6

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

enter image description here

1
  • 4
    So does it still work or not?
    – Justme
    May 26 at 12:16
11

Whatever would have taken place occurred inside the RF modulator:

RF modulator block

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.

Part of C64 RF modulator section

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.

9

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.

2

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.

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  • It's a good point that the output impedance should be factored in as well.
    – Retrograde
    May 27 at 7:19
-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?

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?

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  • 1
    Actually, the RF module does not use composite as input, but separate luma and chroma. Composite signal is made from luma and chroma inside the RF module which then outputs it and also modulates into RF, so yes, if RF output is good, it's a pretty good indication that the composite output is good as well.
    – Justme
    May 26 at 18:01
  • 1
    @Justme And where does this differ from what I said? The composite signal, as outputted to the DIN plug is the very same that is modulated onto the carrier.
    – Raffzahn
    May 26 at 19:22
  • 1
    Sorry, I slightly misread/misinterpreted what you said. You did say that the RF modulator (a sub-block inside the metal module) takes in composite which is true, I don't argue with that, the same composite signal that the module outputs to DIN is also modulated to RF output. What I thought was you meant that the whole metal module takes in composite (which would have been false, as the module takes in separate luma and chroma, and those are combined to composite inside the module).
    – Justme
    May 26 at 20:43
  • 2
    @Justme Sorry, I see, that could have been phrased better. I was not thinking in 'metal blocks' but functional units. As such the modulation happens after mixing, which means that any problem that shows up on the composite plug must also show up in the RF modulated output. Ergo, if the RF output is fine, the composite will not have taken any damage. And that's the whole point why I'm kinda upset about asking such a question before even any problem shows up, making it a free wheeling, opinion based one.
    – Raffzahn
    May 26 at 21:00
  • 2
    I see your point. Knowing the internals of the metal box makes it obvious. But to someone who does not know the internals of the metal box it is not obvious - the composite output could have had a buffer stage and the buffered composite output could be toasted while RF works fine with internal signal. Or a resistor could have overheated and burned up completely or changed its resistance significantly. But fortunately this is unlikely.
    – Justme
    May 26 at 21:11

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