Notices by root42 (root42@chaos.social), page 2

Interesting! The two mechanisms are slightly different! Drive unit 1 is a 101S and drive unit 0 is a 101SM. I wonder what the differences are, except for the slightly different motor.
To me the motors look exactly like the tacho motors used on the Micropolis mechanisms. I do have one or two spares should the need arise...

Another interesting difference compared to the 8050 with Micropolis drives is this weird contraption at the back of unit 0. It is there to support the drive PCB! On Micropolis drives the PCB is simply mounted on top of the mechanism. But the MPI mechanisms only allow for a half length PCB! So the Commodore built PCB hangs over at the end and has to be supported!

You know the drill... Before and after. This time it is not quite as visible. The plastics were already relatively clean.

And another before and after picture for the metal frame. The white crust leaves behind dark splotches. I wonder if this can be treated in some way to avoid future corrosion? So far I have used deoxit, which lubricates with some kind of synthetic oil, so I hope this will keep corrosion at bay.

The motor of the 101SM mechanism is a slightly different variant. All the motors have tachometers, so they have four wires: two for the actual motors, two for the tacho. For the drive unit this black piece fell of, it was glued to the top, same as visible in this pic for the other motor. I wonder what it's for. First I thought maybe it's a magnet for the tacho, but it seems to be a solid plastic piece, not magnetic.Maybe some kind of protection for the cables?

The transformer—rusty though it may be—is humming away quietly and outputting the correct voltages.

Now that we know that we have good voltages let’s have a look at the mainboard. The CBM floppy drives are peculiar in that they are multi CPU systems. The 8050 and its relatives have two 6502 CPUs. One, together with some SRAM, ROM, and a 6522 drives the IEE488 bus, also known as GPIB.
The other 6502 drives the floppy mechanisms and decodes the GCR data.
The ICs have a bit of corrosion on some pins. But luckily all the chips can be replaced if necessary.

There are also two MOS6532A chips, also known as RIOT or RAM IO Timer. The same chip was used in the Atari 2600. Each comes with 128 bytes of RAM. I guess these are used for controlling the floppy drives and act as working memory/buffer for the CPU‘s GCR encoding and decoding. But this is just a guess. I would have to check the schematics…

Yeah, I think I've had it wrong. The RIOTs seem to be attached mostly to the IEEE488 port, and the 6522 seems to drive the floppy mechanisms.

Now the case is to be cleaned! Here are some before and after pictures as well as the cleaning cloth…

Next up: all socketed ICs get reseated and the sockets get deoxit applied. The sockets are dual wipe AMP branded. None of that single wipe rubbish Commodore would later use!

By the way: I found the RRIOT! So there are three RIOTS: two regular ones and one RRIOT!

After I mounted the mainboard back into the case, and verified that the correct voltages were still on the transformer output, I attached one drive unit and switched the device on. LEDs came on, but drive LED didn't turn off! Okay, that is not good, but we can work with that! Motor spins up when closing the latch -- that is excellent!
...to be continued...

So I went to fetch the oscilloscope to start probing. Suddenly -- POOF! Smoke, and the lights in my workspace went out. The mains filter blew -- while the machine was switched off! The mains filters on PETs and CBM Floppy Drives are between live and neutral BEFORE the power switch, so ALWAYS connected!
I quickly pulled the plug (not needed thanks to my fuse panel) and inspected the damage...

The fuse is still intact, as the machine wasn't turned on. Also the two huge capacitors seem to be fine, but probably need a bit of reforming (not due to this event, but because they are decades old).

As a quick sketch: What Commodore did was to put the filter in front of the fuse and switch. Had it put the filter behind the fuse, the fuse would have blown, and no smoke... Or at least LESS smoke.

And here ist the culprit. A mains filter to avoid RF feeding back into the grid. It used to be mandatory back in the day, but isn't used quite that often anymore. This specific type of filter is very hard to find nowadays, and IIRC my other CBM machines don't have one either. So I guess I will skip this and wire up N and L as shown in the schematics.

Let us have a look at the mainboard first. Bench power supply is set to 8V and 2A and feeding into Pin 3 of P4 (+8VDC) and Pin 9 (GND). The board draws 1.8A. So far so good. Nothing gets super hot.

Also good news: RESET goes low, then high and stays high on the first 6502.
However: the clock signal is 4 MHz!
The CPUs are supposed to run at 1MHz. However IIRC the clocks are supposed to be phase shifted so that the 6502s take turns in accessing the memory, as the 4KB are shared.
So something is iffy with the clock generation.

The schematics are there but not REALLY helpful as some of the numbers are totally splotchy! Will have to measure each step. My hope is that some logic chip is borked and the clock is not divided down to 1MHz.