gozzo wrote:I would strongly advise NEVER to connect a zx81 ula video output pin directly to a RCA (or other socket) to connect to a composite input of a monitor/tv - there is a fairly high DC level there which apart from the monitor/TV possibly not liking it, the composite input is supposedly 75 ohm input impedance and if a DC level of,say, roughly 3volts is present at the ULA pin, this means a current of just under 50mA!! Not good for the poor old ULA (or TV)!
Indeed, and if ULA outputs, say, 3V, then (5-3V)*40mA = 80 mW is added to ULA's heat dissipation (since that current comes from the +5V supply, but the 5V - output voltage difference is dissipated inside the chip). That's a -relatively- big addition for a plastic-encased chip which already gets hot under normal operation.
Another reason why not is electrostatic discharge, especially when (dis)connecting big equipment with the Zeddy in between. Say you have a PC soundcard connected to ear input, and you plug in RCA connector to TV. The tip of the connector is normally the first to touch, so
any voltage equalization between TV-ZX81-PC then goes
through the ULA pin. By the time the outside of the RCA connector (ground) makes contact, the damage is done.
Normally the modulator sits in between, with a [transistor buffer / comp. video mod] that cheap & easy to replace transistor would eat such a discharge.
Same story goes for ear/mic connections, but they include series resistors which makes permanent damage much less likely.
PokeMon wrote:Well the capacitor doesn't help much to reduce output current when connected to a 75 Ohm video input as it provides a hf signal and there is no significant resistance of the condensator measureable. So directly 75 Ohm input is too much for the ULA regardless if with or without capacitor.
Wrong! The other end of the capacitor will go to voltage level it's loaded to. For example schematic of my old TV shows a plain 75 Ohm resistor to ground at its composite video input (SCART). So if ULA pin output averages 3V, capacitor would charge to 3V, other end would average near 0V (and thus little point in adding a diode if your goal was a 0V average), and the ULA pin would behave as if it were loaded with 75 Ohm to a 3V DC source. Supplying current when output >3V, sinking current when output <3V, but on average
certainly less than with 75 Ohm to ground. Agree with the value though - 100 uF or more is good, with smaller values you might see some distortions in screens with big black (horizontal) bars & such.
And you should as a parallel ceramic or MKT capacitor with about 1-100 nF to better pass higher frequencies which are maybe affected by an electrolytic capacitor as we are talking of up to 10 MHz when calculating 3rd harmonic of highest possible pixel frequency (for a crisp signal).
You'd think so, but when I tried this once with different capacitor types (like tantalum, which usually has much better high-frequency behavior than aluminium electrolytic), I couldn't find any visible differences. And as long as the pixel frequency gets through, damping higher frequencies may actually improve image quality.
So I guess for this application cheapo electrolytic is good
enough, and other factors (ULA, output circuit, power supply, video cable, TV) are more important for image quality.
Always visually check any work and nearby areas, if necessary using a magnifying glass as short circuits caused by solder threads and splashes can cause all manor of faults 
