[Boatanchors] I was asked, Here's a Primer on using a Signal Tracer. . .

[Philip Atchley]

Hi,

Since I received a couple questions on this subject I thought I'd try to 
answer them all here.

>From: "Eugene Hertz" <ehertz at tcaf.org>
> Hello. You caught my interest. How, exactly, is a signal tracer used?

Glad you asked.

Basically a Signal tracer is a high gain audio amplifier with a 
switchable attenuator and volume control.  The probe usually has a 
switch for "direct" (audio) input and "Detector" (for troubleshooting RF 
stages).

Typical use:

1. On the Signal tracer, set the "Sensitivity" or "gain" switch to the 
most sensitive position and adjust the volume to perhaps midscale.
2.  Put the probe in "detector or RF position and connect the probes 
ground lead to the chassis of the radio under test (here-after called 
DUT).
3.  IF the set doesn't have an internal loop antenna, connect an outdoor 
antenna to the set.
4. Turn the DUT on and tune to what would be the strongest station in 
your area (an AM BCB station is good if the radio tunes that range).
5.  Touch the probe to the high side (signal)of the tuning capacitor 
where the antenna coil and "peak" the tuning control for maximum signal. 
The Signal tracer is acting as a "Crystal set" with audio amplifier.
6.  Assuming you hear the station in the signal tracer, touch the probe 
to the Grid (or base if transistor) of the first RF amplifier stage. 
You SHOULD hear about the same signal in the tracer, about the same 
level.
7.  Touch the probe to the PLATE (or Collector) of the first stage.  You 
should hear the SAME signal, only louder.
8.  Touch the probe to first the grid (base) and then the plate 
(collector) of EACH stage in succession.  The signal should get 
progressively stronger with each stage (may have to turn the Signal 
tracer sensitivity down).

Tip 1. You 'may' have to slightly return the radio as you get further 
down the RF and IF chain and selectivity gets sharper.
TIP 2. IF the signal disappears or gets very weak between the plate of 
one stage and the grid of the following stage, look for a bad interstage 
transformer, shorted capacitors, bad Local oscillator (if a mixer stage) 
etc.
TIP 3.  IF the signal disappears or gets very weak between the grid of a 
stage and it's associated plate connection (same stage), look for 
problems in the plate circuit of that stage, Bad tube, open resistors, 
open transformer, shorted capacitors etc.

OK, now assuming that you've worked your way through the entire RF/IF 
chain and everything seems normal we get to the Audio section (anything 
after the detector stage).  Testing the audio section is similar to 
testing the RF section with only one minor difference.

1.  Put your Probe in the AF or "straight through" position.  Touching 
the tip of the probe with your finger "should" produce a buzz in the 
signal tracers speaker.
2.  Touch your probe to the "high" side of the radios Volume Control. 
You should hear the station the radio is tuned to.
3.  Touch the probe to the center pin of the volume control and rotate 
the control through it's range.  You should hear the station and a 
"smooth" change in volume with no "scratchiness".  If the control is 
scratchy or intermittent, clean the control with "LUBRICATED" contact 
cleaner (with silicon is best).  If the control can't be repaired this 
way, replace it before continuing.
4.  Test the grid and plate of each succeeding stage like you did in the 
RF section.  Listen for hum, noise or distortion on the signal.
    A.  Hum: 'Most likely' due to bad filter capacitors, tubes with 
heater cathode leakage or a miswired stage (if set has been modified).
    B.  Noise:  Most likely due to bad tubes or broken/cracked resistors 
etc.
    C.  Distortion:  Most likely causes are leaky coupling capacitors, 
gassy tubes, incorrect grid bias on a stage (bad resistors etc).

BIG TIP.  On a "dead" set I OFTEN split the "signal tracing" steps in 
half and check THE HIGH SIDE OF THE VOLUME CONTROL before doing anything 
else. This will give me a clue as to whether the problem most likely 
lies in the RF or AF sections.

FURTHER TIPS.
1.  Since it's primarily a high gain Audio amplifier the signal tracer 
may also be used to check other devices.  Things like microphones, 
guitar pickups, home stereo gear, anything with audio to troubleshoot.

2.  IF your signal tracer lost it's special probe before you got it, 
don't despair.  You can make a pair of probes, one straight through for 
audio testing and a crystal detector probe for RF work.  The schematic 
of the detector probe can usually be found in the schematic of a signal 
tracer.  These things are pretty universal.  If your manual doesn't have 
it, the manuals of other units do.  Or even the "RF PROBE" from the ARRL 
Amateur Radio Handbook should work well.

3.  There are other functions that the usual Tracer has too.  Like, a 
"magic eye tube" and circuit to measure the AC current draw of a DUT 
plugged into it's AC receptacle, connections to the internal speaker and 
output transformer to allow them to be subbed for the ones in the DUT, 
etc.  These additional functions are best learned by reading the manual 
for your particular unit.

73 de Phil,  KO6BB
991 Different NDB's heard to date.

From: "Eugene Hertz" <ehertz at tcaf.org>

> Hello. You caught my interest. How, exactly, is a signal tracer used?