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I just did a stupid thing and connected a new cable without inspecting it first. It was from a reputable seller, but has shorted pins 2 and 4.
What is the potential damage it could have caused and what to look for when I get to checking the system for any damages caused by this?
The RF output still works, but waiting for RCA replacement to test composite
Whatever would have taken place occurred inside the RF modulator:
I haven't been able to find a C128 schematic that includes the modulator, however if it's anything like the C64 modulator, there's a current limiting resistor in the path between +9V UNREG, a couple of transistors (only the last of which is shown here) and ground.
For this particular C64 modulator, it seems to be dropping the 9V over a 150 ohms resistor (shorted state, worst case with saturated transistors) so about 60 mA which I'd say is unlikely to cause damage to either resistor or transistors.
If the incident coincided with loosing the composite output, to troubleshoot further, I'd open the RF modulator and try to follow the path back from composite out to power source.
The C128D modulator uses 5V for power supply, and the composite output (pin 4) is driven with luma signal via an emitter follower and a 120 ohm resistor, and with chroma signal via an emitter follower and a 150pF capacitor.
Normally the composite output is disconnected when using the RF modulator, but when using the composite output to a TV, it will be terminated to ground via 75 ohm resistor.
In this case the faulty cable just directly connected it to ground.
So calculating the rough worst case scenario is that there is 5V over the resistor only 42mA flows, dissipating 208mW.
In practice, the highest voltage that an emitter follower can reach is about 4.3V due to about 0.7V Vbe drop, and luma output most likely cannot reach such high levels as it is unlikely that the white level is biased so high. The chroma capacitor would be shorted to ground, but it is charged quite quickly.
Therefore the chance of damage is very small.
After all, the making of the composite signal happens in the RF modulator before being RF modulated, so that is a sign of composite output being OK.
Retrograde has done a good analysis of the available schematics. However, there is a more general analysis that should apply to nearly any retro computer.
Composite video outputs are designed to have 75 ohm output impedance for proper match to the coaxial cables used. You can think of this as a 75 ohm series resistor, even though it can be composed of multiple resistors (like two 150 ohm resistors in parallel in the C64 schematic). There will be another 75 ohms termination resistor to ground in the television set.
Shorting the two pins replaces the termination resistor with 0 ohms. This will exactly double the current flowing through the output resistors and transistors, but not more than that. Most components can handle a short-term 2x overload, so shorting composite output for a few minutes is unlikely to do damage.
What is the potential damage it could have caused and what to look for when I get to checking the system for any damages caused by this?
I'm not sure what this question is about. Is it asking for speculations what could go bad without any indication that there is an issue?
If yes, then closing it would be a must, as it simply asks for opinions.
The RF output still works, but waiting for RCA replacement to test composite
In that case simple logic says there is no damage and composite works as well.
After all, RF isn't a separate interface channel. The composite signal is created before transposed it onto a carrier. If RF gives picture and sound as intended, then this is poof that composite is fine, isn't it?
