Finding out which pin is connected to shield - Page 1
Forum section: electronics / projects

author: MadModder
author_rank: Regular Contributor
timestamp: April 19, 2016, 04:11:50 pm
content: If I have a cable, with connectors impossible to open without destroying them, and I don't want to make any sort of hole in the cable, how do I determine which pin is connected to the cable's shield?Or if there is a shield at all...I was thinking of hooking a pin up to an scilloscope, induce a signal in to the cable with a coil or something, and grounding the other pins one at a time to see if the signal gets any lower.

author: donmr
author_rank: Regular Contributor
timestamp: April 19, 2016, 04:24:17 pm
content: Sometimes you can push a small pin through the outer cover and contact the shield (if there is one).

author: MadModder
author_rank: Regular Contributor
timestamp: April 19, 2016, 04:54:54 pm
content: Yes, but I don't want to puncture the cable jacket.

author: wraper
author_rank: Unknown
timestamp: April 19, 2016, 05:05:30 pm
content: Maybe wrap the cable with a metal foil and measure terminal-foil capacitance. Terminal with the highest capacitance should be connected to the shield.

author: LaserSteve
author_rank: Super Contributor
timestamp: April 19, 2016, 06:20:08 pm
content: Lets see what you have in lab gear...Do you own a magnetic compass?Do you own a home stereo with an aux audio input (RCA plug) ?Do you own an oscilloscope?Do you have a signal generator...Dc power supply with constant current and a multi-meter to measure very low resistances?Capacitance meter?RF signal Generator?Steve

author: MadModder
author_rank: Regular Contributor
timestamp: April 19, 2016, 07:58:43 pm
content: Wraper: Ah, yes, capacitance! Now why didn't I think about that?LaserSteve:magnetic compass - noaux audio input - yesoscilloscope - yessignal generator - yeslab power supply with constant everything - yesvery low resistance meter - nocapcitance meter - yesRF signal generator - no

author: Wobbegong
author_rank: Newbie
timestamp: April 19, 2016, 08:27:31 pm
content: Do you have access to the shield at the connector? If so could you just measure the resistance between the connector shield to each pin?

author: Jeroen3
author_rank: Super Contributor
timestamp: April 19, 2016, 08:49:13 pm
content: Get an xray of the plug.

author: LaserSteve
author_rank: Super Contributor
timestamp: April 19, 2016, 09:36:33 pm
content: Crude ways to find the shield.. My best guess with what you have..Why do I feel a new video from Dave is coming soon...Start with wrapping a few turns of wire around the length of the cable, hooked to the signal generator as a "gimmick" coupling capacitor. .Ground the SG to the stereo aux ground, and apply a few volts at 1-2 KHz to the "gimmick wire"Move the center connection (use a shielded RCA cable) from pin to pin using a jumper cord. At the same time start grounding other pins.. Your using the stereo input as a high impedance probe. One pin will be VERY hot, odds are its the shield or connector shell..The higher the audio frequency, the better this works. You may need to keep your hands away from the setup.If and when you ground the shield, the audio should drop to very low levels...Can try same "gimmick" wire with oscilloscope set to high impedance input at a much higher frequency. Moving scope probe looking for maximum/minimum coupling. OK, to trigger scope using a TEE off the signal generator for synchronous detection.Connect scope probe to each pin in turn. Use your body capacitance to pick up 50-60 Hz and wrap hand around cable shield. Of course you may ground some other pins. My bet is one pin is very hot compared to others...It also helps if you have a spare connector or two to terminate the cable with. Body capacitance from your hand is quite large. Thus it can help to keep the probe outside of your grip by using a connector.Also measure the capacitance, if you can.. Wrap Aluminum foil around the cable if needed...Flow a current thu the wire, and measure the voltage drop across the cable.. This is tricky... May need a Kelvin connection and/or a sense resistor.There is another trick using a car ignition coil, a battery to pulse the coil, and a tiny NE2 neon lamp. But we will save that for last.. It can be quite shocking if you do not know what your doing. Again, capacitive coupling of the pulse.Maybe some other tricks with square waves if you have a fast scope and fast signal generator..Or even a PC sound card with AC coupling from your hand...Note certain mylar shields are designed to be leaky or resistive on the outside on audio "snake" cables .. Those may not be easy to detect.. The reason being the internal drain wire might not show enough capacitance to the outside without a conductive foil.Look up the pF per foot for the wire type if you can..Steve