Russian PCB ZX80 clone problem

[crrush]

This is hires image of ROM selectors
https://ibb.co/fFv7dGw

And those A11 and E pins are already connected to corresponding PCB points

[Erm]

I bet the EPROM is empty for a start! With mine I also got a couple of wrong 74XX chips, so also check those are all correct.

Oh yes, I have those alternate EPROM connections on mine too, never noticed that! Well, in that case, that looks ok.

[crrush]

I bet the EPROM is empty for a start!

Right, that is good enough to start an investigation. How one may check EPROM content? How did you find that out?

[Erm]

I have a Chinese EPROM programmer and before I started putting the sockets/resistors/caps etc on the board I thought I should probably check all the chips as their age and source was unknown (I wasn't bothered about the Z80 as I have some known good spares of those). I was able to use the same programmer to check most of the 74XX logic chips and discovered right away that the EPROM was just full of empty 'FF's. So I burnt the correct ROM image onto it.

Where are you crrush? I'd be happy to check yours and put the right ROM onto it for just the cost of postage, but I suspect you can probably find someone closer and that may be easier.

[1024MAK]

If you don’t have a EPROM/EEPROM programmer, but do have a breadboard, it’s fairly simple to wire up the EPROM to produce an output. This would just be a simple test to check an address where a value that is not 255 (0xFF) should be present.

The pin out of the chips used in a ZX81 are linked to a post in my signature below. The ZX80 and the ZX81 used the same ROM chip pin out. But the ZX80 used a 4K byte ROM chip.

And here is a diagram showing the EPROM pin outs:


Which type number is on the EPROM that you have, 2732 (4K bytes) or 2764 (8K bytes)?

The ZX80 original ROM is 4K bytes. The ZX81 ROM (which can also be used in a ZX80) is 8K bytes.

Mark

[1024MAK]

Here are the contents of the first 21 memory locations in a ZX80 ROM:

ZX80 ROM, first 21 locations

Mark

[crrush]

I'd be happy to check yours and put the right ROM onto it for just the cost of postage, but I suspect you can probably find someone closer and that may be easier.

This is absolutely very kind of you, but unfortunately, I am in Moscow atm. The real problem is that that guy who sells kits does not care of aftersales, he just says he never tried to solder the kit himself, neither he checked ROM before actual shipping.
So thank you, but I think you are right I will try to get a new ROM from someone near me to reduce the shipping price

Alex

[crrush]

Which type number is on the EPROM that you have, 2732 (4K bytes) or 2764 (8K bytes)?

The ZX80 original ROM is 4K bytes. The ZX81 ROM (which can also be used in a ZX80) is 8K bytes.

Mark

I have 2732 shipped with the kit

[crrush]

May be it is a good idea to get EPROM programmer, after all, I may check TT logic chips as well

[Erm]

I'm following this more intently now, to see how you get on with yours. I was inspired to get mine to a working state, so popped the chips on, added a power socket and with ZX8-CCB Video, plus a 2732 ROM with the correct data on it, all I get is a plain white screen (a little flicker, then white)! I've put it away for now and perhaps we can explore our problems together, once you have a known good written Eprom.