Why didn't this work? (Stupid cable tricks)

[+thegoldenband]

This is a little bit tricky to describe, so bear with me:

 

My girlfriend has an old Commodore 64 which hasn't been used in years, and tonight I decided to try hitching it up to see if it worked. Problem is, I don't have a switchbox, nor do I have the special AV cable for the C64. Right now, I have only one other console, a Coco 2, that doesn't have separate A/V outs -- and since I have a Coco 3, that hasn't been an issue.

 

So I tried something a bit goofy. I have no real electrical skills to speak of, but I've replaced the power cable on an appliance before, and have done a couple similar, very small tasks. So I thought I'd see if I could make a cable where one end was RCA, and the other was F Jack (is that the right name?), and see if I could get some sort of image/sound out of the C64. It didn't need to be perfect, just good enough to see that the machine was operational.

 

My girlfriend also had an old antenna that plugged in to the back of her TV, with a cable attached. The connector on that cable had a center pin, but the outer ring is plastic. I cut this cable off of the antenna, and stripped the end, which revealed a single thick, silver wire. Then I took a spare RCA cable, cut and stripped the end, peeled back the layer of copper shielding, and stripped the core. Then I twined the two together, sealed it up with electrical tape, and tried it out.

 

I ended up trying it with the Coco 2, since I know the Coco 2 much better. The result was an extremely noisy, colorless video signal, and a noisy-but-intelligible audio signal. I could just make out the Coco's startup screen, and the audio responded to commands, so I could tell that it was working. But the video was so noisy as to be almost unrecognizable, and certainly unusable.

 

(I didn't have any luck with the C64 -- it was clearly passing some sort of video, but I couldn't make it out, and I don't know any one-line commands to get C64 BASIC to beep.)

 

Now, this webpage tells me that I can go to Radio Shack and get a RCA Phono Plug to F Jack adapter, and that should do the trick. Thus, I'm guessing that impedance and signal level aren't an issue. (And obviously, I'll go pick one of those up, since someday I plan to get a 2600 again.)

 

So, forgive me if this is a stupid question, but: why didn't my splice job work? Is it something in my choice of cables, or is twisting them together too simplistic a solution? I was thrown off by the fact that the antenna cable wasn't shielded, but the RCA was. Where did I go wrong?

Edited June 14, 2009 by thegoldenband

[BigO]

I got lost on the description of the antenna cable. It was coaxial in style, but had only the center conductor?

[+thegoldenband]

I got lost on the description of the antenna cable. It was coaxial in style, but had only the center conductor?

From what I could tell, yeah. There was a center conductor, and the housing around it seemed to be just plastic. I'll go double-check...

 

...yep, just a plastic housing. And it slides on to the connector on the back of her TV, rather than screwing on.

Edited June 14, 2009 by thegoldenband

[BigO]

I got lost on the description of the antenna cable. It was coaxial in style, but had only the center conductor?

From what I could tell, yeah. There was a center conductor, and the housing around it seemed to be just plastic. I'll go double-check...

 

...yep, just a plastic housing.

I suspect there must be something more to the cable from the antenna that you're not able to see. Maybe the shield was attached to the outer sheath that you stripped away? Some cables use an obvious braided tinned copper or similar shield, some have foil but some cheaper cables may just have a thin layer of conductive plastic-y, maybe mylar material. I suppose it's possible that the inner surface of the sheath was actually treated to be conductive. Dunno. I haven't ever seen any coaxial cable in which the shield conductor wasn't obvious.

 

As I suspect you suspect, the lack of that second conductor is a pretty good bet to be the problem with your signal.

Edited June 14, 2009 by BigO

[+thegoldenband]

I suspect there must be something more to the cable from the antenna that you're not able to see. Maybe the shield was attached to the outer sheath that you stripped away? Some cables use an obvious braided tinned copper or similar shield, some have foil but some cheaper cables may just have a thin layer of conductive plastic-y, maybe mylar material. I suppose it's possible that the inner surface of the sheath was actually treated to be conductive. Dunno. I haven't ever seen any coaxial cable in which the shield conductor wasn't obvious.

 

As I suspect you suspect, the lack of that second conductor is a pretty good bet to be the problem with your signal.

That certainly sounds logical. I'll double-check the antenna cable to make sure that there's no sign of foil or other treatment.

 

Out of curiosity, if there had been visible shielding -- let's say copper wrap -- how would one connect the respective cables' shielding together, or otherwise deal with that issue? Or is that a fool's errand?

 

In any event, it sounds, at least, like there was nothing fundamentally wrong with my approach. Which is reassuring, somehow!

Edited June 14, 2009 by thegoldenband

[+poobah]

It's entirely possible to have an antenna with a single conductor (from an antenna design point, that really 'iffies' up where the ground plane is, but anyway...), and this appears the case here, consider a car radio antenna as an example.

Your computer however, absolutely requires both conductors. It 'fakes' a broadcast signal, but it is very weak, and needs the ground run.

Go purchase the correct connector, hook it up, and all should be well in the universe.