New ZX80 boards

[zx81jens]

well, yes, i thought about that, too (PAUL told me before also) - all jumpers are set (5-6/47-48/49-50) - nothing...

there has to be another problem...

all my computers (the ZX81-kit, the APPLE I from VINCE BRIEL and the TL801, my italian ZX80-replica) were build like my motto: put it together and switch it on... they worked (exept for a defect ROM on the TL801). the ZX80-LEGACY is the first one with a problem i can´t solve in the first place.

anyway - it will work!

thank´s for the advice

greetings
jens

[zx81jens]

hi guys!

OLIVER is here (together with KLAUS, who is in my kitchen and who copying some original-ZX80-programs to harddisk - via LOAD/SAVE from BODO WENZEL).

i think, this ZX80-LEGACY is serial number #4 thank you, OLIVER.

from OLIVER @LEE : very great piece of work - an the design ist also great! there are only some miner glitches that required an oszilloscope

how we (he) did it:

the 74LS74 (U18) was defect - after replacing we had a cursor, but the picture was (like the others before) "running".
then we found out that the signal (Q2 - U11-PIN8) was not correct and after putting in a diode (1N4148) the signal was good, but not perfect. i have several monitors to test - i get the best signal with an old surveillance monitor... my simple and small TV/MONITOR (5,-€) didn´t work so well.
after changing R33 to 500 Ohms everything is working!

the heat is a small problem - i will replace the LS into HCT - that will be better.

the original Z80-CPU was replaced by a Z80 (CMOS-type) and this will reduce the heat.

OLIVER did a lot of work! i did the cooking and refill the glasses

KLAUS has already digitized all ZX80-original-tapes that i own - so you will find them here in my next post...

here you can see a picture from the final:

greetings
jens

P.S.: next saturday i will drive to PAUL (serial #3) - so we can compare the two ZX80

[zx81jens]

hello again!

after a very good ZX-day here are the results of the work from KLAUS - he transfred the ZX80-tapes via LOAD/SAVE form BODO to harddisk.

the files in the zip are copy´s from the original tapes from "IMAGE COMPUTER PRODUCTS" - i will send the scan´s of the instructions in the near future...

have fun with the ZX80

greetings
jens

[KnightFire]

Many thanks for the info!
Please keep the chit chat coming guys, I feel like I'm in a null void without it.

[zx81jens]

ooops, i forgot some ZX80-programs

greetings
jens

[Oliver]

Greetings,

I would like to add some details concerning the modification we made to Jens' ZX80-LEGACY last weekend.

Firstly, the design and the quality of Lee's ZX80-LEGACY board is really phantastic! After replacing a broken 74LS74 (it just did not provide the divided 3.25MHz CPU clock), the computer bascally worked immediatelly.


Initially, we had a vertical rolling problem on the b/w monitor we used.
And I have seen that superfo and sirmorris reported similar effects (still?)

So Jens tried another monitor (a nice old-fashoned amber-monochrome one), and there the image was stable.

We looked at the video signal with an oscilloscope and saw that there was no VSYNC in the signal. During the time where VSYNC
is expected, the video signal was at black level, and no HSYNCS during that time. The nice amber monitor was tolerant enough
to accept this for vertical syncronization, but other monitors did not.

From what I understood looking at the schematics and comparing to the original-ZX80-schematics, the vsync bit from U11C/D might not get mixed into the SYNC signal.

So as a quickfix we mixed VSYNC in directly by putting a diode between the /VSYNC bit and the video amplification transistor. This helped to get a stable vertical synchronization on all monitors!

We did not try that, but I would expect that saving data to a tape might currently not work as the VSYNC bit is also not included
in the MIC signal (?). Did anyone try saving to a tape yet?


The second minor adjustment we added was to modify the value of R33. From the oscilloscope, all the video signal levels were
ok but one b/w monitor showed a somehow instable horizontal synchronization. From my experience with
my ZX81-video-output I knew that cheap monitors like a more abundant sync level, and this was what we adjusted with R33.


-oliver

[zx81jens]

Hi!

Here the Photo!

Greetings
Jens

[sirmorris]

Hi guys!

Yes, I still have vertical sync issues. I'll try your mod when I get back to England next week.

Thanks for the information!

[Lee Hart]

Hello fellow ZX80 adventurers,

I am delighted to hear that Jens has his Legacy ZX80 working. My thanks as well to Klaus and Oliver for your help!

I'm Sorry that the 74LS74 I sent was bad. I did not think to test them before sending it out (one assumes that new parts are good -- maybe a bad assumption nowdays). I can send another one if needed (such as if you borrowed the chip from some other computer).

On the weak video problem: Good catch on changing R33 from 1k to 500 ohms to double the maximum video output. I will change the board and schematic accordingly. I think I will leave R33 = 1k, and change R30 to 2.2k, simply because these are values already being used on the board (and we don't otherwise have any 500 ohm resistors).

On the vertical sync problem: I will look into it on my board, too. Right now, I don't see why it didn't work. Here is how it *should* work:

1. /SYNC (U11 pin 8) goes low at VSYNC time:
2. This pulls the base of Q1 low through R37.
3. Q1 thus turns on, and its collector goes high.
4. Since there is a jumper between RXD and RST (P1 pins 5-6), this makes
the input of U13H pin 17 high.
5. This makes the output of U13H pin3 go low.
6. Since there is a jumper between SO and SI (P1 pins 47-48), this pulls the
cathode of D13 low.
7. So the anode of D13 goes low,
8. So the base of Q2 goes low.
9. This forces the video output low (full black; the vertical sync pulse).

10. When /SYNC (U11 pin 8) goes back high and the end of VSYNC time:
11. There is no more current in R37. R36 turns Q1 off.
12. The collector of Q1 goes back low, pulled low by R38 and D12 to ground.
13. This makes inverter U13H input low, so its output is high, so D13
removes the ground on the base of Q2, so video can resume.

Also, I do not understand your modification to make the VSYNC work. In the photo, it looks like you added the yellow wire from the top of R37 to the left end of R31. This would short /SYNC (U11C pin 8) directly to the base of Q2, preventing all video. And the diode you added looks like it goes from the anode of D13 to the left end of R31, which are already connected by foil on the board (so the new diode is shorted). What am I missing?

[zx81jens]

Hi Lee!

I'm Sorry that the 74LS74 I sent was bad. I did not think to test them before sending it out (one assumes that new parts are good -- maybe a bad assumption nowdays). I can send another one if needed (such as if you borrowed the chip from some other computer).

You don´t have to send me a new one - this would be too expensive... Oliver gave that one to me (he had several parts for "reserve" )

On the weak video problem: Good catch on changing R33 from 1k to 500 ohms to double the maximum video output. I will change the board and schematic accordingly. I think I will leave R33 = 1k, and change R30 to 2.2k, simply because these are values already being used on the board (and we don't otherwise have any 500 ohm resistors).

On Saturday I was visiting Frank (near Kologne) and Paul (near Aachen) - this was a nice "Mini-Meeting" - we took some time to compare the ZX80-LEGACY Serial #3 and #4 ... Paul did all the modifications I did - but the video wasn´t doubled ... also he had to change only Jumper W5 to switch between ZX80 and ZX81 - on my board I have to switch W5 AND W4. I also have more heat on U4 than Paul...

... so many things I don´t understand (well, gladly I´m no expert )

In a few days I will order all TTL in HCT - maybe this will do something...


Question one: what is B1 (Battery) for?
Question two: is it possible to get the RAM between 8-16K buffered?


I´m looking forward to see Serial #5 working

Greetting and thank you
Jens