Spectrum Computing

Ok, have some other info. I have another test rom, the one from retroleum. When I pull IC3 but leave all other memory logic chips for upper and lower ram, it has a corrupt screen, but I can read the message saying diagnostics. It is able to check upper and lower ram and says all green. When I put in...

In order to simplify things, I removed all the upper ram and what I thought were the Logic ICs needed for it, do I still need IC24 in place? Yes, IC24 is definitely needed for issue 4A, 4B and 4S boards, as two logic gates are used to improve the timing for multiplexer chips IC3 and IC4. Issue 1 to...

Well, the good news is that it looks like the Z80 is working and can read the ROM/EPROM. The ULA is at least partly working, as the border works and it is trying to display something. The bad news is that the Z80 does not appear to be able to access the lower RAM. If you have checked and tested all...

So, I got a 27C128 EPROM, and flashed it today with the test rom recommend by Mark. Did the wire PIN1 and PIN27 to 5v, then ran the select wire to pin 20. So I turn it on and get this: [media]https://streamable.com/vm3qgh[/media] The first beep is my bench PSU, the rest are what I understand should ...

1024MAK wrote: ↑Thu May 07, 2020 12:58 am Does it always power up and change the border to white (non-bright white)?

And I do mean power up and NOT reset. Test at least eight times.

Mark

Yep, looks good still.

https://i.postimg.cc/JGMKrhjg/signal-attachment-2020-05-06-092515.jpg For you feel better pleasure. Top side just had a few traces, bottom was a disaster ground. I did the full nuke on removing all the upper ram and logic chips and for some reason that section gave me all the trouble (ram). Rest wa...

Wow this thread sounds like mine, gonna try the RAM test, did the OP ever find the issue?

The status so far: I have traced nearly every trace from the ULA, CPU, ROM SOCKETS (Removed the components) either looking for 0 resistance on any trace that has no components (Diodes, Resistors, Caps), or checking for continuity before and after, testing the components. Cleaned both side of the boa...

https://i.postimg.cc/JGMKrhjg/signal-attachment-2020-05-06-092515.jpg For you feel better pleasure. Top side just had a few traces, bottom was a disaster ground. I did the full nuke on removing all the upper ram and logic chips and for some reason that section gave me all the trouble (ram). Rest wa...

Don't feel bad, I managed to lift about 4 traces too, removing all the chips on my board. Most of them from chips that were already replaced by a PO and I suspect damaged them.

Its a tricky board to remove chips from. Patience is key, as in flux and re-flowing - fresh solder.

Thanks, yep nothing else for chips for me to replaced, just testing traces with nothing in line, voltages, etc. ULA is the last thing to rule out!

Thanks again Mark. Just did 100 reads of the ROM, just to make sure I didn't fry it, all returned matching checksum: 4c42a2f075212361c3117015b107ff68 which is also the same for the 48.rom I downloaded referenced earlier in this thread. Found TL866II+ 04.2.112 (0x270) Warning: Firmware is newer than ...

Looking for either one. The only place that sells Nebulas new is closed for now, I dont see ULAs on ebay, or online.

Thank you.

So been going through all the traces, tested all the address and data lines from ULA to lower ram, started from ULA to ROM, etc. Man there are quite a few mistakes in the schematic. One of the other things I did was test voltages around the ULA. They seemed way off based on some other stuff from wor...

me@home:~/tmp$ minipro -p AM27256@DIP28 -r zx.rom -y . . . I have a TL866+ II EEPROM burner BTW, and the GUI is nice, but the open source CLI too lets you bypass all the silly error checks, enabling me to do the dump above! Ah! I tried that... a year and a bit ago, maybe? Couldn't make it work. It'...

I could only find the GI. https://i.postimg.cc/zyBrhthc/signal-attachment-2020-05-01-082936.jpg If that pinout is accurate then the ROM links are correct in the "H" configuration, as they are in your latest photo. Thanks, this is the info that told me this: The Spectrum computer can use o...

I went and pulled the last 2 74ls chips, tested OK as suspected. I now have a 16k spectrum in theory :lol: I scoured the board, and found 1 broken trace from the previous work someone did on the board, it was pin 3, a chroma signal pin with a through hole on the video signal chip. I bodged it below ...

Was finally able to dump my rom. Mine is the zx16k.rom file dumped as a 256bit chip, I then had to split the file to get the upper and lower halves. I guess a trick to get a burner to toggle pin 27 hi and low as a memory select setting and thinking it is reading 2 16k banks... me@home:~/tmp$ minipro...

Thanks again Mark, that is what I thought, and entertained the possibility I am reading it wrong. I am not a professional, but I know enough if I screwed up a through hole, lifted a trace, etc. I check everything after I pull a chip. But yeah will do more passes again looking around where I suspect ...

So, a funny thing happened on the way to the forum. I want to slap myself if this is true. Playing with the rom chip a 23128 GI, I was able to apply power to pin 27 or 20 or 22 and always pin 1 and was only able to do a read when I selected a NEC 27128 chip on my reader, not Hitachi. My spectrum boa...

Thanks, I ordered some blank chips.

I just went and socket the rom, hoping my reader can read it. Does anyone know the specs for that chip?

I also suspect maybe a through hole was damage on the mobo when the CPU was replaced, may have to go through that and check things...

Here is what it should look like with no ram:

https://www.youtube.com/watch?v=izcGFap8HGI

And here is what I get:
[media]https://streamable.com/g7frud[/media]

It looks like something kinda works, but still hangs... Similar but not the same...

Ok, removed all the upper ram chips, last person to work on this used too hot an iron, lots of burned through holes and I have to deal with lifted traces :( Started some bodge wire work on the bottom row, top needs it too. Scoured the area for any shorts I could find. Anyways, after the board was li...

Ok, got some parts in over the weekend. 2 extra ram chips for lower ram. CPU Cap Kit I recapped the whole board, installed the correct ram vs my hacked ram. Same results. :( Swapped CPU. Same results. :( This at this point I assume it is upper RAM or the ULA maybe? The one ram chip that started the ...