HOM rev. new
Heath of the Month #83 - HG-10(B) Variable Frequency Oscillator
Heathkit of the Month #83:
by Bob Eckweiler, AF6C
AMATEUR RADIO - SWL
Heathkit HG-10 / HG-10B
Variable Frequency Oscillator (VFO)
Introduction:
Last October the Heathkit VF-1 VFO was
covered (HotM #79). This VFO, originally
built and styled for the AT-1 transmitter,
also worked with the DX-20, DX-35 and DX40 ham transmitters. In 1960 Heathkit replaced the DX-40 with the DX-60. Besides
some technical changes, the DX-60’s styling
was modernized. With minor changes the
VF-1 would work with the DX-60 though
they clashed in appearance. VFOs were very
popular at the time since novice class licensees were required to be crystal controlled (“rock-bound”) until they upgraded to
the general class license, at which time they
could use a VFO to easily change the transmitter frequency. In 1972 the rules changed
due mostly to technological advances and the
crystal controlled requirement was dropped.
The popularity of the DX-60, and of VFOs in
general, influenced Heathkit to design a
completely new VFO for the DX-60. The result was the Heathkit HG-10 (Figure 1), and
it was a marvel of springs, dial string and
gears. The VF-1 sold for $19.95, but the HG10 was released in 1961 with a price tag of
$34.95. The HG-10 was produced for seven
years before, in 1966, it was replaced with
the HG-10B (Figure 2).
The HG-10 VFO:
The HG-10 includes many changes over the
older Heathkit VF-1 besides styling. The older VF-1 covers the 160-meter band as well as
the (then) amateur 11-meter band. These
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Figure 1: Heathkit HG-10 VFO
Here is a link to the index of Heathkit of the
Month (HotM) articles:
http://www.w6ze.org/Heathkit/Heathkit_Index.html
HG-10 / HG-10B FREQUENCIES
Freq. Range 1:
Band (M) Band (mc) VFO Freq. (mc) ×
80 3.500 - 4.000 3.500 - 4.000 1
Freq. Range 2:
Band (M) Band (mc) VFO Freq. (mc) ×
40 7.000 - 7.425 7.000 - 7.425 1
20 14.000 - 14.850 7.000 - 7.425 2
15 21.000 - 22.275 7.000 - 7.425 3
10 28.000 - 29.700 7.000 - 7.425 4
Freq. Range 3:
Band (M) Band (mc) VFO Freq. (mc) ×
6 50.000 - 54.000 8.333 - 9.000 6
Freq. Range 4:
Band (M) Band (mc) VFO Freq. (mc) ×
2 144.000 - 148.000 8.000 - 8.222 18
Notes:
1. × is the multiplication factor that occurs in the transmitter.
Table I - HG-10(B) Band Operation
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Heath of the Month #83 - HG-10(B) Variable Frequency Oscillator
bands are both missing from the HG-10. Instead, the six and two-meter bands have been
added. The older VF-1 has three individually
calibrated frequency ranges while the HG-10
has four. These are shown in table I.
Frequency is read on a slide-rule dial. The
tubular dial drum has a separate scale for
each of the seven bands; the illuminated
drum rotates with the band switch so only
the scale for the band in use is visible. The
HG-10 dial window is about 5-1/2” wide and
the actual marked scale measures 4-5/8” (for
7.0 to 7.425 mc). Since 40-meters through 10
meters all use the same basic 7.000 - 7.425
frequency range, the length of the in-band
portion of the scale is different for each band.
The 10 meter band uses the full scale length,
while 40-meters uses 70%, 20-meters 40%
and 15-meters 35%. The other three bands,
since they each have a custom frequency
range, use 100% of the scale.
The VFO tuning uses a 28:1 vernier gear drive.
The gears are anti-backlash. Though not complicated to build, due mostly to Heathkit’s
ability to produce clear assembly manuals,
when you were done you might feel like you
could fix your car’s transmission. Specifications for the HG-10 are given in table II.
