Sinclair ZX80 / ZX81 / Z88 Forums

Thank you very much for the very clear informations. I will try that and will report the result.

Do make sure you do the tests I talked about, just in case there is different versions of the Memotech 16k RAM pack...

Good luck

Mark

Yes, I will but ....

I wanted to be sure that the extension is working properly before to do the 9V modification.....

So, I buit a adapter cable for the PSU from my +2. With this adapter, the original +2 PSU is working on the ZX81 and I have 11,6V on the ram.

But the result is the same as with the 9V PSU. White screen at boot.

I tested all the ram, they are all OK.
I un-solder the LS00 and LS14 and test both in my programer. Both are OK.....

I do not know what to try or measure in hope to have it to work....
I do not think it's worth to do the 9V modification if I can't get it working in the original state.... As the ram, voltage and both LS are working, it should boot....

I also redid the soldering on the edge connector just to be sure but it did not helped....

While I am at it, there is also 2X LS157 on the card.... Could be one of those that is perturbing the card ? If yes, i could also test them...

Also, here on the Memotech 16K pictures :

https://secure.flickr.com/photos/264960 ... 402385492/

There is one resistor on each ram IC soldered on the back of the PCB. On my card, those resistors (10 kOhm ?) are not soldered.
It seems my PCB is the same as the one from the picture. I aske me why those resistors are there and not on my card......

Unfortunately, there is also the so called prom... a N82S123N on my card. If this one is broken, the possibly repair will be over for me...... As I've understood, it seems the data to program something equivalent are not available.....

Hourrra !

I still have to test but now the ZX81 boot with the 16Ko ram....

I do not know what hapened with this 16ko but the two LS157 were broken ! Lucky enough it's not the prom...

Some more testing and I will do the mod for the 9V.....

EDIT : The ZX81 also start with the 9V PSU and the 16K pack + HGR. DIM A(3000) return 0/0. But it will probably be not reliable as the ram are powered just with 9V with this PSU.....

Well done

The 74LS157 are multiplexer chips to switch between the 14 bit address from the Z80 to the two lots of 7 bit addresses used by the DRAM chips.

I don’t know why there are extra resistors fitted to the Memotech 16K RAM pack in that photo. They were not present on the unit that I took apart some years ago.

Mark

Hi,

let me describe my experience with this beauty. In this case, there was no boot issue but random characters had started to pop-up at random places on the screen after several seconds. Usually it ended with a blank screen or black stripes. But, before crash, I was able to execute a few lines or code with no problem.
At first, I got voltage levels checked - everything in limits. Just to be sure, I recapped the entire unit but with no success. A next step was to desolder all 4116's, insert sockets and try with known good ones. Nothing.
An oscilloscope showed an unstable CAS signal which was strange so I put the RAM pack in two more ZX's to eliminate M1 and connector issues. Nothing (again). Both 74xx IC's in refresh circuit were OK so everything had pointed to PROM.
Anyways, there were two more IC's left to replace - 74157 multiplexers so I decided to do that as well.

Bingo! At the moment I don't know how to explain all of this - do you have any idea ?

Let me Guess...

Some dust in the dip switch ?

If a dip switch stay many years in a static state, you had to apply a switch cleaner oil and shake it on to off.
An unrecognisable state can hang the memory setting, because of bad contact.

I have tried this as well but in vain.
Anyways, after I had replaced both 74157's, it has started to work.

The most weird thing in this case is defective chips behavior - they are usually either completely functional or completely malfunctional.