Sinclair ZX80 / ZX81 / Z88 Forums

Yeah, planning to play The Hobbit this weekend! Thanks!

Uh, ok, I soldered the resistor but it didn't work. After checking VERY carefully at some image I found on the internet, I found that my clamps were mounted in the wrong way, the throat of the hole must not be on the TOP side but in the bottom side. It may be a bit illogic, but if they must push on ...

Hi, I finally worked out the 48k+ issue 4B that had the fried transistor. I bought a replacement for the keyboard membrane, but once mounted I have a strange issue. The special keys aren't recognized. All keys seems to work fine, but the extended keyboards don't. I see they seems to be seen, but the...

Ok, I DID IT! Changed the 74HCT00 (it's needed for address decoding), desoldered TR6 and TR3 and resoldered them (BC547 is not ok for them). I noticed a change in the video pattern. Now there are a lot of black vertical stripes (like 1 every 8 pixels, sounds familiar?), or sometimes big black bands ...

Yes, the note was about National Semi, I haven't found the reason. And yes, They are from Harris, and again yes it was bought by TI.

Ok I changed all 4 LS157 with 74HCT257. Unfortunately they are of different types and brands, 2 are SN74HCT257 and 2 are CD74HCT257 from two different brands. The difference shouldn't matter, just a small difference in the input levels, especially in the HIGH level (2Vmin vs 3Vmin)... but I hope it ...

I still have doubts about the upper ram, I disabled it by forcing the \CAS low but this works if both the logic and the upper ram work. Err, /CAS should be held high to correctly disable the ‘upper’ DRAM… I’ll repost my comments from another topic... To disable the "upper" 32k of RAM, take a piece ...

Hey Mark! Thank you for your comment. Of course I was so confident it was ok: I had checked it with my other working Issue 4A and it was oriented the same way! My luck was that last time I wanted to check it, it was reassembled and closed, and I didn't want to open it again! Today I did some more in...

There is still definitely something wrong with the section of the DC/DC converter that produces the +12V. Items to check / test / replace are: C78/C80 (22µF), C44, D15 and D17. The two electrolytic capacitors are critical. In both value, polarity and being good quality. Did you replace TR5? Yes, th...

I tested the CPU waveforms and they seem ok. There's a good clock, good \IRQ, no reset/nmi/wait/halt ... everything seems normal. I've disabled the upper ram by connecting pin 5 of IC23 to VCC. no CAS signal to the upper ram, good. So I'm banging my head, I'm unable to see where the problem is. Eage...