after 40 years Sharp MZ abstinence I risked to buy a defective MZ 700 again, the poor version without tape drive with the "bridged" 9 pin connector for external tape IO. After replacement of 2 capacitors in the PSU the computer is booting again, keyboard and TV-out are working fine. So far so good..
The starting basis is a failing loading from tape or ext-in.
My approach was:
- to convert a downloaded mzf to a wav file.
- connect the PC headphone out with the MZ 700 read jack
- start loading @ MZ 700
- play the wav file
- wait for file header printed by MZ
I was repeating the process while I increased the sound volume in small steps from 40-100%
It always failed
Then I luckily was able to test this experimental setup with an other working Sharp MZ 700 using my 9 pin tape bridge and it worked in a wide range of sound volume set (60-90% in my case) (TAPE SW had same position, too).
Later I also tested a working internal tape drive with my MZ 700 and found out that loading from tape is also always failing..
Now I was sure that there is something wrong with my mainboard and the problem is more complicated.
Unfortunately I don't know much about electronic engineering but I checked the schematic diagram and found out (if I am correct) that the analog input is being converted into digital HIGH/LOW passed to only one pin of the intel 8255 PPI. For the conversion multiple inverters of a CD4049 are used to create a kind of simple ADC.
Using a simple multi-meter I checked the operational status of the inverters and found some invalid values:
2 of the used inverters had a 0V in and also 0V out @ Reference value of 5V.
My newbie question are:
- Is it probable that my assumption tested in circuit with this simple multi-meter is correct or would I need to unsolder the IC, first?
- Are there also other (simple) alternatives for file loading not using the analog in, e.g using the MZ IO bus & arduino or similar?
(my idea would be e.g to connect an out port of an arduino directly with the corresponding pin of 8255 and skip the ADC without any synchronization since there is also no sync with the tape drive, interpret the mfz file and stimulate the 8255 the MZ 700 would do after ADC. Does it make sense, and is it feasible to trigger the 8255 from an arduino with a shared GND?)
Thanks in advance for your proposals/recommendations.
BR
Alex