The B+ power requirements for the HG-10
are a little more complicated than the speci-
fications make them out to be. If you connect
the HG-10 to 108 VDC B+ voltage you will
have a problem. The 0B2 voltage regulator
tube regulates at 108 V and it is connected to
the B+ input through a 10K resistor. That
resistor is going to drop some voltage and the
0B2 will not fire and thus not regulate. The
DX-60/(A)/(B) and HW-16 provide about 300 350 VDC to their rear connector where the
HG-10 is designed to plug in. With the internal 10K resistor these voltages will fire the
0B2 and keep the voltage at 108 VDC while
drawing around 19 to 25 MA from the host.
The older DX-40 provides 600 VDC to the
rear connector, but it has a 15K 10W resistor
in series with the wire to the connector. To
use the DX-40 with the 15K resistor (IN
THE DX-40) should be replaced with a 10K
10W resistor. The 10K 10W resistor in the
HG-10 also needs to remain. The HG-10(B)
manual presents a table that specifies the
resistor size you should use in place of the
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HG-10 SPECIFICATIONS
(from Heathkit Ass’y Manual)
Output Frequencies: 3500 - 4000, 7000 - 7425.
(kilocycles) 8333 - 9000, 8000 - 8222.
Calibrated Bands: 80-40-20-15-10-6-2 M
RF Output: > 5 volts RMS (open ckt.)
Load Impedance: > 50 KΩ
Tube Complement: 1/2 6CH8 Oscillator (P)
1/2 6CH8 Cath. Follower (T)
0B2 Voltage Regulator
Power Requirements: 6.3 V AC at 0.75A
108 V DC at 25 mA (See text)
Cabinet Size: 9-3/8” W x 6-1/2” H x 9-1/8” D
Net Weight: 9-1/2 lbs.
Table II - HG-10 / HG-10B Specifications
Figure 2: The Later Heathkit HG-10B VFO
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Heath of the Month #83 - HG-10(B) Variable Frequency Oscillator
10K 10W resistor in the HG-10 for other B+
voltages. This graph is shown in Figure 3.
The HG-10B VFO:!
In late 1966 Heathkit update the HG-10 to
the HG-10B. Why there was no HG-10A is a
mystery. However, the HG-10B came out
around the time the DX-60A was replaced
with the DX-60B, so maybe Heathkit wanted
people to think that the new VFO was designed as a match to the DX-60B. And, while
it is true the HG-10B has at least one minor
circuit modification, most of the changes
were paint and styling changes to match the
new DX-60B. Most obvious is the paint
which went from smooth to textured, and the
colors that changed slightly; both features
that also changed in the upgrade to the
DX-60B.
The HG-10B Controls:!
The layout of the HG-10(B) controls is simple
and straightforward. Table III lists the front
panel controls and the rear panel connections.
The HG-10(B) circuit:!
The HG-10 and HG-10B circuits are nearly
identical. Any differences will be noted during this discussion. the HG-10 VFO may be
broken down into the following parts: Power,
oscillator, buffer and keying. Figure 9 shows
the schematic of the original HG-10 VFO.
Power Supply:!
Power is supplied from an external source,
usually the transmitter, though any external
supply above about 140 volts will work with
the proper dropping resistor as discussed
earlier. Regulation is provided by a 0B2 gas
tube. The 0B2 requires a minimum of 133
VDC to assure the gas will ionize and begin
to conduct. Once the gas fires the voltage
across the tube will remain at a nominal
voltage of 108 volts as long as the current
through the tube remains between 5 and 30
ma. The actual voltage varies by tube and
manufacturer, but is between 101 and 114
volts. Actual regulation is specified as 1 volt
average and 4 volts maximum over the 5 to
30 ma current range. When the tube fires
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Figure 3: Chart from the HG-10 manual showing the proper dropping resistor R11 value for different B+ voltages.
