Search found 2987 matches

by 1024MAK
Sat Jul 04, 2020 1:01 pm
Forum: Hardware
Topic: Memotech 16K Ram pack failure
Replies: 11
Views: 289

Re: Memotech 16K Ram pack failure

Do make sure you do the tests I talked about, just in case there is different versions of the Memotech 16k RAM pack...

Good luck ;)

Mark
by 1024MAK
Sat Jul 04, 2020 1:32 am
Forum: Hardware
Topic: Memotech 16K Ram pack failure
Replies: 11
Views: 289

Re: Memotech 16K Ram pack failure

https://www.sinclairzxworld.com/download/file.php?id=8299&mode=view Remove the following components: D2, D3, ZD1, DZ2, R4, C3, C4, C5, C7, the wire link in D4 position (or other component if no wire link is fitted). Pin 1 is now disconnected on all DRAM chips from the former -5V supply. Connect wha...
by 1024MAK
Sat Jul 04, 2020 12:47 am
Forum: Hardware
Topic: Memotech 16K Ram pack failure
Replies: 11
Views: 289

Re: Memotech 16K Ram pack failure

I’ve drawn most of the schematic.

See this post ;)

About using 4164 DRAM. I think with only a small modification, you may be able to fit the 4164 DRAM chips without having to do a modification for each chip.

Mark
by 1024MAK
Fri Jul 03, 2020 11:46 pm
Forum: Hardware
Topic: low memory fault test
Replies: 27
Views: 564

Re: low memory fault test

If ROM is getting really hot, yes, that’s a problem. The ROM normally runs slightly warm or cool. But your symptoms are confusing, as the border always going white does indicate that the Z80 is reading the code in the ROM at reset and is getting at least to the bit where the ROM code tells the Z80 t...
by 1024MAK
Fri Jul 03, 2020 6:13 pm
Forum: Hardware
Topic: low memory fault test
Replies: 27
Views: 564

Re: low memory fault test

When I try the ROM disable test on a broken issue 2 board (that is awaiting modification) and which has no functioning lower RAM, I don’t get a nice diagonal pattern, but for the first 10 to 20 seconds, the picture does change rapidly. At this stage I would suggest doing some testing with a logic pr...
by 1024MAK
Fri Jul 03, 2020 5:53 pm
Forum: Hardware
Topic: low memory fault test
Replies: 27
Views: 564

Re: low memory fault test

Okay, when powering on, does the border always end up white?
I do mean turning it off, waiting for at least 30 seconds, then powering up again. Do this test multiple times. At least ten times.
If you have any of the temporary wires still connected, remove them first.

Mark
by 1024MAK
Fri Jul 03, 2020 4:06 pm
Forum: Welcome Area
Topic: Found a ZX81 and fixed it
Replies: 13
Views: 677

Re: Found a ZX81 and fixed it

So having got interrupted with work, I did not have time to post this, but better late than never, here is a diagram showing the ZX81 socket pins and their signals compared to the "standard" 28 pin ROM pin-outs (the 27 series EPROMs have similar pin-outs). https://www.sinclairzxworld.com/download/fi...
by 1024MAK
Fri Jul 03, 2020 2:38 pm
Forum: Hardware
Topic: low memory fault test
Replies: 27
Views: 564

Re: low memory fault test

So, I’m concerned that your result of the test I detailed in this post resulted in a similar screen display to trying the computer normally. Can you please try this test again. But before powering on, when the jumper is connected to R33, if you test pin 27 on the ROM with the power off, it should ha...
by 1024MAK
Fri Jul 03, 2020 1:08 pm
Forum: Hardware
Topic: Differentuating a different logic states/pulses on oscilloscope
Replies: 10
Views: 390

Re: Differentuating a different logic states/pulses on oscilloscope

https://www.sinclairzxworld.com/download/file.php?id=8297&mode=view Please note that the control lines and various other circuitry is not shown. The address bus and the data bus are both separated into two sections each in a real machine to support the special way that the ZX81 video system works. ...
by 1024MAK
Fri Jul 03, 2020 11:55 am
Forum: Hardware
Topic: low memory fault test
Replies: 27
Views: 564

Re: low memory fault test

Yes, try the known good ULA from your working machine in the faulty board.
But also try the ULA from the faulty board in the working machine.

In other words, swap the ULA chips between machines/boards.

I also suggest you mark the ULA chips so you always know which one came from where...

Mark