6502.org

I forgot to mention, i already tried that, it only snows, sometimes it can randomly change a random character on the screen to a random one.
When i return i will try to modify the glue logic, so that the phi2 is closer to the output.

I moved phi2 as close to the output as possible, but no change.

EDIT: I also tried slowing down the clock, but no change...

Now i latched the address with phi2, and when i add a capacitor to the ram we input, it kinda works, but then it writes to the crtc's address...
When look to the signals under the scope, i don't see any obvious problem...

Problem solved! I accidentally soldered two address wires just one bit away... I found the bug when i looked all the signals on the ram chip, and there i spotted the bad address signal...
But when i write at some specific locations, that creates unwanted changes to some other locations on the screen.

I still can't find the problem!
When i type POKE 40960(also 40977 and 40979, and maybe more),X (where 40960 is the address of the first element in the crtc ram, and X is any value (EhBasic)), this value also affects the 40978 location, and also there are few other anomalous locations. I tried all kind of tempering with the glue logic, but no luck(the scope doesn't say much either). What other causes there could be?
Also i can't get any output from the chroma modulator, but i suspect that i did something wrong in the analog part of the schematic, so for now i use it in black and white mode (my mini telly is also black and white).

Edit: Since i added a simple routine that sends characters from EhBasic to the screen, i can also write to the problematic location by filling the screen with characters(when the screen fills up, the pointer just goes to the beginning(upper left corner).), and when i overwrite the mystical location, nothing happens, everything works as it should!?
This is also true for the other mysterious locations.

Physical IC damage since you soldered 2 address lines together? I would bet you have a noise issue. Power supply noise entering your circuit.

I found the edited fact after i posted the original post, and that means no physical damage i present?

Edit: I didn't solder the together, but the pin header that they were soldered to had many pins, and i soldered the two last wires one pin away(both were one pin off).

Max power peak at the most disturbed point +/- 200mV max and it lasts 50ns.

A good scope with good probes should reveal most of your problems. The fact that you are not spending quality time with your 'scope and actually discount your 'scope (i.e."...doesn't say much either"...) is not a good sign...
Do not discount good tools, and do not waste good money on bad tools.

Again, I will bet $$$ you have a power supply distribution/bypass cap issue... Assuming everything else is correct. I trust that it is!

There are two problems with analyzing this circuit with the scope:

1. The signals are variating a lot, since the cpu keeps switching control between the cpu and crtc, so when i look at the data and address lines, i can't get a stable reading at both signals at the same time, since they run a at different frequencies, so i can see one stable, but the other keeps shifting

2. When i look at the glue logic signals i can't see anything wrong(except for some ugly signals that are supposed to be square(but the problem is not affected by the cpu clock, the problem persists at 500KHz and at 4MHz), and that it is why it isn't helping...

Edit: I added a 470uf electrolitic and a small 1uf cap at the most disturbed point, but nothing changes.

Is it possible that EhBasic has some kind of glitch?

The interesting fact is that the glitch is closely tied to the address on which the POKE instruction writes.

I also noticed now that the affected location doesn't always receive the value that i poked. Sometimes when i poke 1(that would be A) i sometimes get A, but sometimes i get !, and this also is true for other values but only when the value is less than 32.

Edit: I tampered with the glue logic again(previously i added an and gete that was supposed to assure that the cpu had instantaneous control of the ram when it was writing, now i removed it), but the results are always the same!

Total power consumption of the sbc is ~500mA (due to the NMOS ic).

I think i found the problem with the chroma modulator circuit. When i hooked the chroma modulators output to the scope, and the switch(74hc4053) was set to b/w the scope would show a classical tv signal, but when i switch to color, the signal just disappears.
So i fetched it through my improvised amplifier that I've made for my Eico signal generator and i got something on my telly, but since the amplifier is made for another propose, the output signal was distorted, and without colors...

Video amp schematics:


I also noticed that the diode connecting the pin 13 and 14 is connected contrary to what it is shown to the datasheet, if i got it right this inverts the signal? For now i have the b/w amp shown in picture 2, and it works excellent when the signal comes from the 6847. How could i easily modify the amplifier(or will the amplifier from schematic 1 work fine), so that it works with b/w signal(from the 6847), and color(from 1372)?

As for the poke glitch, i still can't find the cause!

I'm not sure what you're asking, but I would expect schematic 1's 2-transistor amplifier to have a lot more distortion than schematic 2's.

Schematic 2 wracks the 1372 signal completely, i suppose it has the wrong input impedance, so i should modify it so that i match the input impedance (I am no expert with analog amplifiers)? It seems my video amp needs to be modified.

Since i am not an expert in vf analog amplifiers, i would appreciate any useful suggestions.

Do you know how much gain you need, and maybe a corner frequency? Does it need to be good down to DC, or can it be AC-coupled? If it must handle DC, what will the input swing range be, and what should the output be? (I have not studied video at all.) Schematic 2 will be better behaved but it will have less gain, which might be why it wasn't doing the job. It could be that to get the enough gain with low distortion and good stability, an additional transistor will be needed. Again, I've never studied video, but if you can tell me the requirements, I might be able to suggest a circuit to try.

I found these links:

Transistor amp - This is for VGA, but the VGA analog signal is similar to composite.
http://members.optusnet.com.au/eviltim/ ... ga2arc.htm

Op amp:
http://members.optusnet.com.au/eviltim/ ... ga2arc.htm

transistor:
http://www.ee.washington.edu/circuit_ar ... ISCHEM_011

This has one input driving 3 outputs:
http://www.freeinfosociety.com/electron ... hp?id=2194

Hope one of these might help!

Daryl