Zeddy white screen after memory upgrade

[msknight]

OK ... I installed this mod...
https://myretrostore.co.uk/product/zx81 ... expansion/

...and I now get a white screen with no cursor.

The fact that I got the same when I got the Panda working briefly, indicates that I could have likely damaged something on the motherboard.

The following tests all hold... in particular address A14 and A15 are pulsing slow enough to see, with A15 pulsing slightly slower than A14

Pin Function
16 /INT (should be pulsing)
17 /NMI (should be pulsing)
18 /HALT (should be pulsing)
19 /MREQ (should be pulsing rapidly)
21 /RD (should be pulsing rapidly)
22 /WR (should be pulsing)
24 /WAIT (should be pulsing)
25 /BUSRQ (should be high)
26 /RESET (should be high, except for a very short time after power is applied)
27 /M1 (should be pulsing)
28 /RFSH (should be pulsing)

I believe I should be testing all the address pins for continuity?

Of course, if someone has seen this before and knows what it's likely to be, I'd be grateful!

[msknight]

I'm trying to continuity test the address lines on this issue 3 board, and they seem to be jumping around. It's almost as if the memory expansion is mapping to the Ferranti rather than the CPU.

I'm confused.

[1024MAK]

Not all the address lines to the RAM connect directly to the Z80 like for like with the same name. Some have been swapped around to ease PCB track routing.

With SRAM, it does not matter if address or data lines are swapped around. If you check the link to chip pin-outs in my sig below, near the end of that topic, you will see the actual Z80 signal names, as opposed to the RAM manufacturers names.

Or you can look at a circuit schematic.

For the SRAM data lines, these go via resistors (R7 to R14) so that the ULA can override the output from the SRAM.

Again, some data lines are swapped around.

A white screen can be caused by a number of things, please double check all connections.

Please check for short circuits caused by solder splashes on both the top and bottom of the board.

Also, with 16K bytes of RAM, it takes the machine a bit longer to get to the inverse K compared to when it's only got 1K of RAM.

Mark

[Moggy]

I only ask as I can't see it in your photo, but have you altered LNK 1 to LNK 2 ?

[msknight]

I have made the link between the centre point and link 2, but there appears to be nothing "in" a link 1 to begin with, if you know what I mean.. because there's no jumper. ... unless there's a trace i've got to cut.

[msknight]

Please check for short circuits caused by solder splashes on both the top and bottom of the board.

OK - so what you're saying is that it doesn't matter which address lines they map to... as long as they map to "an" address line.

Gotcha.

Will get out the probes now

[mrtinb]

This issue around the holidays, is what we call: White Christmas.

[msknight]

OK - everything tones through to something and I can't find any shorts. It doesn't go to K, it just sits at a white screen even when giving extra time.

[msknight]

This issue around the holidays, is what we call: White Christmas.

Ha! Good one!

[msknight]

I found a short between 6 and 24 ... er... on the CPU that's pins... er... 33 and 37.