Heathkit VC-3 User manual

Heathkit of the Month:
by Bob Eckweiler, AF6C
Heathkit VC-3
Voltage Calibrator.
Introduction:
The August 2013 Heathkit of the Month (#51)
covered the IG-4505 Deluxe Oscilloscope Cali-
brator. It also briefly covered the VC-3 Voltage
Calibrator using information gleaned from cata-
logs. To my surprise Bill - K6WHP recently pre-
sented me with a VC-3 complete with manual.
One look at the unusual schematic and I knew it
deserved an article all its own.
The VC-3 (Figure 1), third in the Heathkit line
of oscilloscope voltage calibrators that were
first introduced in 1952, appeared in late 1956.
Figure 2 shows an ad for the Heathkit VC-3
from the 1958 catalog. It remained in produc-
tion through 1962 at a price of $12.50. In those
days the oscilloscope was the most prolific of
Heathkit’s products. From the O-1 oscilloscope
(Heathkit’s first electronic kit) introduced in
September of 1947 through the end of 1956
Heathkit sold 14 different models, the O-1
through O-11, the OL-1, the OM-1 and OM-2.
All had 5” CRTs except the OL-1 (3”), and all
had AC coupled amplifiers and multivibrator
sweep circuits. DC coupling and triggered
sweep were on the horizon for Heathkit, but
not yet incorporated. The first Heathkit oscillo-
scope featuring these improvements came in
the fall of 1958, the OP-1 Professional Oscillo-
scope Kit. It sold for $179.95, about 2-1/2 times
more than their previous top-of-the line
Heathkit oscilloscope.
The Heathkit VC-3:
Prior to the OP-1, most of the hobby-grade os-
cilloscopes on the market had at best a 1 volt
peak-to-peak test point that one could use to
approximate the voltage of a wave form. Since
the gains were mostly uncalibrated and the at-
tenuator steps a full decade apart, measuring
voltage was difficult without a way to compare
it with a known signal. The VC-3 provides the
ability to make that comparison easily.
The VC-3 puts out a rather impressive ap-
proximate 1 kc square wave that has an accu-
HOM rev. new Heathkit of the Month #64 - VC-3 Voltage Calibrator
Copyright 2015, R. Eckweiler & OCARC, Inc. Page 1 of 5
Figure 2: Heathkit VC-3 ad from the main 1958 Catalog
Figure 1: Heathkit VC-3 Voltage Calibrator

rate peak-to-peak voltage that can be set to one
of eight ranges: 30mV, 100mV, 300mV, 1.0V,
3.0V, 10V, 30V and 100V. Being a square wave
with flat tops and bottoms the calibration can
be more easily set than when using a sine wave.
The VC-3 also has an input to accept the signal
under test and an output that connects to the
oscilloscope input, so the user can switch be-
tween the signal being observed and the cali-
bration signal with the turn of a switch.
The layout of the Heathkit VC-3 front panel is
straightforward with only two switches and two
dual binding posts. On the left is a pair of five-
way binding posts marked SIGNAL, the upper
one being colored red and the lower one black.
On the right is a similar set of binding posts
marked OUTPUT. Dead-center on the front
panel is a nine-position rotary switch. The
switch positions are marked in clockwise order:
SIG[nal], 30, 100, 300, 1, 3, 10, 30, 100. The
second through fourth positions are bracketed
with the nomenclature M/VOLTS, and the last
five positions are bracketed VOLTS. PEAK TO
PEAK is marked below the switch. Below the
rotary switch is the power switch – a slide
switch marked OFF ON. Other nomenclature on
the front panel are: Heathkit VOLTAGE CALI-
BRATOR across the top, MODEL VC-3 at left
bottom and HEATH COMPANY BENTON HAR-
BOR, MICHIGAN on two lines at right bottom.
The VC-3 is housed in a 4–3/4 H x 7–3/8 W x
4–1/8 D gray cabinet and a dark gray front
panel with white nomenclature. The cabinet
(part# 90-39) is the same as used in many
other Heathkit models of the time including
the AM-2 SWR bridge and the QF-1 Q-
Multiplier.
Heathkit VC-3 Operation:
Operation is simple. Connect the VC-3 OUT-
PUT posts to the vertical input of the oscillo-
scope. Connect the signal to be viewed to the
INPUT posts of the VC-3. Place the PEAK TO
PEAK rotary switch in the SIG position and ad-
just the scope to view the signal as desired.
Next, switch the PEAK TO PEAK switch slowly
clockwise until the square wave vertical am-
plitude covers a reasonable amount of the
screen’s vertical real estate and adjust the
scope’s vertical gain a small amount as needed
to have the square wave cover a number of ver-
tical divisions on the scope graticule. You can
then easily calculate the vertical volts/division
at the current vertical gain setting. Now return
the PEAK TO PEAK switch to SIG, adjust the
vertical position as needed, but don’t touch the
vertical gain. Since you now know how many
volts each division is, you can measure the sig-
nal’s peak-to-peak amplitude.
VC-3 Limitations:
There is one caution Heathkit mentions in the
manual about using the VC-3. It involves using
the VC-3 inline with signals having sharp rise-
times or high frequencies over 100 kilocycles. If
you are measuring such signals then the signal
should be applied directly to the scope and not
through the VC-3. The necessitates swapping
the signal and VC-3 at the scope terminals as
needed to make a voltage measurement. The
reason is simple, the signal path through the
VC-3 involves wiring that adds about 35 µµf
(pF in today’s lingo) of shunt capacitance to the
signal and this will cause attenuation of higher
frequency components of complex waveforms.
Still, having to swap leads at the scope vertical
Heathkit of the Month #64 - VC-3 Voltage Calibrator HOM rev.new
Page 2 of 5 Copyright 2015, R. Eckweiler & OCARC, Inc.
Figure 3: VC-3 inside left, showing the vertical chassis,
transformer and three tubes (L to R) 6X4, 0A2 & 6AW8.

