Sinclair ZX80 / ZX81 / Z88 Forums

Hi all Zeddy fans. I have just made the internal 32K mod and something is not working right.
When i run HR software the screen is all corrupted. I am using this memory chip: 62256-LP70 32Kx8 70ns RAM DIP28. I have tested it with 2 new chips and the result is the same. I have attached a pic. Any help would be appreciated. Thanks.

The pic is missing - and what scheme did you use to add the internal 32K RAM ?

By the way, I have another customer who has done the internal mod from http://www.zx81.de/english/hard_e.htm - they want to now use a ZXpand - can anyone confirm whether this scheme still respects the RAM/CS line (so that the ZXpand would disable the internal RAM ?)

If not - is there a way of improving this mod so that it will work with the ZXpand?

Charlie has offered to make a custom GAL and add a jumper to allow the customer to still use the ZXpand (although it will not use the ZXpands memory) - but it would make more sense to update the modification scheme for the future!

Not nice to steal a thread, but anyway: there's a 680 Ohm resistor (R2) between ULA pin and RAMs / edge connector. Which disables /RAMCS as soon as a RAM pack pulls it to +5V.

So any internal RAM mod will obey this scheme as long as it uses the same /RAMCS as chip select for the RAM (or uses its own manner to include this signal). Which the above schematic does.

Yes we need schematic used by 1st poster. Photo to go with it would be nice. Btw: welcome here, Proteus!

The shematic is from :http://www.zx81.de/hardware/32k-ramd.htm.
And here is the link to the screen shot: http://s1174.photobucket.com/albums/r60 ... 256ff8.jpg

ah ah, it's almost working , try adding a 33pf (pico farad) capacitor between Ground and the RAM CS pin.

my reasoning, on some of my Zeddy's i have had problems with Hi-res also, but when using an oscilloscope to see whats going on it starts working with thw load of the probe on the CS pin.

Regards Andy.

If the machine starts & runs software, that means the RAM is working. From the schematic:

if you want to run HRG8, connect pin 27 via a resistor of 10K ohms with 5 volts

Can you check that you didn't forget this?

Also the M1 not circuit might not be working, perhaps the LS00 overheated when soldering, or you picked up /M1 / A15 in the wrong spot. Can you check what happens when you connect the SRAM's pin 1 to a power supply (GND or +5, either will do) instead of to the LS00?

Thank you all for the input. I will try to add the capacitor as Andy suggested and see what happens.

P.S: RetroTechie, i have the resistor in place.

Oops - yes welcome to the forums Proteus.

Thanks for the photo - so it is the same schematic as my other customer (is there any other I wonder?)

Sorry for the rudeness (it has been a long tiring day at RWAP headquarters) but as it's the same circuit, it was at least relevant - particularly if the problem is (as Andy suggested) relates to the RAM/CS pin.

If this doesn't resolve the issue - I guess the other question is what hi-res schemes have you tried (there are several different ones - it may well be that some of the schemes work and not others, which may indicate something else)

Well, today when i came from work i got back to my zeddy and tried the capacitor suggested by Andy and loaded "wrx1k1.p". YEESS it worked!! The circles where perfect! Next i tried "hrg-16k.p" to see it in full glory...damn the same problem again as i had yesterday . Next i loaded a 16K no hi-res and it loaded fine ( so the memory must be fine ). What can i do next? Well, away with the 74LS00 and just try the 16K hi-res. Again no luck. Then i measured the R2 resistance that RetroTechie mentioned. Well, 2,5V. Well at least i have 16K! hehe

so the 1Kwrx program worked ?

have you tried any other hi-res programs such as 25thanni or no limits, or the hi-res only maxdemo ? not all hi-res shemes work the same and some require memory at 8-16K

check out this webpage http://www.swatosch.de/zx81/

and also this thread viewtopic.php?t=289&hilit=maxdemo


Regards Andy