Heathkit IT-12 User manual

Heathkit of the Month:
by Bob Eckweiler, AF6C
Heath IT-12
Visual - Aural Signal Tracer
Introduction:
This month we're going to look at a piece of in-
expensive Heathkit test equipment. If you ever
worked on a ham receiver or even an old AC-
DC radio, stereo or phonograph, the signal
tracer is second only to the voltmeter as a tool
for troubleshooting, especially if you don't own
a quality oscilloscope.
Heathkit IT-12 Signal Tracer
The Signal Tracer:
A signal tracer is a high-gain audio amplifier,
with a built-in speaker and visual signal indica-
tor. Two probes provide input to the signal
tracer. One probe directly connects the input
for troubleshooting audio signals. The second
probe includes a crystal diode for troubleshoot-
ing modulated R.F. signals. Often the probes
are combined into one probe with a two-
position switch for selecting the probe’s func-
tion.
Using a Signal Tracer:
Using a signal tracer is quite simple. A signal
source is input to the 'device under test'
(DUT). For a receiver, a modulated RF signal
generator, or even a local radio station will
work. For an audio device, a record, tape, or
other audio signal is used. The DUT is then
probed one stage at a time. Thus it is easy to
find which stage is malfunctioning, whether it
is a total loss of signal, distortion, noise or low
gain. For example, to troubleshoot a super-
heterodyne shortwave receiver, first apply a
known RF signal. When the RF probe is placed
on the grid (or base) of the RF amplifier stage
you should hear the signal. The probe is then
moved to the plate (or collector) of the RF stage
and the signal checked again. If the stage has
gain you should notice it by the indicator and
the increase in signal volume. This procedure is
repeated in the mixer stage(s) and subsequent
IF stages. At the detector stage the audio probe
used instead of the RF probe and tracing con-
tinues. A sudden loss of signal or other prob-
lem tells which stage or interstage coupling is
at fault. From there it's usually a simple matter
to measure voltages and find the component
that is faulty.
Heathkit IT-12 Features:
The Heathkit IT-12 has a few additional fea-
tures that add to its capability. External termi-
nals are available for the speaker and the pri-
mary output transformer. Front panel switches
isolate the speaker and transformer so they can
the used for substitution in the device under
test. The speaker is handy for radios that don't
have built-in speakers.
A second feature that is very handy is the
NOISE feature. Sometimes components get
noisy and this can be very difficult to trace. Af-
ter you determine the stage where the noise is
originating using regular tracing techniques,
the DUT is turned off and the noise feature on
the IT-12 is turned on. This puts about 140
VDC (current limited) on the probe. When
touched to a component in the DUT a single
click is heard in the speaker of the IT-12. If the
HOM Heathkit of the Month #9 - IT-12 Signal Tracer
Copyright 2008 - 2010, R. Eckweiler & OCARC, Inc.!Page 1 of 3

component being probed is intermittent or
breaking down, the noise is continuous and
that component needs to be replaced. This
method can even detect cold soldered joints.
Heath Signal Tracer History:
To get an idea of what a valuable troubleshoot-
ing device the signal tracer is, Heathkit intro-
duced their first model, the T-1 in May of 1948,
and continued to manufacture updated models
until Heath went out of the kit business in the
early nineties.
Heathkit T-1:
The original Heathkit T-1 uses three octal
tubes: a 6SJ7 audio amplifier, a 6K6 audio out-
put and a 6X5 power supply rectifier. The RF
probe contains a crystal diode and connects to
the T-1 by banana plugs. The original T-1 cost
$19.50 and remained in production for eleven
months until April of 1949, when it was re-
placed by the T-2.
Ad for the Heathkit T-1 from an October
1948 Heath flyer
Heathkit T-2:
The T-2 is similar to the T-1 except it uses a
6SH7 audio amplifier tube and has a multi-
position switch that allows numerous different
matching impedances when using the speaker
output transformer externally. The cost re-
mained $19.50. It remained in production un-
til October of 1952 when it was replaced by the
T-3.
Heathkit T-3:
When Heathkit introduced the T-3 Signal
Tracer in 1952 they made many significant
changes that brought the tube count up to five.
The 6X5 remains as the power supply rectifier.
