ZX Max 128 Issue 3

Hello, you can buy those components from eBay, will add link tomorrow.

Hi, it’s hard to say what’s the problem. For U2, it’s 28C256 or 27C256? After power on, try press reset and spacebar then release reset first and release spacebar after that. Does it boot to DivMMC ?
There is ZX Max 128 issue 5 which includes DivMMC on board.

Not for now, but maybe in the future I will modify to support +3 ROM

That's a great pity - I wanted to add an internal SD or CF card but would really need the +3 ROM to do that..

It need 1 more IC, if you want to try I will think about it and send you schematic to try

Definitely! I'm up for trying that!

Great! I want to try it too.

Let me know if it works or not https://github.com/DonSuperfo/ZX-Max-128/blob/main/Issue%203/Modify%20for%20%2B3%20ROM.pdf

Let me know if it works or not https://github.com/DonSuperfo/ZX-Max-128/blob/main/Issue%203/Modify%20for%20%2B3%20ROM.pdf

ZX Max +3, let me know if you want to try. I have some PCB See picture here https://postimg.cc/47PRq0Hg

ZX Max +3, let me know if you want to try. I have some PCB. See picture here https://postimg.cc/47PRq0Hg

I didn't see this post - do your still have the PCB?

I don't have PCB any more, you can order from here https://www.pcbway.com/project/shareproject/ZX_Max_3_Issue_1_eaec7c7c.html or https://www.pcbway.com/project/shareproject/ZX_Max_128_Issue_5_7ac0f41b.html

U9 is for paging RAM, you may try to check it. If bad memory, it maybe either U3 or U4 but I don't think they are bad.

Thank you for the information. I'll try changing U9 out. I'll come back with the outcome. Cheers.

A quick reflow of the solder joints for U9 fixed this problem. I am very grateful for your guidance. Thank you so much.

Glad it works!

Strangely it has failed again today. I loaded some 128K stuff and it was working fine, then after a while it started to fail to load 128K stuff again, so I used the TestRam program again and it's showing the same fault as before. I have received the new IC for U9 so I will try desoldering the old one and soldering in the replacement. Fingers crossed that fixes it for good. Cheers.

:-( sadly the speccy is still faulty in 128k mode. It's so weird. I change the IC for U9, with difficulty due to my bad skills and tools, but I got it charged. I had some problems with solder bridges and resolved those. Then the system worked fine in 128k mode when I tested. I put the screws in to the case to finalise everything, re-tested and it's back to failing the paging test.
I guess it's some kind of temperamental issue like a bad component or dodgy solder joint somewhere.

Never mind. At least it works in 48K mode.

So... I figured that fastening the 48K case tight seemed to have an influence on whether the system could page the 128K RAM or not. I took out the screws and magically the 128K paging worked fine again. On closer inspection I can see that certain capacitors are a little too tall to allow the circuit board to sit down on the mounting points in the reproduction 48K case, so when the case is screwed together it is forcing the circuit to distort. I've added some spacers to try to prevent this happening and it sort of works. If I press hard enough on the rear corners of the case I can cause a memory faulty, so it's not fixed but at least I know what the cause is.
I didn't assemble the board so I can't comment on the selection of components used but hopefully this little snippet of information can help others if they discover a similar problem.

Thank you for sharing !

I never see this problem before, do you have another CPLD EPM7128 to try?
Or try to reduce R80-R84 from 10K to 4.7K. Let me know if it work or not.

I'm going to change out the keyboard connectors first they seem very flimsy and a bit loose
Failing that I will do as you suggest
Thanks for the reply by the way very kind of you

I just upload "testram.tap" to GitHub https://github.com/DonSuperfo/ZX-Max-128/tree/main/Issue%203 Can you run to see if all 128K ram pass. About keyboard, what keyboard you used? do you have other keyboard to try?

You can put to any 5V point, Use 1K resistor, one side connectto5V the other side connect to Anode of LED, Cathode of LED connect to GND.

You can use standard 10K resistor and solder in vertical and connect one end together as this link https://modwiggler.com/forum/viewtopic.php?t=115629 I don't think it relate to sound, can you check AY chip?

I have ordered one of these 10k resistor's should be with me over the next couple of days. This is the only thing I'm missing on the board I bought off Ebay it all works perfectly apart from the AY sound which is all out of tune on any 128k game I try.. This is my second board now, the first one the tape port audio didn't work.

The AY is a Microchip AY-3-8910 (1232 CCA).

Oh well, I put in the missing component and as you said it wasn't related and didn't make a difference 😕. Maybe it's a dud sound chip or a fake 😔.

It use standard ZX 128 ROM, you can download from here. https://mdfs.net/Software/Spectrum/ROMImages/

Thanks!