Kenwood SM-220 Station Monitor (1980–1988)

A scope, a band-scope, and a two-tone generator in one box - the accessory that turned a Kenwood transceiver into a full station.

Overview

The SM-220 is a station monitor, a multi-function monitor scope that Kenwood built to sit alongside its HF rigs and show what your signal is actually doing. According to Universal Radio's product description, it works as a band-scope (with the optional BS-8 pan display), a monitor, and a tone generator. eHam's write-up calls it a multifunction monitor oscilloscope and two-tone audio signal generator. Both descriptions are accurate. Neither says how much is packed into one chassis.

Instead of a rack of separate test equipment, the SM-220 combines a monitor-scope, an oscilloscope, and a two-tone audio generator. The monitor-scope section covers 1.8 to 150 MHz, so you can watch your transmitted waveform across the HF bands and well into VHF. The oscilloscope section has a 10 MHz bandwidth, enough for general transceiver measurements and waveform observation. The two-tone generator supplies the audio signal you need to tune a linear amplifier or check SSB transmitter linearity. Four sweep ranges are available.

Owner accounts keep coming back to the CRT. The SM-220 uses a B-31 blue-green phosphor tube, and the consensus is that it's pleasant to look at. It's bright, it has good contrast, and it's easier on the eyes during long sessions than some of the harsher phosphors in general-purpose scopes. That counts when you're staring at it for hours in a contest or a long tuning session.

Kenwood built the SM-220 from 1980 to 1988, according to Technotronic Dimensions' repair page for the series. It was discontinued and replaced by the SM-230. It was positioned as the matching station monitor for Kenwood's HF transceiver line.

Key Features

1.8–150 MHz Transmission Monitor

The monitor-scope section covers 1.8 to 150 MHz. That takes in every HF band Kenwood's transceivers covered, plus 6 meters and 2 meters. Both RigReference and KE9PQ's documentation confirm the range. Most operators will use the HF coverage, watching their signal on 80 through 10 meters, but the VHF reach keeps the SM-220 useful if you also run a 2-meter rig.

The monitor displays a variety of test signals, including AF or IF signals from the receiver, as well as transmit envelope and trapezoid displays. The trapezoid display is the standard method for checking linear amplifier performance. With it built into the station monitor, you don't need a separate scope to verify your amp is behaving.

BS-8 Pan Display Option (±100 kHz Bandscope)

The BS-8 pan display adapter turns the SM-220 into a bandscope with ±100 kHz coverage. That's the feature that makes the unit useful during operation and not only during setup. With the BS-8 installed, you see signals across a 200 kHz window centered on your receive frequency. It's the classic pan-adapter view, so you can spot activity without tuning blindly across the band.

The BS-8 is an optional plug-in, not standard equipment, and it was designed specifically for the TS-820 series transceivers. If you're looking at an SM-220 on the used market, check whether the BS-8 is included, since it changes both the utility and the value of the unit. The fact sheet gives a price range of $249.99 to $375.00 and doesn't say whether the BS-8 is included, so ask before you buy.

10 MHz Oscilloscope with Four Sweep Ranges

The oscilloscope section offers 10 MHz bandwidth and four sweep ranges. That's not laboratory grade. A dedicated bench scope has more bandwidth and more triggering options. It's still plenty for transceiver measurements. You can display sine waves, square waves, carrier signals, modulation envelopes, and SSB waveforms. The scope also works as an RTTY tuning scope, which gives digital operators a visual tuning aid that's more precise than tuning by ear.

Even a basic scope built into the station monitor is a big convenience when you're diagnosing a transmitter problem. The SM-220's scope won't replace a proper bench instrument for serious repair work. For routine monitoring and adjustment, it does the job.

Two-Tone Audio Generator (1000 Hz and 1575 Hz)

The two-tone generator produces 1000 Hz and 1575 Hz tones, available individually or simultaneously. That's the standard two-tone test signal for SSB transmitters. Feed it into the microphone input and watch the resulting waveform on the monitor scope to check for flat-topping, distortion, or other linearity problems. The fact sheet doesn't give output voltage specifications.

B-31 Blue-Green CRT Phosphor

The B-31 phosphor CRT produces a blue-green display that owners consistently describe as bright and easy to read. The B-31 formulation suited a station monitor that might be left on for extended periods, since it's legible without being fatiguing. The fact sheet doesn't specify the CRT size, so check that if you're considering a purchase.

Compatibility with Kenwood Transceivers

The SM-220 was designed for use with the TS-520 and TS-820 transceivers, according to eHam. KE9PQ notes that it's also compatible with several other Kenwood models, including the TS-830S. The BS-8 pan display adapter was designed specifically for the TS-820 series, so if you're running a TS-520 or TS-830S, verify which pan adapter option works with your rig. The fact sheet mentions only the BS-8, not a BS-5 or other variant, so the pan display compatibility appears to center on the TS-820 series.

The SM-220 isn't a universal station monitor that works with any transceiver. It was built for Kenwood's own line. It may function with other brands, but the pan display and some monitoring functions are specific to the Kenwood IF frequencies and signal levels.

Historical Context

Kenwood manufactured the SM-220 from 1980 to 1988, according to Technotronic Dimensions' repair documentation for the series. That eight-year production run spans the peak of the HF transceiver boom, when serious operators built stations around a main rig and a collection of matching accessories. The SM-220 was part of that setup, a station monitor designed to complement Kenwood's transceivers and not to stand alone as test equipment.

Universal Radio's listing notes that the SM-220 was discontinued and replaced by the SM-230. The SM-230 presumably offered updated features or improved performance, though the fact sheet doesn't say what changed.

The fact sheet doesn't include original pricing, advertising copy, or contemporary reviews, so the SM-220's market positioning at release is hard to reconstruct. It was a specialized accessory aimed at operators who wanted integrated station monitoring instead of a collection of separate instruments.

Collectibility & Value

Second-hand SM-220 units trade in the $249.99 to $375.00 range, according to current listings. That's a wide spread, and it reflects the usual variables: condition, whether the BS-8 pan adapter is included, and whether the unit has been tested or serviced. The fact sheet doesn't say whether the higher end of that range includes the BS-8, so confirm what's in the box before committing.

The SM-220 is not rare. It shows up regularly on the used market, and instruction and service manuals are available from multiple sources. That makes one easy to find, and it also means there's no scarcity premium. This is a working tool, not a collector's trophy.

The common failure points, according to Technotronic Dimensions' repair page, are the usual suspects for CRT-based equipment of this era: dead/no power, no deflection, no sweep, no display, no vertical height, no trace, focus problems, intermittent display, blown fuses, a dot in the center of the screen, and weak or dim trace. These are age problems. Capacitors drift, solder joints crack, CRTs wear out. Repair services are available for the SM-220 series.

A working SM-220 with a bright CRT and the BS-8 adapter is a useful piece of gear for anyone running a TS-820 or similar Kenwood transceiver. A non-working unit without the pan adapter is a project. The price difference between those two should be significant, and buyers should factor in the cost of repair or restoration.

The fact sheet includes one owner comment from an eBay UK buyer (johfrase0, 27 Dec 2018) who reported the unit "worked as expected," and another listing describing a unit as "in very good condition and tested." These are typical used-market descriptions, not detailed reviews.