Kenwood TS-870S (1996–2004)

Kenwood's first HF transceiver with a fully digital IF signal path - and the front-end overload problem that made early serial numbers a gamble.

Overview

Kenwood put a tagline on the TS-870S box: "Intelligent Digital Enhanced Communications System." It was more than ad copy. The TS-870S was the first Kenwood HF transceiver to run Next Generation Digital Signal Processing at the IF stage on both transmit and receive. That is a different architecture from the analog-IF rigs it replaced. The service documentation confirms it. In receive mode the fourth 11.3 kHz IF signal is converted from analog to digital and handed to the DSP. On transmit the microphone input goes through an analog-to-digital converter before it reaches the modulator. By far the greatest change, as one period write-up put it, is that the new radio has a fully digital signal path.

Kenwood built the TS-870S as an all-mode HF transceiver covering 160 through 10 meters. Output is 100 Watts in SSB, CW, FSK, and FM, and 25 Watts in AM. It requires 13.8 VDC at 20.5 Amps, so it needs a serious supply, not a wall wart. The rig shipped with the MC-43S hand mic. The built-in automatic antenna tuner works on all bands in both transmit and receive modes, which is unusual, since most tuners in this class only engage on transmit. The RS-232C serial port on the back made computer control a first-class feature. The "extensive user-adjustable filtering functions" were the headline selling point for contesters and DXers who wanted to shape passbands in software rather than swap crystals.

Production ran from 1996 to 2004, according to the only source that dates it. The radio is long out of production. Universal Radio lists it as "Discontinued!" and eham.net notes the product is not in production. The original MSRP was 2399.00. It is still a popular radio with hams today, and Nifty Accessories describes it as a highly regarded transceiver. eham.net's own product description was blunter: "The Leader Of The Pack."

Key Features

Next Generation DSP at the IF Stage

The DSP is the whole point of the TS-870S. Kenwood's own materials call it the first Kenwood HF transceiver with Next Generation Digital Signal Processing at the IF stage on transmit and receive. The service manual lays out what that means. The DSP handles modulation and demodulation, filter processing, AGC processing including S-meter data output, mic amplifier AGC, VOX, voice equalizing, noise processing (noise reduction, beat cancel, automatic notch), speech processing with COMP meter data output, squelch, and sidetone and beep generation. That is a lot of the signal chain moved into firmware for a 1996 rig. The digital section runs two DSP56002FC40 processors alongside RAM, three ROMs, and a gate array.

The payoff was supposed to be flexibility. "Extensive user-adjustable filtering functions" means you dial in bandwidth and shape rather than ordering a crystal. The catch, as owners discovered, is that DSP selectivity is only as good as the analog filtering in front of it. The stock 2nd IF crystal filter and 3rd IF monolithic ceramic filter are mediocre. When strong signals blow past them, they modulate the following stages, including the DSP circuits themselves. One well-known modification write-up puts it plainly: Kenwood's engineers decided to rely on obtaining selectivity mainly by using the DSP. That is theoretically great until the crystal filters let a loud neighbor through.

Front-End Overload and the Serial Number Fix

This is the TS-870S's defining flaw, and any buyer needs to know it. Earlier production units before serial number 801xxxxx had front-end overload and consequent distortion and crossmodulation in later stages, including the DSP. It is especially noticeable on the low bands. Owners report the rig folding up when a strong signal appeared a few kHz away. One account describes a contest on 160m where signals 4 kHz distant caused constant interference. Kenwood's response was a modification to the gain distribution in the RF and mixer stages, implemented in later production models. So the serial number matters. A unit from the affected range is not a bad radio. It needs the factory mod, or a competent technician to apply the same fix.

