16k Sinclair issue 1 pack

[dr beep]

Try this game without RAM-pack.
Does the ZX81 show the right picture then?

You can do it, the game is my 1K 3D Random maze (Game 5 of 81 1K hires games)

[msknight]

Everything works fine without the ram pack. The trick is to convert the RAM pack to WRX. That's what I'm trying to achieve with this one.

The other thread contains the other problem which is the vintage ram, and trying to get that to work.

Once I've got these two problems licked, I'm good to go.

[1024MAK]

Well, if no one has any ideas to help on this...

Which diodes did you use in the end?

Standard silicon diodes have a forward voltage of between 0.55V and 0.7V at low currents.
Germanium have a much lower forward voltage, but are rather hard to find now.
Hence why I recommended Schottky diodes (BAT types) which have a forward voltage of between 0.15V to 0.45V.

The value of the resistor is less critical.

Maybe you should show us some photos of what you have done.

Mark

[1024MAK]

Simulation of the added circuit using 1N4148 diodes:

Link

Same circuit, but using Schottky diodes:

Link

Mark

[Moggy]

Whilst I cannot add anything of use I have to say that this thread is a most absorbing, interesting read plus I'm learning things into the bargain and look forward to a what I'm sure will be a satisfying conclusion.

[1024MAK]

In both the simulations, click the H inputs to flip each one to L in turn, and observe the output, both the logic level and the voltage.

Mark

[msknight]

Ah. Looks like I took the feed wrong somewhere in following the diagram..

This is what I did...

1) Cut the line from RFSH that went to the chip and hooked that line into the 5v supply that was going to the chip anyway.

2) Cut the RD line and put the silicone diode across to bridge it.

3) Put some insulating tape down and linked the 5v, through the 4.7K resistor, and then through another diode, to go to the RFSH pin.

Suplemental... At that point, the ram pack worked, but didn't work with hi res. So I linked the 5v, after the resistor (blue line) to the RD line, after the diode. But at that point, the RAM pack stopped working at all. - the result on boot is a white screen with no K.

[msknight]

The obvious question is... do I have the diodes the correct way around?

[1024MAK]

Not having a RAM pack in front of me, I can't answer your question directly.

Both the black band on both diodes on both diodes indicates the cathode, and should be connected to the circuit that goes to the relevant edge-connector socket pins.

The other end of each diode should form a connection with the resistor and the circuit that the /RD line goes to on the relevant chip.

Mark

[msknight]

That is what I believe I have done. I'll tear it down and check it all.