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.