Still faulty 80K

hlide
Posts: 243
Joined: Thu Jan 25, 2018 9:31 pm

Re: Still faulty 80K

Post by hlide » Sun Aug 12, 2018 6:54 pm

I'm interested. What would be the shipment fee for France?

gregctaylor
Posts: 27
Joined: Tue May 29, 2018 5:38 pm

Re: Still faulty 80K

Post by gregctaylor » Sun Aug 12, 2018 7:07 pm

About £15 plus insurance dependant on value but that is free up to £50.

S_U_C
Posts: 12
Joined: Sun Feb 17, 2019 6:41 pm

Re: Still faulty 80K

Post by S_U_C » Wed Feb 20, 2019 7:28 pm

Hi, I did know a think or two about the MZ80K but that was ages ago.

The screen display is a self contained unit on the main board that has it’s own memory chips ( type 2114 IC 41 IC 42 soldered to the main board ) mapped to $D000 to $D3FF
It consists of a free running counter with a /8 and followed by /1024 sections and is clocked at 8MHz, the /8 outputs forms part of the address lines to the GG ROM.
The /1024 are the address lines the screen ram and rests at a count of 1000dec .
The screen ram contents form other GG ROM address lines.
It is so self contained it will still produce a display of random characters even if the CPU is missing.
The CPU is clocked a 2MHz by diving the the 8MHz crystal output by 4, not the 555 timer.

On boot up the K monitor does
1) sets the stack pointer (must have a good ram block 1)
2) sets up the 8255 ports. For keyboard / tape etc.
3) clears the screen by filling $D000 + with $00s, it does it a line at a time i.e 40 characters 25 times.
Then latter on displays the title, prompt and flashing cursor.

The rate of the cursor is set by the 555 timer and is monitored though the 8255 ports and flashed with code.
The 8253V is used as a clock i.e time and used to produce sound.

So your screen of alternate characters for the full screen baffles me as it indicates the the monitor has tried to clear it.

The sound on key press is turned on with [S] then [G] then [CR] and the MZ80K has a volume control on the far left of the video board.
The volume may be turn off so try it again SG [ENTER].
If still no sound when pressing keys, it is most likely the CPU is crashing in an end less loop due to partial address lines buffers failing.
Low address for ROM ok but higher address for Screen / ports some problem
If you get sound then it’s most likely the video ram 2114 chips.

I hope this helps.

gregctaylor
Posts: 27
Joined: Tue May 29, 2018 5:38 pm

Re: Still faulty 80K

Post by gregctaylor » Sat Feb 23, 2019 10:17 pm

Thanks for that. I gave up over Christmas but it is time I started again. I am determined to get it fixed. Something serious had happened to the computer. I had no crystal clock (now fixed) and no clock from the 555 timer (also now fixed). I can now see clocks throughout the CPU and graphics systems. I will try the sound again tomorrow but think I tried this without success earlier but let's hope not! I have tried the video RAM chip at a time but perhaps I will need to purchase some more so I can swap out the whole lot in one go.

Any other ideas you have please pass on. I have a decent workbench including oscilloscope and analysers so can check most things. They will be much appreciated.

Greg

gregctaylor
Posts: 27
Joined: Tue May 29, 2018 5:38 pm

Re: Still faulty 80K

Post by gregctaylor » Sun Feb 24, 2019 3:07 pm

Hi, Have checked, I replaced both the video RAM chips before Christmas. Pressing SG <CR> produced no sound, nor if I tried it again. Volume is on maximum.

Any other ideas? I shall get back to the service manual tests to see if that gives me anything else.

Greg

hlide
Posts: 243
Joined: Thu Jan 25, 2018 9:31 pm

Re: Still faulty 80K

Post by hlide » Sun Feb 24, 2019 7:42 pm

CPU and Monitor ROM?

gregctaylor
Posts: 27
Joined: Tue May 29, 2018 5:38 pm

Re: Still faulty 80K

Post by gregctaylor » Sun Feb 24, 2019 7:59 pm

New monitor ROM, swapped out for a newer 6502, no difference so replaced the old one. Also swapped out the 8253 which had some very irregular waveforms on its output.

hlide
Posts: 243
Joined: Thu Jan 25, 2018 9:31 pm

Re: Still faulty 80K

Post by hlide » Sun Feb 24, 2019 9:51 pm

6502!? you mean a Z80, right?

gregctaylor
Posts: 27
Joined: Tue May 29, 2018 5:38 pm

Re: Still faulty 80K

Post by gregctaylor » Sun Feb 24, 2019 10:19 pm

Whoops! Yes, Z80! I could say I was confused as I am also fixing a ZX80 but that might not take to a 6502 either. :D

hlide
Posts: 243
Joined: Thu Jan 25, 2018 9:31 pm

Re: Still faulty 80K

Post by hlide » Sun Feb 24, 2019 11:31 pm

So the situation is you have no screen activity and no sound activity. Sounds like the CPU is not running. The more I think, the more I believe we need an arduino to take the Z80 bus ownership and be able to access RAM, ROM, VRAM and Intel chips to make some tests and diagnostics.

- Monitor ROM ($0000-$0FFF) : could read a ROM image to compare with the expected one byte by byte or compute and check a checksum.
- DRAM ($1000-$CFFF) : could read/write a DRAM per chip to check which chips have issue and at which rows.
- VRAM ($D000-$D3FF) : could read/write a VRAM per chip to check which chips have issue.
- NEC chips : tape, sound tests.

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