Sorry but this is winding up being a difficult thing to find with Google... I'm getting every kind of sniffer/sensor probe coming up except for what I'm looking for.
Does anyone have a schematic for a device that can detect and amplify hum emitted by chokes, transformers, wires, etc. I've seen guys who made these. I suspect It's basically a coil pickup probe point, hooked up to a small op amp and small speaker, maybe a VU meter too. It is used to probe around the empty space of a chassis to find where hum is being emitted. It can be used to position chokes and transformers, detect what sides of a coil might be emitting the most or least hum field, etc. It would be a handy device for valve construction.
I would like to make one out of junk box parts on hand, if anyone has a good schematic for one of these. Could they share it?
Does anyone have a schematic for a device that can detect and amplify hum emitted by chokes, transformers, wires, etc. I've seen guys who made these. I suspect It's basically a coil pickup probe point, hooked up to a small op amp and small speaker, maybe a VU meter too. It is used to probe around the empty space of a chassis to find where hum is being emitted. It can be used to position chokes and transformers, detect what sides of a coil might be emitting the most or least hum field, etc. It would be a handy device for valve construction.
I would like to make one out of junk box parts on hand, if anyone has a good schematic for one of these. Could they share it?
A cheap MM phono cartridge taped to a stick. Use the short cartridge leads soldered to phono cables and use a phono pre to a the amp. It'll hum like hell.
Thanks, I already have the Fluke version of this same probe, I use it to trace and install long runs of CAT6 cable, using tone detection. My Fluke filters out 60hz completely, it is focused on detecting the warble tone at the far end of a single cable in a bundle of many cables. So this makes it useless for tubes where you actually want to detect 60hz. Does your 200FP have a way to omit the 60hz filter?
Get a small iron core RF choke and connect it to an oscilloscope. All transformers and chokes are going to emit some some energy.
Thanks, this does seem the best way. I can epoxy/encapsulate a small choke into a fatter old pen shell or probe shell after soldering it to a shielded cable. Then just terminate the cable with a BNC connector. Hang it up with my other scope probes. I can tell so much more about what garbage is in the air using a scope rather than a speaker.
Take an inexpensive small DC choke.
Remove the E's and I's (Laminations).
Cover the I's with shrink tubing to insulate.
Install the I's in the coil opening.
Fasten the I's securely with tape.
You now have a sensitive magnetic pickup to drive a scope, or a small Radio Shack speaker/ amplifier, the one that had a 9 V battery in it.
Build an N-channel JFET Source follower.
Connect a 10Meg Ohm resistor from gate to source.
Connect the gate to a 1 Inch wire (the probe end covered with shrink tubing).
Use shielded wire for connecting the Follower output to the scope.
Connect a source resistor, 1k Ohm from source to shield.
Connect a 9V battery, + to JFET Drain, and battery - to shield.
Connect the JFET source to the center conductor of the shielded wire.
Connect the center conductor output of the shielded wire to the scope input;
and the shield to the scope input shield.
Set the scope to AC coupling.
You now have a sensitive capacitive E-field pickup.
Or connect a coupling cap from the center conductor to the input of a small Radio Shack speaker amplifier,
and connect the shield to the speaker amplifier input shield.
If you can build a Vacuum Tube Amplifier from scratch, you can build these very useful probes.
Happy hum and noise "fishing".
Remove the E's and I's (Laminations).
Cover the I's with shrink tubing to insulate.
Install the I's in the coil opening.
Fasten the I's securely with tape.
You now have a sensitive magnetic pickup to drive a scope, or a small Radio Shack speaker/ amplifier, the one that had a 9 V battery in it.
Build an N-channel JFET Source follower.
Connect a 10Meg Ohm resistor from gate to source.
Connect the gate to a 1 Inch wire (the probe end covered with shrink tubing).
Use shielded wire for connecting the Follower output to the scope.
Connect a source resistor, 1k Ohm from source to shield.
Connect a 9V battery, + to JFET Drain, and battery - to shield.
Connect the JFET source to the center conductor of the shielded wire.
Connect the center conductor output of the shielded wire to the scope input;
and the shield to the scope input shield.
Set the scope to AC coupling.
You now have a sensitive capacitive E-field pickup.
Or connect a coupling cap from the center conductor to the input of a small Radio Shack speaker amplifier,
and connect the shield to the speaker amplifier input shield.