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Heath of the Month #83 - HG-10(B) Variable Frequency Oscillator
you can see a soft blue-violet glow within the
tube. If you are having problems like the
VFO changing frequency during keying,
check that the tube continues to glow
throughout the cycle; some change in intensity will occur.
Oscillator:!
Like the older VF-1, the HB-10 uses a pentode
tube in an electron coupled Clapp oscillator
circuit. The Clapp oscillator is a variant of the
Colpitts oscillator with an added capacitance
in series with the coil CC. This capacitance is
usually made variable and adjusts the frequency. Figure 4 shows the basic circuit. C1
and C2 are often identical in value and made
up of multiple capacitors in parallel. These
capacitors have different temperature coefficients and are chosen to keep the oscillator
from drifting over temperature variations. C3
is also often made up of multiple capacitors,
trimmers for setting the oscillator calibration
as well as fixed capacitors for padding and
temperature compensation.
Variable capacitors C1 and C2 are the twosection main FREQUENCY control. Each sec-
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HG-10 / HG - 10B
Controls and Connections
Top half of front panel (centered):
Slide-Rule Dial - (Only selected band’s scale shows)
(Spacing: [minor tick], [mid tick], [major tick] - fig 5)
80 METERS: 3.5, 3.6, 3.7, 3.8, 3.9, 4.0 (mc)
Spacing: 10 kc, 50 kc, 100 kc.
40 METERS: 7.0, 7.1, 7.2, 7.3, 7.4 (mc)
Spacing: 10 kc, 50 kc, 100 kc.
20 METERS: 14.0, 14.1, 14.2, …, 14.7, 14.8 (mc)
Spacing: 20 kc, (none), 100 kc.
15 METERS: 21.0, 21.1, 21.2, …, 22.1, 22.2 (mc)
Spacing: 25 kc, 50 kc, 100 kc.
10 METERS: 28.0, 28.5, 29.0, 29.5 (mc)
Spacing: 50 kc, 100 kc, 500 kc
(ticks continue to 29.7)
6 METERS: 50, 51, 52. 53, 54
Spacing: 100 kc, 500 kc, 1 mc.
2 METERS: 144, 145, 146, 147,148
Spacing: 100 kc, 500 kc, 1 mc.
Bottom half of front panel (L to R):
FUNCTION: (rotary switch - 3 position)
STandBY, OPeRate, SPOT
FREQUENCY (dual 5 - 25 µµf variable capacitor)
28 : 1 geared vernier drive
BAND: (rotary switch - 7 position)
80, 40, 20, 15, 10, 6, 2
Rear panel skirt (L to R from rear):
RF OUTPUT (RCA Phono jack)
KEY (1/4” phone jack)
POWER (Cable with strain relief)
8-pin octal plug at far end of cable:
Pin 1: Ground (black)
Pin 2: Filament - 6.3 VAC (white)
Pin 4: B+ (red)
Pin 8: Key line (green)
Table III
Figure 4: Basic
Clapp oscillator
circuit.
Figure 5: Typical slide rule scale (10-Meters selected) identifying tick mark types for Table III
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Heath of the Month #83 - HG-10(B) Variable Frequency Oscillator
tion has a capacitance of 5 to 25 µµf; C2 is in
parallel with C1 when on the 80-meter band
and not used on the other bands.
Table IV shows the frequency determining
capacitors that make up the four different
frequency ranges of the VFO. For frequency
stability C11, C13, C17, C22 and C23 are
N750 type that have a negative temperature
coefficient of 750 parts per million (PPM) per
degree Kelvin. This corrects drift from the
rest of the capacitors that either have positive or no temperature drift (NPO).
Feedback for the oscillator is from the cathode
of the pentode section of V1A. The RF choke
(RFC2) prevents the RF from being bypassed
to ground while R2 provides the DC path to
ground when the VFO is operating.
In an electron coupled oscillator the screen
provides voltage for the pentode and the RF
signal passes through the grounded suppressor grid to the plate. This circuit tends to isolate the plate load from the oscillator, providing more stability. The plate is untuned and
the RF is isolated from the B+ line by RFC1.
