Sinclair ZX80 / ZX81 / Z88 Forums

Hi everyone

I've opened up a ZX81 I bought as not working on eBay and found the following user modification to the RAM.
I am assuming that the original owner has (tried to) upgrade the internal RAM but I cannot see how the modifications that they have made correspond to the internal RAM upgrades I have read about elsewhere.

Does anyone have any ideas as to what kind of RAM upgrade this might be and whether or not it looks like it should work. From what I've read it looks as if some of the pins should have been bent up but I am just guessing.

It is possible that this is a botched upgrade and that this is the reason why the machine does not work. I don't know. I don't even know for certain that the machine does not work as I have not applied power yet or made any other tests. I wanted to inspect visually to see if there were any obvious issues with the motherboard before I did so and found this which baffles me (but being new to this, many things baffle me so I thought it wisest to ask!).

Thanks in advance for any suggestions.

(I took these photos earlier and am not in front of the machine right now so I'll have to take any suggestions in batches and do them all at once when I next get time to sit down to do so. I will update this thread with any new information as and when I get a chance to investigate further.)

It looks underneath like this one I have done myself a good few times:

http://8and16bit.blogspot.com/2015/08/m ... o.html?m=0

Not sure about the wire on top though - maybe a track got lifted and that is a repair?

Actually I can see that it is a fixed track - see below.

Not sure about these:


Maybe more fixes?

Hi Lardo

Thank you. That is amazingly helpful. Is the site you link to yours? Those pictures are very useful.

Next time I get a chance to sit down with it, I am going to risk powering it on and then, if there is no display, start investigating further. Now that I know what this mod is supposed to be doing, I'll have a better chance of working out if it is working as intended.

I'll keep you posted on my progress.

(And thank you for your welcome on the other thread too!)

twoangrybears wrote: ↑Thu Jul 25, 2019 9:47 pm Hi Lardo

Thank you. That is amazingly helpful. Is the site you link to yours? Those pictures are very useful.

Next time I get a chance to sit down with it, I am going to risk powering it on and then, if there is no display, start investigating further. Now that I know what this mod is supposed to be doing, I'll have a better chance of working out if it is working as intended.

I'll keep you posted on my progress.

(And thank you for your welcome on the other thread too!)

You are welcome! No the site isn’t mine. I am ok at following other peoples instructions and expertise but not in coming up with ways to do these things! I am also familiar with lifted tracks when modding these things.

Has it been composite modded? Daft as it sounds I sold one (of quite a few) on eBay where I had done the RAM as shown and fitted a ZX8-CCB composite mod board and the buyer got in touch to say it didn’t work. He had tried to plug it into the RF input of the telly and not a composite socket! Once plugged into the correct socket all was well.
After that I always bundled a composite to SCART adaptor with them to be sure it was plugged in correctly.

twoangrybears wrote: ↑Thu Jul 25, 2019 9:47 pm Hi Lardo

Thank you. That is amazingly helpful. Is the site you link to yours? Those pictures are very useful.

That be my blog. I really must get around to posting up some more content. But real life and distractions keep consuming my time...

Mark

Has it been composite modded? Daft as it sounds I sold one (of quite a few) on eBay where I had done the RAM as shown and fitted a ZX8-CCB composite mod board and the buyer got in touch to say it didn’t work.

Lardo, can you believe it, yes it has - perhaps this is one of yours? - and it turns out that plugged into a composite socket it *does* work (mostly)!

PRINT PEEK 16388+256*PEEK 16389 shows 32768 so it looks as if the RAM upgrade is working.

The machine is very unstable however so I am going to try replacing my original 700mA power supply with a more modern one first before I explore other avenues.

Fortunately I bought a second ZX81 which is working beautifully with a portable UHF TV Monitor so I have one to enjoy using and another to enjoy trouble shooting!

Life is cruelly busy but hopefully it won't take me another five months before my next post. This thread though can be considered closed. Thanks everyone for the help!