EPROM Versions and SSB Decode Problems

There is a second revision trap. Older versions of the TS-870S exhibit distorted or muffled audio on SSB during pileups compared to the TS-850. The fix is replacing three EPROMs - 27C256PCJJTF, 27C256PCJJUF, and 27C256PCJJVF - located on the bottom left half of the RX/TX board behind the shield. The version indicator is the letter second from last in the part number. T, U, and V mark the latest revision. These chips are not cheap. One source quotes $29.53 each from Kenwood parts centers, and authorized service centers reportedly would not sell them directly. The original owner of that account was not impressed with Kenwood's customer service, arguing the company should have been replacing the old PROMs free of charge as a design problem. Either way, check the PROM revision before buying, because the difference in SSB decode performance is audible.

Filter Modification

The single biggest improvement available to a stock TS-870S is replacing the stock 2nd IF crystal filter and 3rd IF monolithic ceramic filter with high-quality International Radio or equivalent filters. Better-shaped filters make a tremendous improvement to selectivity, overload behavior, and DSP performance. Depending on intended use, 2.7, 2.4, or 2.1 kHz bandwidth filters are the options. One contester chose 2.1 kHz primarily for contesting and found it worked very well even for CW, a significant improvement over the stock arrangement. Narrower filters for CW are possible, but the selectivity selection scheme is microprocessor-controlled switching. That makes it complicated and throws the front-panel control markings off their indicated positions.

Built-In Antenna Tuner and Electronic Keyer

The built-in automatic antenna tuner works on all bands in both transmit and receive modes. Receive-mode tuning is a convenience most competitors of the era skipped. The rig also includes a built-in electronic keyer. Combined with the RS-232C serial port, the TS-870S was designed as a complete station rather than a component, with computer control, keying, and matching all handled internally. The supplied MC-43S hand mic covers the SSB side.

Frequency Accuracy and the Optional TCXO

The reference oscillator used in the TS-870S is accurate to 10ppm across -10 to +50°C. If the optional temperature-compensated crystal oscillator SO-2 is installed, accuracy improves to 0.5ppm over the same range. Owners considering serious digital-mode work should note this: 10ppm is adequate for SSB and CW but marginal for narrow digital modes, and the TCXO upgrade is the fix. The SO-2 is switched in via slide switch S800, and the switch must be set to the SO-2 position once the oscillator is installed.

Historical Context

Kenwood introduced the TS-870 as a replacement for the TS-850s and TS-850sat. The company's own marketing claimed the performance characteristics of this radio had set entirely new standards, unmatched by any other product in its class. The TS-870S marked the point where Kenwood moved its HF flagship from analog IF filtering to a digital signal path. The rig's reputation in the years since has been shaped as much by the early-production fixes as by the architecture itself.

Production is dated to 1996 through 2004 by the only available source, a single video description, so treat those endpoints as approximate rather than confirmed. The radio is definitively out of production. A 104-page user manual exists in English and Dutch as a PDF. Nifty Accessories publishes a laminated mini-manual measuring 4.5 by 8 inches across twenty pages for field reference.

Collectibility & Value

The TS-870S remains a popular radio with hams, and the used market reflects that. A second-hand listing for a Kenwood TS-870SAT DSP HF Transceiver was priced at £999.00, and one sales index tracked 85 Kenwood TS-870S listings at a single point in time. The original MSRP of 2399.00 gives a sense of where the used price sits relative to new. That is roughly a forty-percent discount off original in the UK example, which is strong retention for a transceiver of this age.

Buyers should ask three questions about any example. First, what is the serial number - is it past 801xxxxx, or has the front-end overload modification been applied? Second, which EPROM revision is installed - are the T, U, and V chips present, or is the radio still running the older versions with known SSB decode issues? Third, has the stock crystal and ceramic filter chain been upgraded, or is the rig still relying entirely on DSP selectivity? A TS-870S with all three addressed is a substantially better radio than a stock early unit, and the price difference should reflect that.

Low power output appears as a known issue category, and a help forum page dedicated to it exists. No specific failure components are identified in the available documentation. A troubleshooting table is referenced in at least one service resource, but the specific faults it lists are not documented here. Prospective buyers should test transmit power across all bands before committing. The modification and EPROM work described above is well-documented enough that a competent HF technician should have no trouble with it.