Piggyback RAM - TMM41464AP??

So you can run 256K software now on internal RAM? Nice!

Seems that MA18/KDBIR double function is interfering... To fix, you'd need a small circuit that holds S1985 /X5 pin (a keyboard return signal) low until reset line has gone from active to non-active, and doesn't interfere with /X5 after that. I don't have a ready-made / dead simple circuit for that, but shouldn't be too hard to figure something out.

To other people following this thread: suggestions are welcome...

For an example: check Sony HB-G900AP service manual page 75 on how this machine does it on /X1 and /X3 signals. I suspect something similar could be done with 2 or 3 suitably connected components (resistor + capacitor + diode or something).

You should find R.van Gans epifiet
He made ma18 work and KBdir. !

Wait..!! HB-F1XD already has a suitable circuit, and it appears very easy to extend.

Look in the service manual on the page (21) that has joystick and printer ports wiring, above (or on the right, depending on view) black marking "IC11". There's a transistor (Q5? looks a bit blurred) with base connected to RESET signal (S1985 pin 85, active-high). There's 2 diodes (D13 and D14) connected to that transistor, and the other side of each diode to a /X.. keyboard return signal. When RESET is active, the transistor pulls that common connection low, and (through those diodes) the corresponding /X.. signals. When RESET goes inactive, the transistor 'releases' those signals. The transistor + diode adds a small delay, so that /X.. line is seen as 0 when S1985 RESET signal return to normal (inactive).

So all you'd have to do is add a 3rd diode between that common point, and /X5 keyboard return signal. Optimal would be a small Schottky diode like BAT42/43/46/82/85, but possibly a plain 1N4148 works too. Connect the side that's marked with a ring to the common point / transistor pin, the non-marked side to /X5 pin. Just be careful not to solder directly to that transistor, these parts overheat easily (and diodes too, but less quickly). Solder to circuit board traces there (or diode D13/D14 / resistor array RB4). When machine works fine as before (check whether X, P, H and 5 keys work normal!), re-connect MA18 to the SRAM.

Btw: elsewhere in the service manual I see that common point would be that transistor's center pin. Don't take my word for that though, check it on the board!

Good luck!

Try this. You're an idiot for trying all this stuff without the service manual. ;) I assumed you had it all along.

Anyway, there's that common point where transistor pin & ring-marked side of D13 / D14 connect. That's where you connect ring-marked side of a 3rd diode, connect non-marked side of that to /X5 keyboard signal.

Thats the place I thought you mean't, at least I can follow instruction well ;)