Weird memory fault

[Orac]

Added a 62256 to a ZX81 for the 16K internal memory upgrade but I’m having issues.

Here is a pic of the 62256 memory in a socket on the board, LK2 is in place, wires lead to the board:

Here is what I see:

You can view a video of it here:

https://youtu.be/Yfr8rp579no

One thing to note, the machine works fine with a 16K RAM pack installed so it is working, just not with the internal memory.

I have two 62256 chips, one wired as recommended on ZX81.de here http://www.zx81.de/english/32k-rame.htm
But it didn’t work so I removed it.

It’s currently installed a different way, I think I saw that here. Same fault both ways.

So I’ve tried two 62256 chips, wired it two ways, both show the same issue.
But a ram pack works fine

Any help because I’m stuck.

[Lardo Boffin]

When an external RAM pack is plugged in the internal memory is not used.

I have a dead zeddy that works fine when a RAM pack is plugged in but otherwise won’t boot (I will be doing the 16K RAM internal mod to fix this).

Have you done the usual checks for solder splashes etc.? Also do you know for certain whether the RAM chips are working?

[Moggy]

I notice the board looks like a ZX81 replica, does anyone know if that makes a difference?
IE wired slightly differently but works in the same way as a real board if anyone follows my meaning?

Sorry if that's not clear.

[1024MAK]

I notice the board looks like a ZX81 replica, does anyone know if that makes a difference?
IE wired slightly differently but works in the same way as a real board if anyone follows my meaning?

Sorry if that's not clear.

It depends on if the replica is wired with A0 to the A0 pin on the RAM, A1 to A1, A2 to A2 etc. Only the original Sinclair boards are not wired like this, they take advantage of the fact that it does not matter which address line from the Z80 goes to which address pin on the RAM chip, as long as the required number of lines are used. Same with the data bus D0 to D7 lines.

Can a photo of the underside be shown. Also a overhead photo of the board with no RAM chip fitted?

Mark

[Orac]

When an external RAM pack is plugged in the internal memory is not used.

I have a dead zeddy that works fine when a RAM pack is plugged in but otherwise won’t boot (I will be doing the 16K RAM internal mod to fix this).

Have you done the usual checks for solder splashes etc.? Also do you know for certain whether the RAM chips are working?

I used the ram pack as a test to switch off the internal ram, yes.

I’ve checked for splashes, shorts that I can see. No, I don’t have a chip tester to check the ram.

[Orac]

I notice the board looks like a ZX81 replica, does anyone know if that makes a difference?
IE wired slightly differently but works in the same way as a real board if anyone follows my meaning?

Sorry if that's not clear.

It’s an exact duplicate of an Issue 3 and others have it working.

[Orac]

It depends on if the replica is wired with A0 to the A0 pin on the RAM, A1 to A1, A2 to A2 etc. Only the original Sinclair boards are not wired like this, they take advantage of the fact that it does not matter which address line from the Z80 goes to which address pin on the RAM chip, as long as the required number of lines are used. Same with the data bus D0 to D7 lines.

Can a photo of the underside be shown. Also a overhead photo of the board with no RAM chip fitted?

Mark

The German mod wires the address lines correctly, the current mod does not but is a valid mod.

Neither work. Both give the same result.

[1024MAK]

Sorry, I was meaning the wiring of the PCB tracks. But you have since said that the PCB is an exact copy of an actual board, so the PCB tracks should be the same.

Mark

[Orac]

I’ve now removed the new ram and put in an HM6116 in the lower part of the socket and changed the link back to LK1.

That doesn’t work either. Lol

So it’s a memory fault on the board somewhere, any tips?

16k external ram pack still works so the address and data lines must be ok. What might not be?

[1024MAK]

It must be a broken PCB track or poor connection somewhere. Test using a multimeter (using the continuity range or the appropriate resistance range) between each RAM pin/socket pin to the relevant Z80 pin, or edge-connector contact, or the appropriate resistor (R7 to R14 for the eight data lines).

Also test from the /CS pin for the RAM to resistor R2, the value of R2 (should be 680 ohms) and from R2 to ULA pin 12.

Mark