Interestingly, RFC1 is the only component
I’ve found that has changed value between
the original HG-10 and the HG-10B. It was
changed from 390 µH down to 28 µH. Why
this was done may have been for cost reduction? However, in 1973 Heathkit released
service bulletin HG-10B-1 which states:
Hum On 40 Meters
Replace RFC-1, 28uH [PN 45-18]
with a 350 uH choke coil, [PN
45-82]. This change will be put
into production soon.
Cathode Follower Buffer:!
V1B, the triode section of the 6CH8, is a
cathode follower. They have been covered
many times in HotM articles. It should suffice to say it provides isolation by having a
high input impedance and a low output impedance with a voltage gain of somewhat less
than one. R3 through R6 set the tube’s oper-
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HG-10 / HG-10B
Frequency Determining Components:
On all bands:
C20 and C221 act as CA
C21 and C231 act as CB
When on 80 Meters (3500 - 4000 kc):
L3 acts as LA
C15, C2 5, C7 6, C131 and C14 act as CC
C19 also affects the frequency 2
When on 40 thru 10 Meters (7000 - 7425 kc):
L2 acts as LA
C15, C6 6, C111 and C12 act as CC
C18 also affects the frequency 2
When on 6 Meters (8333 - 9000 kc):
L1 acts as LA
C15, C5 6, C9, and C10, act as CC
C16 and C171 also affect the frequency 2
C15 also affects the frequency 3
When on 2-Meters (8000 - 8222 kc):
L1 acts as LA
C15, C5 6, C9, C10, act as CC
C16 and C171 also affect the frequency 2
C3 6, C4 6, and C8 also affect the frequency 4
Notes:
1. These capacitors have a negative temperature coefficient (N750) for temperature compensation.
2. This capacitor is not shown in Figure 4. It is across
the series capacitors CA and CB.
3. This capacitor is in series with CC reducing its overall capacitance.
4. These capacitors are in series and parallel with C1,
the main tuning capacitor, reducing its overall tuning
range.
5. C1 and C2 are the dual-section main tuning capacitor.
6. These capacitors are trimmers for calibration.
Table IV
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Heath of the Month #83 - HG-10(B) Variable Frequency Oscillator
ating parameters. The RF output is taken
across R5 which sets the output impedance
as R6 is bypassed to RF by C31.
Keying Circuits:!
The DX-60 series and the HW-16 use gridblock keying, while the DX-40 uses cathode
keying. The HG-10(B) is designed to work
with either with some minor wiring changes.
The oscillator tube V1A can be prevented
from oscillating by R7, the large resistor in
the cathode circuit. This resistor raises the
cathode voltage enough to prevent oscillation; shorting it out will key on the oscillator.
The oscillator can also be prevented from oscillating by applying a negative voltage
across R9 to bias the tube off.
When using grid-block keying the keying
should be done at the transmitter. A low current, fairly high negative voltage (usually a
bit greater than 100 VDC) appears across
the open key. This negative voltage is applied to the necessary stages of the transmitter to cut them off; thus no signal appears at
the output of the transmitter. When the key
is closed the negative voltage is shorted to
ground and the stages operate normally.
This negative keying voltage appears on the
HG-10(B) VFO keying line that comes in via
the octal plug from pin-8. This voltage is
connected through an NE-2 neon bulb which
lights with the key up and drops the keying
voltage by 75 to 90 volts. With the FUNC-
TION switch in the OPR mode this lower
negative voltage appears across R22 and biases off the oscillator tube V1A. At the same
time the FUNCTION switch shorts R7 to
ground, bringing the oscillator cathode near
ground; now the only reason the oscillator is
not functioning is the negative voltage across
R9. When the key is closed in the transmitter
the negative voltage drops to zero, the neon
bulb goes out, and the voltage across R9
drops, causing the oscillator to start, and
output to appear at the RF OUTPUT jack.