The rest of the tubes were changed to 12-volt
filament types. The 6K6 audio output tube was
changed to a 12A6 and the 6SH7 audio ampli-
fier to a 12SH7. The T-3 has separate inputs
(and probes) for audio and RF. The RF section
has an additional stage of gain using a 12C8
tube. On the T-1 and T-2 there is no visual indi-
cation of signal level; however they have jacks
where you may connect a VTVM. This changed
on the T-3 with the addition of a 1629 "Magic
Eye" tube. The 1629 was used in the WW-II
Command Set transmitters and was very com-
mon and inexpensive on the surplus market at
that time. The eye-tube provides indication of
relative signal strength. The T-3 also has an
unusual feature. It can measure the power con-
sumption of the DUT. The device is plugged
into a special AC outlet on the T-3 and turned
on. Once the device warms up, a control on the
T-3 is adjusted until the eye just closes but does
not overlap. The power consumption of the
DUT can then be read on the scale of the con-
trol. The 12C8 RF amplifier doubles as a diode
for the wattmeter function. The added features
of the T-3 raised its original price to $22.50.
The T-3 remained in production until the end
of 1957 when it was replaced by the T-4.
Heathkit T-4:
The T-4 was a totally new design. The price of
the T-4 dropped back below $20 and it re-
mained in production until 1962 when it was
given a new cabinet color scheme and renamed
the IT-12. Other than the new case the two cir-
cuits are identical.
Heathkit IT-5283:
In Early 1978 Heathkit stopped production of
the IT-12. In its place they introduced a line of
five solid-state low-cost test equipment pieces
that run on batteries or a common power sup-
ply. This is the 5280 line and includes the IG-
5280 RF Oscillator, IB-5281 RCL Bridge, IG-
Heathkit of the Month #9 - IT-12 Signal TracerHOM
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5282 Audio Oscillator, IT-5283 Signal Tracer,
and IM-5284 Multimeter. When Heathkit went
out of the kit business only two of these kits
were still being made, the audio oscillator and,
showing its versatility, the signal tracer. Near
the end the IT-5283 sold for $59.95. up some
from its original $42.95 price.
The Heathkit IT-12:
The IT-12, and its electrically identical prede-
cessor the T-4, was the first in the signal tracer
line to use miniature seven and nine pin tubes
(except for the 1629 eye-tube). A 50 ma sele-
nium solid-state rectifier replaces the rectifier
tube. The IT-12 uses a single probe with an RF
- AF switch. As in the T-3, 12-volt filament
tubes are used. This is driven by the 1629 hav-
ing a 12-vollt filament. The IT-12 uses three
tubes, the 1629, a 12AX7 dual triode that per-
forms as two stages of cascaded audio amplifi-
cation, and a 12CA5 audio output tube.
Front panel controls are few and simple. A two
position rotary switch selects OFF/ON. In the
OFF position the primary of the output trans-
former is isolated from the power supply so the
transformer may be used as a substitute trans-
former. (Many older consumer devices have
their speaker and occasionally their output
transformer mounted to the cabinet so when
the chassis is removed from the case they are
not present on the chassis). A LEVEL control
adjusts the gain of the audio amplifier. A SPKR
slide switch disconnects the speaker from the
internal transformer so it can be used as a sub-
stitute speaker. A NOISE slide switch places
DC voltage on the Probe tip (Audio Position
only) for checking components. Another slide
switch on the probe, AUDIO/R.F. switches in
or out the crystal detector circuit in the probe.
Five binding posts are located on the front
panel. Two connect directly to the speaker, and
three, marked B+ CT and P, connect to the
three primary leads of the audio output trans-
former. Other front panel items are the 3-1/2
inch speaker and "Magic Eye" display tube.
The IT-12 may be used with transistor circuits
with one warning. The noise circuit can dam-
age solid state and low-voltage components if
used indiscriminately. However many compo-
nents may be checked if one end is discon-
nected from the circuitry prior to the check.
The manual gives recommendations for doing
this.
The IT-12 originally sold for $19.95. When dis-
continued at the end of 1977 the price had risen
to $34.95.
Summary:
This is a very versatile piece of test equipment
for people troubleshooting receivers and audio
electronics. Long after Heathkit became his-
tory, its simple signal tracers remain a common
tool for repairing communications receivers
and audio circuits and devices.
73, from AF6C
Remember if you come across any old
Heathkit Manuals or Catalogs that you do
not need, please pass them along to me.
Thanks - AF6C
This article originally appeared in the Octo-
ber 2008 issue of RF, the newsletter of the
Orange County Amateur Radio Club -
W6ZE.
HOM Heathkit of the Month #9 - IT-12 Signal Tracer
Copyright 2008 - 2010, R. Eckweiler & OCARC, Inc.!Page 3 of 3