Post by dbster on Nov 21, 2011 20:15:12 GMT -5
I'm putting this in the "Newbie Stuff" section even though it is somewhat technical. The title refers to why you need a volt-ohm-meter, often referred to as a VOM. Even if you buy reasonably priced Emotiva equipment, this $50 investment can be handy. You can buy a VOM new, but get more for your money at a ham radio flea market or Craig’s List or Ebay or similar. I prefer the classic mechanical meter ones like a Simpson, but there are many choices in analog and digital. Nothing sophisticated is needed for the applications described below. Several common uses in audio:
1) Check the batteries in your remote.
Put the meter on a range that goes to 2 Volts, DC, but not a lot more.
Put the red (positive +) lead on the top of the battery (bump out in center) and the black (negative -) lead on the bottom. A new battery will read more than 1.5 VDC. A used up one will read less than 1.25 VDC
2) Check continuity
This is useful for fuses – it can be hard to tell visually if a fuse is good or not. Take fuse out of equipment before doing this! Put VOM on Rx1 range, touch the two test leads, one to each end of the fuse. Zero ohms or close to it is good, otherwise bad. Flick the fuse with a free finger – reading should not change.
Cables.
Here we check for two things – continuity of the same conductor, and leakage.
Completely disconnect the cable from equipment on both ends before doing this!
So pin to pin of an RCA style interconnect for example should read zero ohms, meaning no resistance. Check both center pin to center pin and shield to shield at each end on Rx1 range of your VOM. This shows no resistance (e.g. good conductor)
Now put it on the highest R range (Rx1K, Rx10K depending on how good a VOM you bought). At this sensitivity, you may get some kind of reading just by accidentally touching the two metal ends of the probes in your hands, since your body is a poor but measurable conductor of electricity. Put the two leads on the pin and shield of the same end, holding the plastic part of the probe not the metal part. You should see a very high reading (hardly any movement of needle if a classic analog type VOM). This shows no connection between the two conductors of your cable – what you hope for.
So the cable conducts between the two similar pins (shield or center) and doesn’t conduct between them - that is good.
Now, the best problems are intermittent, so while doing these two resistance tests, flop the cable around and make sure readings are stable. Intermittent connections plague all electronic equipment.
Another continuity check is matching ends of unmarked cables. If you have 100 feet of parallel speaker wire that is not coded, test the continuity between one wire at one end and a wire at the other end. One will read close to zero ohms, meaning it is the same wire. Checking two wires that aren’t the same will show infinite resistance – no connection
3) Check audio output
With the speakers connected, put the VOM on a scale of around 50 V AC.
Put the probes on the speaker terminals, being sure not to cross them or cause a short circuit. A crude measure of voltage is the square root of power times resistance (impedance for simplicity here) (EEs: I’m writing this for the newbie section, so I’m doing great simplification - I do know better). So 100 watts going to an 8-ohm speaker is very roughly 28 volts AC. You can see the meter move with the music, though it is much too slow to follow the music dynamics, so it is averaging. But it gives you an idea of signal presence from the amp. If you have a test record, like a 1 KHz tone for several minutes, and identical speakers, you would expect the same steady voltage reading at each speaker if the amplifier output was the same for each channel.
1) Check the batteries in your remote.
Put the meter on a range that goes to 2 Volts, DC, but not a lot more.
Put the red (positive +) lead on the top of the battery (bump out in center) and the black (negative -) lead on the bottom. A new battery will read more than 1.5 VDC. A used up one will read less than 1.25 VDC
2) Check continuity
This is useful for fuses – it can be hard to tell visually if a fuse is good or not. Take fuse out of equipment before doing this! Put VOM on Rx1 range, touch the two test leads, one to each end of the fuse. Zero ohms or close to it is good, otherwise bad. Flick the fuse with a free finger – reading should not change.
Cables.
Here we check for two things – continuity of the same conductor, and leakage.
Completely disconnect the cable from equipment on both ends before doing this!
So pin to pin of an RCA style interconnect for example should read zero ohms, meaning no resistance. Check both center pin to center pin and shield to shield at each end on Rx1 range of your VOM. This shows no resistance (e.g. good conductor)
Now put it on the highest R range (Rx1K, Rx10K depending on how good a VOM you bought). At this sensitivity, you may get some kind of reading just by accidentally touching the two metal ends of the probes in your hands, since your body is a poor but measurable conductor of electricity. Put the two leads on the pin and shield of the same end, holding the plastic part of the probe not the metal part. You should see a very high reading (hardly any movement of needle if a classic analog type VOM). This shows no connection between the two conductors of your cable – what you hope for.
So the cable conducts between the two similar pins (shield or center) and doesn’t conduct between them - that is good.
Now, the best problems are intermittent, so while doing these two resistance tests, flop the cable around and make sure readings are stable. Intermittent connections plague all electronic equipment.
Another continuity check is matching ends of unmarked cables. If you have 100 feet of parallel speaker wire that is not coded, test the continuity between one wire at one end and a wire at the other end. One will read close to zero ohms, meaning it is the same wire. Checking two wires that aren’t the same will show infinite resistance – no connection
3) Check audio output
With the speakers connected, put the VOM on a scale of around 50 V AC.
Put the probes on the speaker terminals, being sure not to cross them or cause a short circuit. A crude measure of voltage is the square root of power times resistance (impedance for simplicity here) (EEs: I’m writing this for the newbie section, so I’m doing great simplification - I do know better). So 100 watts going to an 8-ohm speaker is very roughly 28 volts AC. You can see the meter move with the music, though it is much too slow to follow the music dynamics, so it is averaging. But it gives you an idea of signal presence from the amp. If you have a test record, like a 1 KHz tone for several minutes, and identical speakers, you would expect the same steady voltage reading at each speaker if the amplifier output was the same for each channel.