The HG-10 may be converted to cathode keying by two simple changes in the HG-10B
circuitry as shown in the schematic. First,
move the green wire on terminal strip A from
terminal 2 to terminal 3. And second, remove
the wire between terminals 2 and 3 on the
KEY phone jack. The green wire mentioned
above is the keying lead from the octal plug.
When the preceding modifications are made
to the HG-10(B) it can be used with cathode
keying. The two key jacks are now in parallel
and a key may be used in either. With the
FUNCTION switch in the OPR position the
key is connected across R7, the large cathode
resistor that is keeping the oscillator biased
off. When the key is closed R7 becomes
shorted and the cathode voltage drops allowing the oscillator to start. Thus the oscillator
is keyed along with the transmitter.
When the FUNCTION switch is in the STBY
(standby) position the key jack and keying
lead are open and the oscillator is biased off;
when the FUNCTION switch is in the SPOT
position the key jack and keying lead are
open and a short appears across R7 causing
the oscillator to operate independently of the
transmitter keying. The oscillator can be
heard in the receiver and the receiver or VFO
may be adjusted to the other’s frequency.
Mechanical Design:
It would be amiss to discuss the HG-10(B)
without commenting on the mechanical layout. Like its predecessor the VF-1, the
HG-10(B) has heavy ceramic coils, a double
bearing tuning capacitor and rigid construction to improve stability, but is also has features that made up a part of Heathkit design
in the early sixties ham products. Someone on
the Heath design team must have been a me-
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Heath of the Month #83 - HG-10(B) Variable Frequency Oscillator
chanical engineer who loved gears and pulleys. Much of the ham gear of that period features a rotating drum that turns with the
band-switch, showing only the current band
to the user. They also feature a smooth geared
drive for the tuning capacitor. This multi-gear
tuning system mounts between the front panel and a sub-panel. Equipment of the day featuring such mechanical wonderment include
the MR-1 (“Comanche” mobile receiver), MT-1
(“Cheyenne” mobile transmitter), RX-1 (“Mohawk” HF receiver), TX-1 (“Apache” HF
transmitter), HR-20 (mobile SSB receiver),
HX-20 (mobile SSB transmitter), and possibly
others.
Figures 6 and 7 show the mechanical layout
of the HG-10(B) gear reduction for the FRE-
QUENCY tuning capacitor. The sub-panel
mounts behind the front panel
with the gear mounting plate
towards the front. The shaft
(#9) protrudes out the front and
holds the main tuning knob.
The pulley (#16) drives the dial
pointer across the slide-rule dial
via dial-cord and a bunch of
idler pulleys not shown in these
drawings.
Figure 8 shows the drum assembly. The pulley on the bandswitch (#29) connects to the pulley (#23) on the dial drum via
dial-string, causing it to rotate
to the correct dial scale as the
band-switch is turned.
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Figure 7: Drawing showing the HG-10 VFO capacitor.
The long shaft #3 mates with the coupling #10 of figure 6.
Figure 6: Drawing showing the
HG-10 VFO sub panel with gear
mounting plate and set of reduction
gears. The geared shaft #9 protrudes
through the front panel and holds the
FREQUENCY knob.
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Heath of the Month #83 - HG-10(B) Variable Frequency Oscillator
Proper alignment and lubrication is required
to achieve the best dial results. A little extra
time here pays off with smooth tuning.
Summary:
While the change from crystal to VFO operation for the novices occurred in 1972, a lot of
novice rigs were in use and many were still
being sold due to their low price compared
with a transmitter with built-a in VFO. The
more expensive radios used frequency synthesizers locked to a standard crystal providing
frequency control as well as crystal stability.
These radios were expensive and so VFOs
continued to sell. It wasn’t until older synthe-
sized radios began to come up on
the used market at reasonable
prices that the need for an external
VFO began to decline. The HG-10B
continued in production until 1976,
four years after the rules changed.
