exxos's Atari, Amiga & retro forum

I think I've tied it down to the READY line never going high and that's controlled by a quad AND gate (74LS21). All the inputs to the lines which ultimately generate the signal are high but the output is low. When I connect one of the inouts with the output the screen goes black, suggesting that something happened on the processor side.

So, I need a replacement chip.

More components ordered...

Then I tried to remove the old chip.

Most of the pins did free but one wouldn't desolder at all. Clipped the pin and got the chip out, then after a lot of effort the last pin was removed from the board, but I can't for the life of me clear the hole. I'm going to have to get my tiny drill and pin vice and try to gently clear it that way.

With the 74LS21 removed and a socket installed the processor is actually running, though it's not doing anything useful as it can't switch between the I/O bus and the memory bus. Still, it's an improvement.

The ground pin hole was very difficult to clear, even with a drill. I'm glad I don't need to do that again, at least on this chip.

OK. Looks like a CPU issue too as the M-/IO line wasn't operating correctly, merely giving spikes rather than proper signals.

Hopefully, after the new chip arrives tomorrow I should get a bit further.

Well, UPS still haven't turned up with my Farnell delivery and it's still showing on the tracking that it will be delivered by 13:15.

I'm guessing that I won't be getting those 74LS21s today then.

stephen_usher wrote: 12 May 2021 15:55 Well, UPS still haven't turned up with my Farnell delivery and it's still showing on the tracking that it will be delivered by 13:15.

I'm guessing that I won't be getting those 74LS21s today then.

Boo to UPS!! (Hopes own Amazon package actually arrives today!!)

They now say that they attempted delivery and the building was closed, which it's not.

Apparently they'll try again tomorrow. :-/

P.S. The driver reported that he tried delivery at 4:33pm. I was in the reception area at that time and no-one tried to get into the building.

Although I couldn’t get the machine to run properly I was able to do one thing this evening, test the IR receiver board.

I didn’t know if the IR remote control for the TV would have the correct 940nM wavelength but when pressing the buttons I can see a signal on pin 13 of the Z80 SIO chip, so all is happy on that board!

I’ve also started writing some of the code for use on a Raspberry Pi so as to try to use it to translate key presses on a USB keyboard connected to the Pi into IR packets that the F1 will understand. This evening’s code basically allowed me to create the Hamming encoded packets. I’ve still got to make a 7 bin ASCII to Apricot scan code lookup table, including whether shift and/or control has been pressed.

Of course, once this has been done I’ll have to (a) build the IR sender board based upon the original keyboard schematic circuit and (b) work out how to drive the single-wire synchronous serial port on the Pi!

UPS have finally delivered the chips, so playtime later.

Popped in the 74LS21, switched it on and... success! We boot to the BIOS.


Unfortunately the floppy is no longer working. This may be a problem due to me taking the floppy apart and not putting it back together properly or any number of things. I've also got another (new) crack in the case to repair.

So, two steps forward and half a step back.