The active signal tracer probe has AGC

[Electronics Old and New] One of his old projects has a new version. The original project was an active search. He took what he learned to make that probe and put it into a new probe design. He added automatic profit control or AGC. You can watch a video explanation of the design below. The probe basically uses a JFET with a high-impedance input that can amplify the audio or demodulated RF signal, which is a simple device when solving radio problems.

The audio amplifier is a common LM386 circuit. The real work is at the input stage and the new AGC circuit. True, we have used the amplifier itself for similar functions, although the chip’s raw input impedance is about 50K and much lower in many circuits that use a pot at the input. Having a JFET buffer and an RF demodulating diode is certainly easy. You think the AGC block will be at the audio level. However, the design uses it before the detector which is great as long as the amplifier can handle the RF frequency of your interest. In this case, we think he’s working on most old tube AM radios, so the maximum signal is probably around 1 MHz.

A similar device was the head of a radio shack for many years

The module is designed to amplify an electret microphone using an MAX9814 that contains AGC. The module had a microphone that was off for this project. The datasheet does not specify a high frequency limit, but a similar maximum part indicates that its gain is more than 5 at 600 kHz, so it is likely that the type of signal it is used for will work well. We wondered if you could use the module and distribute it with JFET input. The chip probably has a pretty high input impedance, but the datasheet doesn’t give a great indication.

Over the years we have used a signal tracer from Radio Shack – if we can still find it – it now has an LM386 inside after the original electronics failed decades ago. In those days, to fix an AM radio was to use a device like this to find out where you were and to find out if there was a signal or injection signal at different points on the radio. Both sides of the same coin. For example, if you hear a signal in the volume control – indicating that the RF steps were good and there was a problem with your audio side. Conversely, if you inject a signal into the volume control, not hearing means the same thing. Once you know if the problem is on the RF or AF side, you will divide that part roughly in half and repeat the operation until you get to a bad stage. Of course, you could use signal generators and scopes, but in those days you were unlikely to get them.

Heathkit, of course, had their own version. It even had that amazing magic eye tube in it.

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