The content of this website and blog has been created and posted here solely for the purposes of reading interest by Rob's Radio-Active, LLC. The content is the thoughts of the author and is not intended to be an authoritative guide. There may be errors and omissions. Electronic and electrical devices pose a potential hazard to life and property if improperly handled, used, or serviced. The author cannot possibly think of, and warn of, all the ways one can get into trouble with electricity, and recommends one learn all of the technical and safety aspects of electrical/electronics before attempting to work with or use any of it. There are hazardous voltages and currents involved in almost all electrical and electronic equipment—especially vacuum tube devices or anything that plugs into mains current for operation. Furthermore, vintage equipment was never designed or built to be up the safety standards of modern times.
Here is a handy troubleshooting tool you can make. A small plastic project box is handy for the main amplifier box. The probe handles can be made from PVC tubing or something similar, but it is not a bad idea to shield the probe handles with a metal or metal foil shield grounded to the tracer's ground system. (Note the caution on the schematic, however, on connecting the ground clips to any electrified point, especially if the ground shield is in your hand.)
For the cables connecting the probes to the main box, I used shielded audio cable taken from one of those TV to stereo connector harnesses sold in the electronics department of most discount stores.
This works well to find signal loss, weakness, excessive distortion, or noise that is not supposed to be there in a stage of most AM radio receivers or audio amplifiers. It is sensitive enough, that when connected to an outdoor antenna, it can pick several local AM radio stations...all at once!
The principle of operation is that the radio frequency voltage is applied to the RF probe tip, and the ground clip connected to a safe signal ground. The 100pF capacitor on the input blocks DC current which may be present on the circuit under test and feeds the RF signal to the gate of a J310 (or similar type) field effect transistor (FET). This provides a high input impedance that will only minimally "load down" the signal voltage on the circuit being tested. The FET circuit amplifies the small signal, typically from between 6 and 20 times, depending on frequency. (I measured a voltage gain of 6 at about 10 MHz.)
This amplitude modulated signal is then rectified ("detected") by the small signal diode and its coupling capacitor into an audio frequency (AF) signal voltage. The FET amplification drives the diode and makes the tracer more sensitive than a simple diode detector going to an audio amplifier. A 1N34A or similar type of germanium or shottky diode with their low forward voltage threshholds will allow detection of signals of only a few, or a few tens of millivolts by "square law" action...in spite of what you often read about a diode not conducting anything at all until it reaches its forward knee voltage. The AF goes through a shielded cable to the input of the audio amplifier. The 2N3904 gives a boost in AF voltage of several times before feeding it to the LM386 audio power amplifier IC. The level is controlled by the 100K volume potentiometer to help avoid overdriving the IC on strong signals. The RF probe handle assembly also has a high/low sensitivity switch, which bypasses the FET, RF preamplifier circuit altogether and feeds the signal straight to the detector circuit. Again, this is for strong signals. The signal tracer works in much the same way with audio probe, except that an audio signal requires no detection, and usually does not require as much amplification, so it is fed to the volume potentiometer to the LM386. Double throw switches in both cases connect the signal where desired and disconnect from previous amplifier stages to avoid signal loading or undesired feedback.
As always, know your electrical safety rules and use caution when working. Rob's Radio-Active, LLC cannot be held responsible for the use or misuse of this or any tool or technique.
© 2016 Rob's Radio-Active, LLC
All Rights Reserved
For the cables connecting the probes to the main box, I used shielded audio cable taken from one of those TV to stereo connector harnesses sold in the electronics department of most discount stores.
This works well to find signal loss, weakness, excessive distortion, or noise that is not supposed to be there in a stage of most AM radio receivers or audio amplifiers. It is sensitive enough, that when connected to an outdoor antenna, it can pick several local AM radio stations...all at once!
The principle of operation is that the radio frequency voltage is applied to the RF probe tip, and the ground clip connected to a safe signal ground. The 100pF capacitor on the input blocks DC current which may be present on the circuit under test and feeds the RF signal to the gate of a J310 (or similar type) field effect transistor (FET). This provides a high input impedance that will only minimally "load down" the signal voltage on the circuit being tested. The FET circuit amplifies the small signal, typically from between 6 and 20 times, depending on frequency. (I measured a voltage gain of 6 at about 10 MHz.)
This amplitude modulated signal is then rectified ("detected") by the small signal diode and its coupling capacitor into an audio frequency (AF) signal voltage. The FET amplification drives the diode and makes the tracer more sensitive than a simple diode detector going to an audio amplifier. A 1N34A or similar type of germanium or shottky diode with their low forward voltage threshholds will allow detection of signals of only a few, or a few tens of millivolts by "square law" action...in spite of what you often read about a diode not conducting anything at all until it reaches its forward knee voltage. The AF goes through a shielded cable to the input of the audio amplifier. The 2N3904 gives a boost in AF voltage of several times before feeding it to the LM386 audio power amplifier IC. The level is controlled by the 100K volume potentiometer to help avoid overdriving the IC on strong signals. The RF probe handle assembly also has a high/low sensitivity switch, which bypasses the FET, RF preamplifier circuit altogether and feeds the signal straight to the detector circuit. Again, this is for strong signals. The signal tracer works in much the same way with audio probe, except that an audio signal requires no detection, and usually does not require as much amplification, so it is fed to the volume potentiometer to the LM386. Double throw switches in both cases connect the signal where desired and disconnect from previous amplifier stages to avoid signal loading or undesired feedback.
As always, know your electrical safety rules and use caution when working. Rob's Radio-Active, LLC cannot be held responsible for the use or misuse of this or any tool or technique.
© 2016 Rob's Radio-Active, LLC
All Rights Reserved
Below are some pictures taken of the unit as I constructed it, before putting it into any housings. The first is of the RF probe handle, the second is the main (audio) amplifier, and the third is the whole mess together as it was being tested to confirm performance.
