Sinclair ZX80 / ZX81 / Z88 Forums

Update!

Making progress, I think. I replaced all the power related diodes, and socketed the memory chips. Still no video. I tested the ROM in my 1500, and it worked fine. Good news! I also tested the Z80 on my 1500 and got a black screen. Hmm...maybe bad Z80.

I tried my working Z80 in the bad machine...nothing. I tried an older Z80 that i have from a TS1000 and I noticed we go voltage on R20. Plugged up the composite mod and we got video!! But, only a nice and bright white screen. No K cursor Added a 16K Ram pack. Same.

How sensitive is the 1500 to a specific Z80?

Memory chips were tested and voltage is within range. Logic probe arrives tomorrow and I have my eye on an ossciliscope!

Don

A white screen means the the ULA is working.. and that's very good news for a 1500 as is the only one part you can't replace this days!!!..
Z80 from a TS1000 should be the same.
If ICs were ruled out. Check all data and address lines. Pin by pin . Have in mind that some lines have resistors in them so continuity will fail if tested directly. (Check from the spected resistence) it is better if you test from the IC pins as that will rule out a miss contact too.
Rampack disconnect internal ram on ZX81 and TS1000 but I think it is no the same on TS1500.. so you can still have ram problems.. I guess..

Great, so with a good ula, should be able to fix about anything else I would think. Practically everything else is already replaced - matter of elimination! lol

So, I'm in the the process of tracing data and address lines.

I notice they run through 2 separate ICU's. 74LS244 and 74LS245 data buffers. How often do these things fail? I haven't traced them yet, but I'm curious.

computergui wrote: ↑Wed Mar 24, 2021 8:40 pm So, I'm in the the process of tracing data and address lines.

I notice they run through 2 separate ICU's. 74LS244 and 74LS245 data buffers. How often do these things fail? I haven't traced them yet, but I'm curious.

And even with good continuity from the pins, if the ICS are bad, I assume the machine wouldn't boot to the cursor.

74LS244 and 74LS245 data buffers are normally reliable, but just like anything else, they can fail.

Mark

1024MAK wrote: ↑Wed Mar 24, 2021 9:12 pm 74LS244 and 74LS245 data buffers are normally reliable, but just like anything else, they can fail.

Mark

I've ordered a few and should have them in a couple days. I'll let you know.

https://www.electronicsrepairmadeasy.co ... ester.html

This website abovementioned can test the TS1500/ZX Spectrum 16/48's inductors. I am trying to help all of you.

Cheers,
DesSony

Thanks. Eventually we will have replaced every chip lol...