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?
Memotech 16K RAM pack preventing startup
Re: Memotech 16K RAM pack preventing startup
Last edited by SabreWulf on Mon Aug 23, 2021 12:44 pm, edited 1 time in total.
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Re: Memotech 16K RAM pack preventing startup
Well done
Mark
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.
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.
Re: Memotech 16K RAM pack preventing startup
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.