RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling
2 MB. Its primary purpose was to provide me with ready access to commonly needed
formulas and reference material while performing my work as an RF system and circuit
design engineer. The World Wide Web (Internet) was largely an unknown entity at
the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps
while tying up your telephone line, and a nice lady's voice announced "You've Got
Mail" when a new message arrived...
All trademarks, copyrights, patents, and other rights of ownership to images
and text used on the RF Cafe website are hereby acknowledged.
vintage Heathkit IM−2420, 512 MHz Benchtop Portable Frequency Counter kit is
one of the latest unbuilt
Heathkit kits which appeared on eBay. I have been saving the images in order
to preserve the history (see image below). The constantly growing list is at the
One of the first instances I could find for IM−2420 being offered for sale was
in the Winter 1981 Heathkit catalog, at a cost of $239.95 ($754.51 in 2021 money
BLS) as a kit or $299.95 ($943.18 in 2021) assembled and tested. A comparable
benchtop frequency counter today is the RISEPRO
Benchtop Frequency Counter for 10 Hz-2.4 GHz, at a cost just south
"UHF capability up to 512 MHz, low frequency capacity down to 5 Hz
and a budget-pleasing price are all yours when you choose the newest Heathkit
Digital Frequency Counter. Service technicians and do-it-yourselfers will find its
extended frequency range idea for servicing UHF 2-way radios now used by police
and fire departments - as well as most other communications equipment.
Our engineers have designed more features and added capabilities into
this laboratory quality Heathkit instrument. For example, the IM−2420 can do more
than measure the frequencies of input signals - its period function can give cycle
time in seconds, while the frequency ratio function provides the ratio between two
Make more accurate measurements: A Standby power switch can
keep the crystal oven warm, giving you maximum frequency accuracy as soon as the
IM−2420 is turned on. Proportionally-controlled crystal oven keeps the internal
time base extremely accurate - within 0.1 part per million (ppm) - over a wide temperature
range. The crystal-controlled time base provides excellent long-term stability,
with drift controlled to less than 1 ppm per year. And the external time base jack
allows the use of an external time base signal.
Four gate times and a large, 0.43 (1.09 cm)-high, 8-digit LED display provide
the resolution you need to measure UHF signals. The pivoting stand places the IM-2420
at a comfortable, convenient viewing angle.
The IM−2420s excellent 4-15 mV typical sensitivity, over the
entire 5-Hz-512 MHz range, allows counting of weak signals. Trigger level control
assures stable counting when noise is present, and provides more accurate measurement
of complicated waveforms. Effective limiting circuits at both inputs (5 Hz-50 MHz
and 40 MHz-512 MHz) make sure counting is stable, even when the input signal level
varies by as much as 60 dB.
Measure signals with a direct connection. Or, for frequency
counts without a direct connection, order the SMA-2400-1 Swiveling Telescopic Antenna
Extra protection is built into the IM−2420, too. Its rugged, compact metal cabinet
provides improved shielding from radio-frequency interference (RFI). Full voltage
protection assures worry-free counting.
Extra-wide frequency range for maintenance of UHF, VHF radios
Crystal-controlled time base for excellent long-term stability and accuracy
Features four gate times and 8-digit resolution to give you more precise readings
Provides you with two separate inputs and trigger level control for added versatility
Kitbuilding is easy: the IM−2420 takes as little as four evenings to build, with
a fully detailed Heathkit manual to guide you along the way, step-by-step. ICs are
standard and easily available, for faster servicing.
Heathkit IM-2420 512 MHz Frequency Counter Kit Contents
Heathkit IM-2420 512 MHz Frequency Counter Kit Parts
Heathkit IM-2420 512 MHz Frequency Counter Printed Circuit
Heathkit Winter 1981 Catalog Cover
Heathkit IM-2420 512 MHz Frequency Counter Components
Heathkit IM-2420 512 MHz Frequency Counter Rear Panel
Heathkit IM-2420 512 MHz Frequency Counter Schematic
Heathkit products were well known for the completeness of its instruction manuals,
with clearly illustrated instructions. During the writing and editing process, Heathkit
employees were given pre-production kits to take home and build, while annotating
any difficulties or errors encountered. Doing so helped minimize the situation where
the writer inadvertently assumes his own familiarity with the process is shared
by the customer. Having built a few Heathkit products myself in the 1970s when I
did not have a lot of experience with electronics assembly, I can attest to the
user friendliness of the instructions. Heathkit still sells many of its vintage
manuals for around $15, which is what you would pay on eBay. You can download the
IM−2420 manual here.