Composite mod voltages

[PokeMon]

I try to use LVL logic gates with about 3.6V supply voltage, maybe the inputs are better designed.

So just want to mention this because we discussed that stuff earlier.

Yes - they are better designed. Found out today trigger point at 1.5V with hysteris of 100mV.
So that sounds great.

LVC_char.jpg (89.97 KiB) Viewed 3073 times

More: http://www.ti.com/lit/ml/scba010/scba010.pdf

[ArtemKuchin]

Okay, it seems i have done it.

Here is the board now:

zx81-v.jpg (91.86 KiB) Viewed 3039 times

it is only 30x40 mm but high because of the caps.
Eventually i used 11K/6.8R input divider, 150R/150R output divider, and 47R/22R ouput R (instead of 33/15)

I've done two of them (i have 3 zx81, one is broken though) Both seem to work.

Then i tried to load anything. After playing around with volume and finally made it to react.

Problem one: when i type load"" and hit enter the screen goes black, tv show "no signal". So, no nice loading noise.

Then i tried to load hi-res for testing and another game. It failed many time until i realized that i did not plug 16K ram pack

So, i plugged a ram pack only to see a completely white screen. Unplugged it - everything is ok again.
Took another 16K ram pack and plugged it. Here is what i see:

zx81-a.jpg (22.08 KiB) Viewed 3041 times

Unplugged it (turned everything off before of course). The keyboard did not work some 10-20 seconds, they came alive again. I repeated - the same
effect, keyboard freeze for 10-20 seconds. Tried with another ram pack - same effect. Strange. They i plugged the ram pack which gives the stripes pictures and waited a minute. The screen went black. Unplugged it. Keyabord freeze, then everything work again.

So, i loaded a 1K game and it worked.

Problem 2: does this video circuit affect the ram pack or my ram packs are dead? I really never used them before.

I want to dig depper with a o-scope, but afrain to leave it ON for more than a minute like this.

[1024MAK]

The ZX81 does a crude memory check to work out how much RAM it has (but only in the 16k normal RAM area).
It takes a little while for it to do this, during which time you just get a white screen.
Once the memory check is done, you get the normal K on the input line.
If after 1 minute you do not get this, the RAM pack is most likely faulty or there is a poor connection.
If you get anything other than a white screen, again the RAM pack is most likely faulty or there is a poor connection.

Mark

[PokeMon]

Problem one: when i type load"" and hit enter the screen goes black, tv show "no signal". So, no nice loading noise.

Problem 2: does this video circuit affect the ram pack or my ram packs are dead? I really never used them before.

I want to dig depper with a o-scope, but afrain to leave it ON for more than a minute like this.

First: As you have a digital TV connected (not just an CRT with loose running sync when no signal available), you won't see pictures you know before with CRT. It does go black because the video signal is not present (or better not valid). Even some TVs switch off after some time. That's why I developped a new circuit with a standby sync generator which will leave the screen visible (white) when running in FAST mode. The same you have when trying LOAD SAVE commands. You should be able to press SHIFT SPACE for break and it should come back to a visible screen and abort the LOAD command.

Second: It does not affect in any way the work of the memory packs regarding a bit more power consumption. In most cases old memory packs are dead or not running for long time. Sometimes it will boot with the memory pack but fail when programs are loaded and executed in memory.

Third: I didn't see a SAMSUNG in wooden (plastic) case before. Looks funny.

[ArtemKuchin]

1: i thought so about TV, thanks for confirming. I usually use just SPACE w/o shift to return from loading. It works

2: What i meant is that when ram pack is plugged maybe the video screen works somehow differently and the output of ula is shifted down a bit, so i again
don't get enough voltage for LS07 to switch. But well, it was probably stupid theory.
Well, then if my ram packs are dead then i am after to make internal ram mod for 16-32K. This is off topic, but will this mod http://www.zx81.de/english/32k-rame.htm work with hi res?

3: well, the shot is made on my xperia ion and it never failed white balance that bad before. the tv is actually WHITE and light grey texture and very nice looking. Where yellow comes from i have no idea since my room light is 4200K T5 lamps.

Forgot to mention. I measured current consumption of the video board. It is 40mA.