How to Use A Signal Tracer
How to Use A Signal Tracer
In this article, we will explain how to use a signal tracer. A Signal Tracer is an essential diagnostic tool for restoring vintage tube radios. It is essentially a high-gain audio amplifier with a loudspeaker and a probe, allowing a technician to "listen" to the signal as it progresses through the various stages of a receiver. By tracing the signal from the antenna to the speaker, one can pinpoint exactly where the signal is lost or distorted.
1. Understanding the Signal Tracer Probe - How To Use A signal Tracer
With respect to using a signal tracer, typical ones use the following two modes of operation, often toggled by a switch on the probe or the device itself:
- RF/IF Mode (Diode Detection): Uses a to demodulate radio frequency (RF) or intermediate frequency (IF) signals into audible audio. This is used for the "front end" of the radio.
- Audio Mode (Direct): Bypasses the diode to trace signals in the audio amplification stages.
2. Using A Signal Tracer Procedure
To begin, connect the radio to an for safety. Apply a modulated signal to the antenna terminals using a (typically 1000 Hz tone at the tuned frequency). If a generator is unavailable, tune the radio to a strong local station.
Stage 1: The Converter/Mixer Stage
The signal enters the antenna and is processed by the converter tube (e.g., 12BE6 or 6SA7).
- Test Point 1 (Grid): Place the probe (RF mode) on the signal grid of the converter tube. You should hear the station or tone faintly.
- Test Point 2 (Plate): When using a signal tracer, place the probe on the plate of the converter. The signal should be significantly louder. This confirms the tube is oscillating and mixing correctly.
Second Stage: The IF Amplifier Stage
At this point, the signal is now at the Intermediate Frequency Transformer (usually 455 kHz).
- Test Point 3 (IF Grid): Touch the grid of the IF amplifier tube (e.g., 12BA6 or 6SK7).
- Test Point 4 (IF Plate): Touch the plate of the IF amplifier. If the signal is present at the grid but absent or distorted at the plate, the IF tube or the associated Intermediate Frequency Transformer is likely defective.
Stage 3: The Detector and AVC Stage
The Detector (often part of a 12SQ7 or 12AT6) strips the RF carrier, leaving only audio.
- Test Point 5 (Detector Input): Use RF mode on the diode plate of the detector tube.
- Test Point 6 (Volume Control): Switch the tracer to Audio Mode. Touch the "high" side of the volume control. You should hear clear audio. If the signal is clear here but the radio is silent, the fault lies in the audio power stages.
Stage 4: Audio Power Output
- Test Point 7 (Power Grid): Touch the grid of the power output tube (e.g., 50C5 or 6V6).
- Test Point 8 (Output Transformer): Touch the plate of the output tube. Caution: High DC voltage is present here; ensure your tracer probe capacitor is rated for at least 600V.
3. Interpreting Results
By following this linear path, when using a signal tracer, the point of failure is identified as the stage immediately following the last "clean" signal.
| Observation | Likely Fault |
|---|---|
| Signal at TP2, but not at TP3 | Open primary or secondary in the 1st IF Transformer. |
| Signal at TP4, but distorted | Leaky coupling capacitor or biased-off IF tube. |
| Signal at TP6, but not at TP7 | Open coupling capacitor between 1st Audio and Power Amp. |
| Signal at TP8, but no sound from radio speaker | Open primary or dead speaker voice coil. |
4. Safety and Precautions
When working on "All-American Five" (AA5) sets or any vintage tube gear:
- Always use an Isolation Transformer when using a signal tracer. Many vintage sets have a "hot" chassis connected directly to one side of the AC line.
- ICheck the voltage rating of your tracer's input coupling capacitor. It must exceed the highest B+ voltage in the radio (typically 150V to 350V).
- Keep the tracer volume low initially to avoid sudden loud bursts that could damage the tracer's speaker.
Hopefully you’ve benefited from this article on How To Use A Signal Tracer. If you would like your Radio repaired, check out Retro Radio Shop. Antique Radio Repair Services.