg.sourceforge.io/) to convert this to a WAV file. I have some more explanations on how to use this program in this post. Just for my own notes, here’s the command I use to run the program:
LD_LIBRARY_PATH=../wav-prg-libaudiotap/:../wav-prg-libtap/ cmdline/prg2wav -s -w logic_diagnostic.wav logic_diagnostic.prg
If the test goes well, the floppy drive’s LEDs will blink in a random pattern.
Don’t have a datassette either? Well, the program is written in BASIC so you could copy it by hand. ;)
Since they were socketed, I decided to give the 2114 SRAM chips inside the floppy drive another separate test. I made a 2114 RAM tester a couple years ago, but reused the breadboard for something else. So I just took out the 2114 chips one by one and put them in the PET’s VRAM socket. I didn’t see any issues.
PEBKAC #2
I didn’t really know whether my media was good or not, so I put it in a working 1541 drive attached to a C64, formatted it, saved a small program, and took the floppy back to the 3040. This time, the initialize command worked! So I did LOAD “$”,8. The drive said it was loading, but never finished. (Also the LEDs were doing something weird?) After that, I tried formatting the disk again (on the 3040), and it finished without errors! However, LOAD “$”,8 still didn’t work.
At this point I decided to read up on how Commodore floppy drives actually worked. I found a book called “Inside Commodore DOS” and put it on my Kindle and read the first few chapters on the train on my way to work and other places: https://archive.org/details/Inside_Commodore_Dos_1984_Datamost_a
This book is mostly about the 1541, but most things didn’t change much, and it does mention some things specific to older drives as well. It explains how the BAM/filesystem works, and includes short example programs that do ~mid-level sector reads and print the result on the screen. I ran the program in chapter 5.3 (“Block-Read Command (U1)”) on page 74. And to my surprise, I got a decent result! The magic numbers looked plausible and the disk name (specified when formatting) too!
Next, I tried something that was mentioned earlier in the book… On the C64 you usually do LOAD “$”, 8, sure. But you can also do LOAD “$0”,8. The book said that this is a relic from earlier times when there were two drives per drive. And the 3040 obviously is from those earlier times! So the correct commands are…
LOAD “$0”,8
SAVE “0:FILENAME”,8
VERIFY “0:FILENAME”,8
LOAD “0:FILENAME”,8
And guess what, these commands fricking worked! Dammit.
Except the drive sometimes get stuck somehow, but that’s a topic for another day. (It seems like it’s in the communication part. I can turn the drive off and BASIC will return with a DRIVE NOT PRESENT error.)