Many HG-10(B) VFOs are also used
on VHF. The oscillator has the stability to operate AM, and CW on 6
and 2-meters, and reports can be
found that claim they were useable
on 1-3/4 and even 3/4-meters on
AM, and CW.
General HotM Comments:
V-6 Voltmeter Restoration:
My V-6 voltmeter project is on hold
while I wait for a part. Can you
believe that a 0.01 µf 2KV axial
lead capacitor would be hard to
find? Mouser was out until just recently and so I have a few on the
way along with some other parts
for another Heathkit project. I was
able to find a 200K 1% precision
resistor that is part of the range
attenuator. The original was open.
Why not a Link to the HotM Index?:
While looking over some old emails I came
across correspondence from Dave, W6OVP
(Dave provided me with a lot of very useful
information for the HA-14 Compact KW Amplifier article for HotM #58). I don’t remember seeing this before, but Dave commented:
Suggest you could maybe put a link at the
bottom of EACH of your review pages to
steer viewers […] back to the 'Orange County
Club Heathkit Articles' […] link. Potential customers should not have to hunt for your
products. :-! .
Great idea Dave! Sorry it took me so long to
implement it. Starting with this article, I
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Figure 8: Dial drum assembly shown mounting to the
front sub-panel of figure 6.
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Heath of the Month #83 - HG-10(B) Variable Frequency Oscillator
will include the link to the Heathkit of the
Month index on the first page of each article.
From that index the articles can be accessed.
Adding a link to previous articles may be difficult to do for reasons I may explain in a future article.
What’s Up Next?:
With taxes due and the Baker to Vegas race
looming I may not get time to do an article
next month. However, I was talking with
Chuck Penson (author of those great Heathkit books, and he suggested I write on the O12 oscilloscope. I actually have one here that
is in need of restoring. I thought I’d do even
more and cover the whole ‘O-#’ oscilloscope
line, (there are twelve of them altogether)
spending more time on history and evolution
and less on circuit description. I was thinking of doing it in three parts, the O-1 to O-4.
the O-5 to O-8 and the O-9 to O-12. I was
able to obtain from Chuck a schematic of the
O-1, the first Heathkit. It’s missing some information but is reasonably complete. Unfortunately, it is not a good enough reproduction for an article, but the circuit is simple so
maybe I can redraw it if I can’t find a better
copy. I don’t think the manual (it was really
a schematic, parts list and some assembly
notes, I’ve been told, is available for sale.
The three parts won’t appear consecutively;
I’ll try to write on another kit or two in-between sections.
73, from AF6C
A ! Short
Heathkit VTVM Probes:
Over the years the probes that came with
Heathkit VTVMs changed. In older units
there were three connectors on the front
panel, Two banana jacks (red and black) for
AC and Ohms measurements with appropriate red and black test leads, and a phone
jack for a shielded cable with a black probe
holding a 1 meg Ω resistor that is in series
with the lead, for DC voltage measurements.
With the release in 1961 of the IM-11, which
replaced the V7-A, the three leads were combined into one that plugs into a phone jack.
The plug-in test lead is a single shielded cable that goes to a new test probe. On that
test probe is a switch that switches the 1
meg Ω resistor in for DC measurements and
out for AC and Ohms measurements. A black
test lead with an alligator clip comes out of
the phone plug separately and provides the
common lead.
Those probes get broken or lost, thus there
are numerous articles on replacing them.
Some suggest placing the switching of the 1
meg Ω resistor inside the VTVM. Don’t do
this. That resistor is located close to the test
point to isolate the cable capacitance from
the circuit under measurement. This capacitance can affect the circuit operation, even if
you’re measuring DC voltages.
73, from AF6C
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This article originally appeared in the
March 2018 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.
Be sure to update> Thanks - AF6C
A simple replacement probe for late VTVMs. Use a
simple slide switch & Belden 8899-BLK for clip lead.
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Copyright 2008 - 2018, R. Eckweiler & OCARC, Inc.
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Figure 9: Heathkit HG-10
Schematic
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