Panasonic DMR-EX89 HDD/DVD recorder composite input fault

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Martechnology
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Joined: Jul 22 2005

A fault occurred while I was using my above-mentioned recorder on the AV3 input (on the front of the unit) to check the output of a mixer. Everything was OK using an s-video link, so I then tried the composite output of the mixer into the composite input on AV3. A picture appeared for a few seconds, then disappeared.

My first reaction was to blame the cable which I had made up myself from a length of 75 ohm aerial cable, but metering the suspected cable followed by a change of cable disproved that idea.

Next, I blamed the mixer (recently acquired), but there was no ready way to check this without moving equipment about to get at other connections.

While I figured out a way to connect the mixer directly to the TV set without putting my back out in the process, I checked the AV3 input on the recorder with a known good camera and cable. The result was conclusive. The s-video input worked, but the composite input did not. 

Although I haven't dismantled the unit yet, I suspect a component failure rather than a dry joint on the back of the socket. 

A final thought has just occurred to me. The chassis of the mixer, a Panasonic MX12, is earthed via it's mains plug. I checked this with a meter. The rest of the AV system is not, so it could be floating a generous handful of volts above ground due to induction from mains wiring. This difference could have caused a voltage spike on the input when the composite lead was plugged in. If the dv/dt of this spike was faster than any internal filtering or bandwidth limitation could take out, the input device could have been destroyed. I admit at this stage to having "hot-plugged" the lead, although I did the same with the s-video lead and that input survived.

Or it could just be that the composite input was less than 100% reliable from the day the unit was born, ie a disaster just waiting to happen. The unit has seen a lot of use in the last 4 years, so each time something was plugged in (always hot-plugged) it could have been provoked ever-closer to failure.

Any advice on tracng the fault would be gratefully received.