Heathkit of the Month:
by Bob Eckweiler, AF6C
The HD-1250
HOM Heathkit of the Month #7 - HD-1250 Solid State Dip Meter
Solid-State Dip Meter
(and Its Predecessors)
the GD-1. The GD-1 is a handheld unit that
plugs into an AC wall socket. The front of the
unit has a meter, a phone plug and a SENSitiv-
ity control with an OFF/ON switch. Also on
the front is a scale for the capacitor which is
turned by a thumb knob that extends out of the
case on each side. On the back side is a switch
that selects either OSCILLATOR (GDO)
mode or DIODE (absorption wave meter AWM) mode.
Introduction:
The Dip Meter, known as the Grid Dip Oscillator (GDO) in the days of vacuum tubes, has
always been a practical piece of test equipment
for the ham who builds, tinkers or constructs
antennas. With such popularity it is a product
Heathkit produced in various forms over a long
part of the company’s existence.
The GD-1 was short-lived, being replace by an
improved GD-1A in 1953, and then the GD-1B
later that year, each offering circuit improvements. All three versions are very close in appearance and cover the same frequency range
of 2 MHz to 250 MHz using five plug-in coils.
Later in the production Heath offered the 341A low frequency coil set. The set contains two
additional coils to cover the lower frequencies
of 350 KHz to 2 MHz. The 341-A includes a
calibration chart that relies on the “Logging”
scale of the GD-1 series tuning knob. In 1956
the GD-1B sold for $19.50 (the same price as
the original GD-1 cost in 1952) and the LF 341A coil set sold for an additional $3.00. A European version of the GD-1, the GD-1U was also
produced; it was designed to run off of 220
VAC. The GD-1B remained in production until
1960, when the growth of solid-state devices
necessitated a design change.
The Heathkit HM-10 Tunnel Dipper:
As a replacement for the GD-1 series, Heath
came out with the HM-10 Tunnel Dipper in
1961, followed shortly in 1962 by the HM-10A.
The Tunnel Dipper uses a then revolutionary
new semiconductor device called the tunnel diode. This is a specially designed diode that has,
over a narrow range, a negative resistance.
That is, over part of the diode’s range, as the
current increases the voltage across the diode
decreases. This phenomenon allows the diode
to act as an oscillator when in the proper circuit. The tunnel diode oscillator is followed by
The Heathkit GD-1/1A/1B Grid Dip Meter:
The first Heath Grid Meter appeared in the October 1952 Heathkit Flyer and was designated
Copyright 2008 - 2010, R. Eckweiler & OCARC, Inc.! Page 1 of 3
three stages of transistor amplifiers to drive a
0–1 milliampere meter. The tunnel diode has
one problem; being sensitive to temperature, it
won’t operate in cold temperatures below
Heathkit of the Month #7 - HD-1250 Solid State Dip Meter HOM
freezing. This is a minor disadvantage because
the Tunnel Dipper runs off a single AA battery
and you aren’t tethered to AC power when
working on outside antennas and feedlines.
The HM-10A covers 3 to 260 MHz using six
color-coded coils. The tuning capacitor has a
large vernier drum dial offering long dial scales
for reading the operating frequency. The HM10A case is made of rugged extruded aluminum
and has a snap-on cover that protects the meter
and controls, and also houses the six coils. The
three position slide switch selects OFF, OSCillator or DIODE. The DIODE position allows
HM-10 to be used as an absorption wave meter
utilizing the meter for indication. This is the
only Heath dip meter that doesn’t have a headphone jack, so you can’t listen to a modulated
signal. In 1961 the Tunnel Dipper mail order
price was $34.95. It disappeared from the
Heathkit catalog around 1970. The HM-10A
Tunnel Dipper was replaced in 1975 by the HD1250 Solid-State Dip Meter.
The Heathkit HD-1250 Solid-State Dip Meter:
The last of the Heathkit dip meters is the HD-
1250. It was manufactured from 1975 through
1991. The HD-1250 uses a Motorola MRF502
high-frequency transistor for the oscillator, and
an RCA 40673 MOSFET and two hot-carrier
diodes for the detector. It has a phone jack for
earphones; an item missing in from the Tunnel
Dipper. The HD-1250 runs off a standard
NEMA 1604 9-volt battery. It covers 1.6 MHz
to 250 MHz utilizing seven color-coded plug-in
coils. The two pound dip meter measures 2” x
2-5/16” x 5-7/8” H x W x L excluding the plugin coil.
In the oscillator mode the oscillator transistor
runs as a Colpitts oscillator using a split tuning
capacitor operated by a large circular thumb
knob that protrudes through a slot on each side
of the case, similar to the GD-1 series. Two
plastic windows allow viewing the seven colorcoded scales of the dial on the knob, corresponding to the seven coils; an eighth Logging
scale is marked on the dial. A smaller OSCillator LEVEL adjusting knob also protrudes
through the right side of the case. This control
sets the voltage for the oscillator transistor and
thus the strength of oscillation.
To operate the dip meter in the absorption
wave meter mode you don’t need to throw a
switch as in the earlier units. Instead you decrease the OSC LEVEL until the meter reads
zero. At that point the oscillator is not oscillating but acts as a Q-multiplier for the tuned coil
and capacitor. In both oscillator and wave
mode, the MOSFET detector is directly coupled
to the tuned circuit. This is practical because of
the extremely high impedance of the MOSFET
transistor. The detector circuit amplifies the
voltage on the tuned circuit and demodulates
any AM modulation. The voltage is then fed either to the meter or to the headphone jack.
Page 2 of 3 Copyright 2008 - 2010, R. Eckweiler & OCARC, Inc.
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