input is a small price to pay for being able to
make reasonable voltage measurements on an
otherwise uncalibrated piece of test equipment.
For audio and low frequency measurements the
oscilloscope can be left permanently connected
through the VC-3.
VC-3 Circuit Description:
The schematic diagram of the VC-3 is shown in
Figure 6. Probably what catches the eye first is
that the tubes are drawn upside down! In fact
looking a little closer the whole power supply is
wired to produce a negative voltage. The VC-3
contains three tubes, a 6X4 dual diode rectifier,
an 0A2 VR (voltage regulator) tube and a dual-
section 6AW8 triode - pentode tube. The first
two tubes function in the power supply while
the third, dual-section, tube functions as a 1
kilocycle, symmetrical, free-running, square
wave multivibrator. Output from the multivi-
brator goes through a switched precision volt-
age divider to produce proper calibrated volt-
age. Each of these three sections will be dis-
cussed separately.
The VC-3 Power Supply Circuit:
The power supply is transformer based, provid-
ing 6.3 VAC filament voltage to two of the tubes
and 400 VCT to the plates of the 6X4 rectifier,
producing about 230 VDC relative to the center-
tap of the transformer. The DC is filtered by a 20
uF electrolytic capacitor. The negative side of
the 230 volts goes to the the cathode of the 0A2
“cold-cathode” VR tube, and the positive side
goes to the plate of the 0A2 VR tube through a
dropping resistance of 3.4!K"(composed of two
6.8 K"2-watt resistors in parallel). “Cold-
cathode” refers to the fact the tube has no fila-
ment. Instead the tube contains an inert gas
mixture that conducts at a given voltage (150
volts nominal for the 0A2); as it conducts it
draws more or less current through the 3.4K"
resistance keeping the voltage at the anode con-
stant with respect to the cathode. A jumper be-
tween pins is used to disconnect the circuit
should the tube be removed, preventing higher
and possibly damaging voltages appearing.
Placing a large capacitance across a VR tube
often leads to instability, and the 20 µF electro-
lytic is 200 times higher than the recom-
mended maximum capacitance, so a 100 "re-
sistor is placed between the two for isolation.
At first it was baffling as to why the resistor and
capacitor are where they are. A simple C-R-C
filter before the 3.4 K"dropping resistance
would be a better choice - as the 100 "resistor
reduces the regulation of the VR tube. Opening
the unit revealed the answer - cost. The two 20
µF electrolytic capacitors are in the same pack-
age and share a common negative terminal.
Hence the circuit was designed to fit this crite-
ria. Using two separate capacitors would add
significantly to the price of a kit that sold for
$12.50, especially since the capacitor used was
one common in many kits and bought in large
quantities, likely at a good price break.
The output of the power supply is across the
second 20 µF capacitor. The positive side is
grounded and the negative side is passed
through the jumper in the VR tube and on to
the cathodes of the dual section multivibrator
tube. Notice that neither side of the filament
winding is grounded; instead one side is con-
nected to the negative side of the power supply.
This is done so the voltage between the cathode
HOM rev. new Heathkit of the Month #64 - VC-3 Voltage Calibrator
Copyright 2015, R. Eckweiler & OCARC, Inc. Page 3 of 5
Figure 4: VC-3 inside right; the dual electrolytic capacitor
is lower right, cal. pot lower center and rotary switch with
precision resistors center towards left.

