ZX81 Downsway 64k memory expansion

[Gryzor]

My friend was given, by his father, a box full of assorted "old stuff". His dad was seriously in computing in the early eighties, thankfully graduating from a ZX81 to a CPC464 which my friend still has to this day (though the tape deck is broken), as are lots of his tapes and disks *nostalgia mode on*.


Anyhow, in the box were a couple of ZX81 PSUs, a ZX thermal printer along with five sealed printer rolls (the ones with the aluminum coating - I wonder how much these go for), a standard Sinclair 16K mem expansion (he told me that when his father was working he would ask his mom to keep him from jumping around because of the wobbling - that's quality for you! Sinclair did parenting too!), and... this:



I'm only posting it because I haven't seen it before and I can't find anything online, so I suppose it's pretty rare (and I guess it would've cost him quite a bit more than the computer!). Inside it's got a stamp stating June 1982. Here's the insides of it:



That's the back in all its non-linear beauty.



It's got too boards! One houses the controller and stuff (TM) and the other the memory chips. Unfortunately the "connector" is unlike everything I've seen and it's soldered, so I couldn't pick it apart



Vertical take



The mem board. Strange old little chips...


Anyhow, anyone's seen this expansion before?

[Bryce]

Nope, but what a cool little device. The Mem chips are Ceramic package, but what really got my attention is that 3W (or possibly 5W) resistor in there!! What's that all about?

Bryce.

[Gryzor]

Ceramic - ah yes, I knew they were reminding me of something (namely, CPUs)...


As for the resistor, well, *I* should probably be asking *you*


With extreme caution I realised the connector is quite pliable, so I took a couple more photos:











And an ad that was tweeted to me:







Btw, full-res photos over at Downsway 64k - Album on Imgur .


Interestingly, a whopping 8KB of memory is not usable.

[Bryce]

I assume it's on the power line to the memory chips to limit it. At 3W 5V means they were expecting some seriously high currents to be flowing. My only explaination for this could be that they were expecting the RAM chips to fail to a short, in which case this resistor would limit the current to 150mA to save the ZX81 PSU from getting damaged. However, this would only produce 0.75W so maybe it's only a 1W resistor?

Bryce.

Edit: If it's on the 9V a short would produce 272mA across this resistor = 2.45W which sounds more realistic, but nothing on that PCB should need 9V. Eitherway, they weren't very confident of the circuit not shorting?

[EgoTrip]

Maybe you should ask over on World of Spectrum, they might know more about it

[Gryzor]

Yeah, I'm not that interested so as to join the dark side I just found it cute and interesting...

[Gryzor]

Edit: If it's on the 9V a short would produce 272mA across this resistor = 2.45W which sounds more realistic, but nothing on that PCB should need 9V. Eitherway, they weren't very confident of the circuit not shorting?

Good point


Btw, do memory chips really fail to a short? I thought they just stopped working, not blowing up!

[Bryce]

Well it depends on (A) What type of RAM they are, (B) what part of the RAM circuitry inside the chip has failed. Yes, they do fail to a short quite often, but not always.

Bryce.

[The Last Bandit]

Interesting piece of kit alright, any chance it still works ?


RAM can definitely fail short alright, recently repaired a Colecovision where the RAM looked to have failed short and nicely scorched the PCB.

[Gryzor]

No idea if it still works unfortunately, my ZX is stowed away... somewhere...

[Bryce]

Easy repair even if it doesn't work:

Test: If ZX won't start with it's connected then the RAM has failed
Repair: Swap RAM, finished.

The caps look to be decent quality and still in good condition. 4164s are still readily available, but you won't be able to add sockets due to the sandwich design, so they will have to be soldered in.

Bryce.

[highlander]

No idea if it still works unfortunately, my ZX is stowed away... somewhere...

Just found the box with the ZX81 that my father gave to me, I think it still works and I have both power supplies here (ZX & printer).
BTW I just talked to my father and he told me that this RAM upgrade cost 100$ in 1983, he actually bought it using dollars from a shop in UK...

[Gryzor]

Ladies and gentlemen, please welcome our other system admin and congratulate him for his first post in years - even if it's a Spectrum-related one

[Bryce]

Yeah! Now there's two people we can complain, moan and grumble at!

Bryce.

[Gryzor]

Please do; I can give you his email