Memotech 16K RAM pack preventing startup

Discussions about Sinclair ZX80 and ZX81 Hardware
SabreWulf
Posts: 19
Joined: Mon Jul 26, 2021 6:31 pm

Re: Memotech 16K RAM pack preventing startup

Post by SabreWulf »

C3 was removed and it tested closed-circuit. The 7805 was also removed and tested with a 1200 supply: 14.3v in, 4.9v out. I had some 22uF, 25V electrolytic capacitors of suitable dimensions and replaced C3 with one of those. No short circuit was present between 9v and ground with C3 removed, nor with C3 replaced.

With the RAM pack attached to the ZX81, it powered up to a white screen, a short delay, then the magic K. A quick check of ramtop and...

32768!

Fantastic, I've never had 16K before. Thanks for your generous help Mark, it turned out to be a simple fix but I learned a lot along the way.

Will the 22uF, 25v electrolytic capacitor be sufficient or should I add a 35v tantalum or >22uF, 25v electrolytic to my next component shopping list?
Last edited by SabreWulf on Mon Aug 23, 2021 12:44 pm, edited 1 time in total.
User avatar
1024MAK
Posts: 5101
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Memotech 16K RAM pack preventing startup

Post by 1024MAK »

Well done 👍 :D
SabreWulf wrote: Mon Aug 23, 2021 1:05 am Will the 22uF, 25v electrolytic capacitor be sufficient or should I add a 35v tantalum or >22uF, 25v electrolytic to my next component shopping list?
The 22uF, 25v electrolytic capacitor will be fine. No need to change it.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
SabreWulf
Posts: 19
Joined: Mon Jul 26, 2021 6:31 pm

Re: Memotech 16K RAM pack preventing startup

Post by SabreWulf »

Thanks again for your help. I'll leave the capacitor in. It could do with a composite modification, although without a back-porch this could be less than useful - but on the bright side (groan) it does give me something else to think about.
Post Reply