The Feiler Electronic "Stethoscope" Signal Tracers



Most signal tracers are non-tuned probing devices.  They include two or three stages of audio amplification with some form of demodulation circuit in the probe. Most of the non-tuned signal tracers use a variety of rectification demodulation such as a germanium diode similar to that of a crystal radio. Then there are signal tracers that employ a vacuum tube inside the probe to demodulate or couple the probe to the tracer. This tube can also serve to amplify the signal. These signal tracers include the Philco 7030 and 7031 Dynamic Tester and the Feiler Engineering TS-2, TS-3A and TS-5 Electronic "Stethoscope" Tracers.

What is special about these signal tracers is that they are very sensitive and at the same time they can be used to probe fairly high voltage circuits without damaging the probe or the tracer. Also, these probes have a unique feature in that they will demodulate RF signals leaving the audio. But unlike a diode, they will not distort audio frequencies thereby enabling the tester to clearly hear the audio while tracing audio circuits. The probe itself has a high impedance of 1 meg ohm.

Feiler TS-3A Electronic “Stethoscope” Signal Tracer

From the February 1947 addition of Radio Retailing: Model TS-3 is an a-c signal tracing Stethoscope. The 1" diameter aluminum probe contains a miniature tube in a detector-amplifier isolating circuit, while the larger case houses 3 more tubes, including a power output which drives the 5" PM speaker for audible signal tracing. An a-c voltmeter may be connected to front panel jacks which terminate VTVM and output meter circuits, while earphone jacks are also provided for extremely sensitive work. Model TS-2 is a similarly constructed Stethoscope for portable battery operation.

Besides the ubiquitous VOM, this signal tracer is one of the most used pieces of test equipment in my lab. It is a late 1940s  Feiler Signal Tracer. They labeled it an Electronic “Stethoscope” …and rightfully so, I use it for just that. Not only do I use it to trace signals through a device or seek out noise, I also use it to analyze both digital as well as analog circuits. Most of what I probe is well below 250 volts, so I’m not worried about accidently burning out the probe while tracing through a circuit.

This signal tracer can be used for AF and demodulating AM RF signals. There is a tube detector built into the probe that can handle high voltage signals. My Feiler "Stethoscope" probe has been modified by the prior owner to use a 6BA6 instead of a 6C4 triode.

My Revised Upgraded Probe

The probe cable is made up of a piece of shielded 2-conductor cable. Only three wires are needed by the probe, one of which is the tube heater, the other is the biased signal return, and a common ground (the cable’s shield). The cathode and one side of the filament are connected to ground. One wire carries the 6.3 volts for the filament and the other is the plate B+ biasing and its return signal leading to a coupling cap and B+ load resistor located inside the TS-3A. The control grid in the probe has a 1 meg grid-leak resistor and a small 1800 pf cap connected from the control grid to the probe’s tip.

Modified TS-3a

I was given this TS-3A Signal Tracer in the late 80s by a then retiring owner of Haze TV on Ludlow Avenue in Clifton in Cincinnati. The owner who was closing his shop said he had bought it new sometime in 1950. After I got it home, it ended up sitting in my basement for over ten years when I decided I had enough of being afraid I would blow out an oscilloscope or its probe while poking around inside tube amps. I found that, though the TS-3A was in a poor condition, it never-the-less allowed me to find problems much quicker. In using it, I had some problems with it feeding back internally.

Schematic With Modifications

Original TS-3a

Obtained from the radio Museum ( website.

I took it out of its cabinet and noticed that it had been – well, repaired many times. Since then I’ve made a number of modifications to, so-to-say, clean up the mess as well as quieting it down hence improving its performance. These included rectifying and filtering the 6.3 volt heater voltage along with moving various parts around. Now its signal to noise ratio is quite good. I basically turned it from a piece of junk into something very usable.


The original probe was an aluminum tube capped at either end with Bakelite caps and some soft tin like shielding inside wrapped around the tube and wires. I have subsequently rebuilt the probe using a small length of ¾ inch copper pipe and two copper caps. I did reuse the original tip. Though all the TS-3s seen on the web have their probes hardwired into the tracer, mine came with a ¼ inch stereo type jack and plug. Along with the probe holder clip mounted atop the tracer using one of the cabinet’s fasteners, I’m pretty sure the prior owner added this socket to separate and stow the probe.



This signal tracer was found in a pile of stuff at a Hamvention in early Y2K. I’ve made no modifications to it except that I did recap the thing and I rewired the probe. It’s basically original except for the replaced caps.

This signal tracer still has all of its diseases including a mild hum and is susceptible to ground fault and AC noises while probing.  I did check for AC line leakage, but it is isolated. The three tubes in the box (35W4, 50B5, and 12BA6) are series strung with a power resistor. It does have a small power transformer for the B+ and 12 volts for the probe's tube filament. I checked filament circuit and found there was infinite resistance (open) between the line cord and the chassis ground.  I suspect the AC noise is leaking through the 12BA6 tube filament because when I ground the input grid of the 50C5, most of the hum goes away. There's less than a volt on the control grid of the 50C5.

This probe does have a 6C4.  From the pictures I could find, The TS-5 probe was not hardwired but instead it originally did have a three-pronged socket for the probe.

My electronics shop is only about 10 by 14 feet.  Most of my test gear is rather compact.  I sold my CRT based oscilloscope and bought a much smaller LCD scope.  The rest is true for most of my test gear.  The TS-3A does take up a lot of shelf space.  The TS-5 is really small and it easily fits in a drawer

Right now the TS-5 is just a collector novelty.  If it didn't have any antique value and I wanted to use it as I now do the TS-3A, I would get rid of the tubes and either make or use an existing small solid state amp powering it with the 12 volt winding, design a biasing supply for the probe and use the existing probe with its existing tube.

I sure could use the space.


SteveS November 28th, 2018