and filament will not exceed the filament to
cathode maximum rating of 100 VDC.
The VC-3 Multivibrator Circuit:
The two 6AW8 tube sections are wired as a free
running multivibrator. The triode section acting
as one half the multivibrator and the pentode
section as the other. The pentode section is
wired as a triode with the screen-grid acting as
the multivibrator plate with the actual plate iso-
lated as in an electron-coupled oscillator. This
isolation significantly reduces the influence of
plate load changes on the frequency and wave-
form of the multivibrator. The frequency and
duty cycle of the multivibrator are determined
by the 470 K"and 4.7 K"plate resistors and
the 0.0013 and 0.003 µF mica coupling capaci-
tors. These values were selected for a nominal
frequency of 1,000 cps and 50% duty factor.
They vary significantly from each other due to
the characteristics of the different tube sections.
VC-3 Output Switching Circuit:
The plate load of the pentode section of the
6AW8 is provided by a 20 K"precision voltage
divider in series with an internal 10 K"vari-
able calibration resistor. The positive side of
the power supply is at ground potential, so one
of the output terminals may be also. This is the
reason that the power supply was designed
with the positive side connected to ground.
The actual voltage at the plate (pin 9) of the
pentode section varies from zero when the tube
is cut off to approximately 135 volts negative
when the tube is conducting. The calibration
resistor connected between the voltage-divider
chain and the plate is adjusted so the top of the
chain is at -100!VDC when the tube is conduct-
ing. The resistor chain is composed of eight 1%
precision resistors that sum to 20.0!K". The
resistor values and corresponding output volt-
ages are given in Table I.
One obvious idiosyncrasy of the output signal is
that it is a negative going square wave. This, how-
ever, should make no difference for AC coupled
oscilloscope amplifiers, and eliminates capacitive
coupling within the VC-3 that would only distort
the waveform and introduce inaccuracies.
Since the impedance of the precision voltage di-
vider is low compared to the one-megohm imped-
ance of most Heathkit scopes of the day, negligible
attenuation occurs. When the switch is in the SIG
Heathkit of the Month #64 - VC-3 Voltage Calibrator HOM rev.new
Page 4 of 5 Copyright 2015, R. Eckweiler & OCARC, Inc.
Switch Resistor
Position Resistor Sum Voltage
1 n/a n/a (SIG)
2 6Ω6Ω0.03 V
3 14Ω20Ω0.1 V
4 40Ω60Ω0.3 V
5 140Ω200Ω1.0 V
6 400Ω600Ω3.0 V
7 1.4 KΩ2.0 KΩ10 V
8 4.0 KΩ6.0 KΩ30 V
9 14 KΩ20 KΩ100V
•Column Two is the values of the resistor chain. If you
sum them all they add up to 20.0 KΩ
•Column Three is the sum of the resistors up to and
including the switch position of the row.
•Column Four is the output voltage determined by:
((value in column 3) / 20 KΩ) * 100 V.
Table I - VC-3 Precision Voltage Divider Output Chain
Figure 5: Modern oscilloscope screen showing the VC-3
output set to 3 V. (1 v/div Vert., 500 µS/div Horiz.)

position, plate voltage is removed from the 6AW8
preventing calibrator noise on the output.
Summary:
Perusing the manual, the kit appears easy to as-
semble. The actual step-by-step instructions go
from the bottom of page 6 to about an inch onto
page 12, and two of those pages are full-page picto-
rials. Also included on those pages are seven fig-
ures and a detail drawing.
Looking at this manual, earlier manuals and
more recent manuals it is evident the manuals,
for which Heathkit is so well known, have
evolved substantially over the years. This might
make a good topic for a future Heathkit article.
Next month is the dreaded April issue. I have a
few ideas but have not chosen one. Between now
and then I’ll be supporting the Baker-to-Vegas
race and completing some family responsibili-
ties that are time critical, (oh and taxes too.)
I’ve gotten numerous requests to write on the
HW-12 & 12A series. If anyone has a PDF copy
of the HW-12A, 22, 22A 32 or 32A manual
please contact me and I’ll try cover the series in
a future Heathkit article; I have a copy of the
HW-12 (non-A) manual.
I’d like to especially thank Bill - K6WHP for
providing the VC-3 for this article, with a man-
ual no less!
73, from AF6C
HOM rev. new Heathkit of the Month #64 - VC-3 Voltage Calibrator
Copyright 2015, R. Eckweiler & OCARC, Inc. Page 5 of 5
Figure 6: Heathkit
VC-3 Schematic
This article originally appeared in the March
2015 issue of RF, the newsletter of the
Orange County Amateur Radio Club - W6ZE.
Remember, if you are getting rid of any old
Heathkit Manuals or Catalogs, please pass
them along to me for my research.
Thanks - AF6C