If you can build a Vacuum Tube Amplifier from scratch, you can build these very useful probes.
Happy hum and noise "fishing".
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The smaller the pickup the better, as well as using twisted pair and shielded cable from as close to the pickup coil as possible, is my recommendation. And try to keep the cable connecting to the pickup as far from the likely flux emitter as possible, so that it is the pickup, and not the connecting cable, that generates the most signal being observed on your scope. You may also want to test how grounding the pickup shield to the chassis under test affects the signal, as that may reduce electrostatic signal relative to magnetic coupling to the sensor.
I like the sensitivity a FET amp would add to the probe with your idea here. But as far as the pickup coil goes though... I think if that can be kept to the size of a peanut it would have better pinpoint accuracy over a coil with a cross section approaching an inch or more. I do have a few peanut sized surplus RF chokes here wound with hair thin wire. Time to experiment, I suspect I'll learn something about positioning and distances with inductors that I just guessed at before.
Lots of great ideas here. I do have plenty of quad shield RG6 for this level of shielding. I'll have to use an F connector with RG6 quad I have plenty of waterproof F connectors, then use an F to BNC adapter to plug it into the scope.
Hence my suggestion, which seems to have been summarily dismissed, for using a cheap MM phono cartridge. Ever torn one of those apart to see how small the coils are? Get one of those $17 P-mount AT cartridges. I've had a MM cartridge pick up power transformer magnetic radiation from 2 feet away requiring TT repositioning to remove it. Yes grounded.
These sensors are ideally suited to the task:
TELEPHONE PICK-UP | All Electronics Corp.
They have been around for decades, and have been proven and tested.
If you just want informal/sniffing information, just connect it to a signal-tracer, otherwise tie it to an AC millivoltmeter (you can do both at the same time).
If you want to really pinpoint the source of a perturbation, you can use a SMD coil as sensor (it has to have an open magnetic circuit, thus generally <1mH)
TELEPHONE PICK-UP | All Electronics Corp.
They have been around for decades, and have been proven and tested.
If you just want informal/sniffing information, just connect it to a signal-tracer, otherwise tie it to an AC millivoltmeter (you can do both at the same time).
If you want to really pinpoint the source of a perturbation, you can use a SMD coil as sensor (it has to have an open magnetic circuit, thus generally <1mH)
Even a spare miniature relay would also likely do the job, although like the phono cartridge, you want to retain the existing coil wire terminations and use the provided terminals, as the winding wire size gets very fragile as you go to smaller coils.
Keep in mind, an amplifier that is turned on and operating will have very large magnetic and electrostatic fields.
Many of these "probes" will be overloaded (clipping JFET, magnetics that are saturated, etc.).
But even so, they are useful to find the cause of problems that you did not even think of, or suspect.
I work hard to get my amplifiers to have 100uV or less at the output terminals (good enough for most loudspeakers, but not good enough for sensitive headphones and earbuds).
Many of these "probes" will be overloaded (clipping JFET, magnetics that are saturated, etc.).
But even so, they are useful to find the cause of problems that you did not even think of, or suspect.
I work hard to get my amplifiers to have 100uV or less at the output terminals (good enough for most loudspeakers, but not good enough for sensitive headphones and earbuds).
Detecting is fine, but that isn't the same as probing imho - where you may need to position a search coil in a particular location and the coil oriented in any of 3 orthogonal directions (x,y,z), and then relate that back to say a pickup loop (eg. an output transformer winding, or a circuit loop) that also has a nominal orientation and location (which you may be able to then adjust to minimise the transfer).
Wrt Rod's link, such an amplifier can be good to have around, but for this type of snooping there may be a lot of merit to using a soundcard interface, as that then gives he opportunity to look at the signal spectrum using suitable software. For example, driving a power transformer primary winding with a frequency that is different from the mains frequency (eg. 50 Hz for a 60Hz mains) then allows the search coil to differentiate the PT leakage field from spurious 60Hz mains frequency related fields in the vicinity. And a soundcard would be sensitive enough to search for low level signals if using an audio amplifier to generate the 50Hz signal voltage but not capable of driving say a 110Vrms output.
Any coil will do. Connected via shielded cable to the input of some guitar amp. A wide field for experiments....