Flashing Floppies

So I installed a Gotek on my Roland S-750. It worked, but it didn't automatically boot the system image when powering on. I had to select the image and then it would boot, which is peculiar.

I spent a lot of time tracking this down. I made extra sure that the flashfloppy config was absolutely correct and pointing to the right image file. I even copied the exact same config from my Roland S-760 that was working correctly and booting automatically. But no dice.

What eventually ended up working was setting pin34 = nchg in the FF.CFG file, though I'm not entirely sure why. In the schematics, it's quite clear that pin34 is active-low, and so "nchg" should be correct.

Or, well, I suppose "D_CANGE" is supposed to be active low. The overbar is clearly visible, at least.

But what gets me is that it's always supposed to be active-low, and I don't really understand why flashfloppy very clearly sets the pin to active high for the IBMPC mode.

This is very peculiar. I'm glad it works, I guess, but it feels unsatisfying that this contradiction doesn't really make sense.

It's All About Control

As mentioned in my previous S-750 blog post, my unit is reading a ghost-mouse on the EXT CTRL port, which results in a phantom cursor that slides down to the bottom-right of the screen at all times, disrupting the UI control. This is due to IC14 on the mainboard failing, and while I don't yet have a fix for that IC, I can at least try to work around it.

First off, the simple test: does bypassing the blown fusible resistor help?

Well, it would be nice, but no it does not. It still detects a phantom mouse and will not speak to the RC-100. So, it's time to get drastic. Let's have a look at the backside of the connector.

It's a bit grimy with flux from when the board was put together back around 1991, but other than that nothing looks too amiss here. I did see what looked like a blob of conductive something that might have been shorting pin 8 to ground.

But cleaning it off, as I expected, did not yield any improvement.

So the drastic step I'm going to take here is to completely disable this port. I'm doing this by shorting out pins 1, 2, 3 and 4 to ground. These are the pins used by the MSX mouse to report its movement, and by shorting them to ground rather than leaving them high, the movement will be reported as 0x0,0x0 instead of 0xF,0xF. We'll still end up with a mouse cursor, but it won't move, and thus won't interfere with the button-based navigation.

It's not the most beautiful solder job, but it'll do.

While I'm in here I'll also pull the blown fusible resistor, since it's not doing anything at all useful.

And then leave a note for whoever opens this next wondering why the EXT CTRL port isn't working and half the pins are shorted to ground.

Also while we're here, since this is now in a usable state, let's get rid of the floppy drive.

I do actually have the physical boot floppy for this, which is nice, but I'd rather not have to rely on a 35 year old mechanical drive. So in goes a Gotek.

Conveniently the set of thumbdrives I got to use with the Goteks was a 3-pack, so I only needed the floppy emulator itself.

Next up, the feet need some love. The front two are in good shape, but the rear two have exited the chat.

A little heat and scraping gets the remains of the tape off.

And after cleaning the remaining adhesive with some alcohol, the new feet go right on.

Weirdly this set of feet came in a set of 18, which is very much not divisible by 4, so I'm glad I found a use for the remaining two.

On the good news front, the video-out works, and gives a much better view of what's going on than the built-in front panel LCD.

The composite-out circuit is a bit primitive, though, and only displays a B&W image. It looks a bit fancier in colour, like when I hook up to my S-760.

Though you do only get 8 colours, since the display has only 1 bit each for red, green and blue.

Seven Fiddy

I've been on the lookout for a certain piece of hardware for a little while, and just recently a good deal came up on eBay, so I jumped at it.

This here is the Roland S-750. It's a much older rack sampler than the Roland S-760 that I purchased previously.

And it is a chonky boi. It's 3u tall instead of 1u and a bit deeper than the S-760. What's somewhat amusing is that the boards inside don't really take up all that much more actual space than what would fit in a 1u case, but whatever, it is what it is.

Why did I want an S-750? Well I kind of didn't, actually. I wanted what it came with, which is this.

Wait, no, that's just a regular IEC power cord. I mean this.

This is the RC-100 remote controller, and it's actually worth more than I paid for the whole bundle together. So I essentially got a discounted RC-100 and a free S-750, which is kinda cool.

The RC-100 works with a number of Roland samplers, and in particular it works with the S-760, thus why I was interested in picking one up.

Anyway, fun will be had with the RC-100, but first let's see what we can do with the S-750. Inside we can see that it lacks the wave memory expansion, which is also something I'll be tinkering with later. It slots into the two grey connectors up at the top of the board.

But first, the volume knobs were looking a little misaligned and were binding somewhat, so let's see about fixing that.

The diagnosis of this issue is at least pretty simple.

This metal bracket is not supposed to be shaped like this. Let's fix that real quick.

And that's much better. Not perfectly straight, but it'll do. Back in the unit, things are looking much more aligned.

Though I do need to deal with one of the volume knobs, which managed to separate into two pieces while I was trying to pull it off the shaft.

A bit of not-so-super glue does the job just fine.

And the knobs are looking much better now.

So let's see what the RC-100 does. Just plug it in and hold the right buttons on the front panel to select it.

Except hmm, no dice. It's not powering on. Let's have a look at the jack board to see if it yields any clues.

It's the board on the right, there. Pulling it out we can see the problem pretty easily.

R35, a fusible resistor, has fused. This is not really a surprise given that it's on the +5v rail which supplies this port with power.

And as you can well imagine, with these male pins sticking out of the front of the unit, it's only a matter of time before something conductive comes in contact with them and lets all the magic smoke out of the fuse.

So a new pack of fuses is on the way, but in the meantime there's another problem: the sampler keeps auto-detecting a mouse, and the UI is scrolling uncontrollably when nothing is connected to this port.

After doing a fair bit of diagnosis, it looks like IC14 on the mainboard has fried. It's reading a very clean logic-high on the unconnected port as a logic-maybe, and interpreting that as (junk) data from a nonexistent mouse.

This is unfortunately a Roland-proprietary ASIC, and so no direct replacement is available unless I can find one to salvage from a parts unit. The other option would be to reverse engineer the ROM and OS to determine how it talks to this chip and then replace it with an FPGA. The chip doesn't do any particularly complicated tasks, it mostly just does the front panel matrix scanning, drives the LEDs, and acts as an IO expander.

Anyway, that project, assuming it ever happens, is pretty far down the to-do list so we'll see if I ever get around to it.

Oh and just in case you're wondering, the RC-100 works perfectly with my S-760.