Sinclair ZX80 / ZX81 / Z88 Forums

Whelp, it wasn't the PCF1306P or the RAM. It's the ULA. I went through every RAM chip I have with the tester (great device btw!) and it turns out I only had 4 bad chips, and none of them were on the ZX. Installing the ZXMUX had no effect and I still got the no RAM loop. However, I have a friend work...

Hi Mark! If the board suffered power supply problems, yes. The 4116 (or equivalent) "lower" DRAM chips often die if the +12V is present but the -5V is missing. And if a 4116 dies and the +12V or -5V leaks out to other parts of the board, that can damage other chips. I believe that for the 4116s that...

Hi Mark! Thanks for the tips. Sorry, I didn't notice the add files option at the bottom. I'll do that going forward. I went ahead and got that ZXMUX: https://myretrostore.co.uk/product/zxmux/ I have bought from him before, and I got the chip tester I'm waiting on from him. Nice guy and he's got some...

So I desoldered the PCF1306P. I have never had a 40 pin chip come out so easily. This IC27 wanted *out*. :lol: Now, instead of the no memory loop, I'm now just getting this on the screen: https://i.postimg.cc/G3xvFGL4/2025-03-21-5-01-43-AM.png So, progress, I guess? 1024MAK is in line with what you ...

Hi 1024MAK! Thanks for taking the time to reply. :) Do you mean 22nF rather than 22µF (which are rather bigger!) for C6, C7 and C57? Oops! Yes, that was a typo, I meant 22nF. Alas, everything is pointing to IC27 (PCF1306P / ZX8401) being toast. This chip does the multiplexing for all the address lin...

Hi everyone! I inherited a ZX 48k that was in really rough shape and was destined to be a donor board. The ZX is a 48k 6a model. Almost everything was busted on this motherboard... Here's what I have done so far: - Bad voltage regulator - replaced with TRACO - 2 bad rams were bringing down the 12v a...