Collins KWM-2 / KWM-2A
Restoration Deep-Dive

Arthur Collins’ Masterpiece • 1959–1977 • The Compact S-Line That Went to War

Of all the rigs to emerge from Cedar Rapids, Iowa during the golden age of amateur radio engineering, the Collins KWM-2 occupies a singular position. It was the world’s first commercially produced amateur mobile SSB transceiver, it served military communicators in Vietnam, it flew phone patches for US troops halfway around the world, and it remains — over sixty years since its introduction — one of the most capable and sonically satisfying HF transceivers ever built. This deep-dive restoration guide covers everything from dating your unit by serial number and IF can codes, through the complete Collins Service Bulletin set, to the specifics of 70K-2 PTO rehabilitation, relay service, capacitor replacement, PA neutralization, and final alignment. Whether you’re looking at a 1959 KWM-2 that’s never been opened or a late-production 1975 KWM-2A fresh off a MARS station desk, the approach is the same: methodical, document-driven, and unhurried.

Production History & Serial Numbers

The KWM-2 debuted in 1959 at a list price of US$1,150, making it roughly the cost of a small car at the time. It was replaced in the amateur market by the KWM-2A in 1961, initially priced at US$1,250, though by 1977 — the last year of catalogue listing — the price had climbed to US$3,533. Across approximately 18 years of continuous production, well over 25,000 units were manufactured, making it the most successful transceiver Collins ever produced.

Serial Number Dating Sometime before 1963, Art Collins decreed that serial numbers be randomised on the production floor — he was troubled that dealers were grading S-Line equipment by serial number alone. For the KWM-2/2A this means that chassis serial number alone is not a reliable production date indicator. The most accurate dating method is to read the date codes stamped on the IF transformer cans and fixed capacitors inside the chassis; these are typically formatted as YY MM (two-digit year, two-digit month). A chassis with cans coded 74 08 was assembled no earlier than August 1974.

Model	Introduced / Price	Notes
KWM-2	1959 / $1,150	Amateur bands only. 11 crystals supplied (80–15 m + WWV). One 10 m crystal supplied; two additional sockets provided.
KWM-2A	1961 / $1,250 → $3,533 (1977)	Adds secondary crystal board for up to 14 out-of-band crystals. MARS, military, and RTTY capable (with 516F-2 and external cooling). Front-panel board-select switch and indicator added.
KWM-2A (Military) AN/FRC-93	Circa 1963–1976	Deployed as part of the AN/FRC-93 radio set (also including 30L-1, 516F-2, 312B-4/5, 180S-1). Extensively used by MARS stations during Vietnam. US Army TM 11-5820-554-12 covers this configuration.

Identifying Factory KWM-2A vs Field-Converted KWM-2

Service Bulletin 9 (Revision 2) provided the field conversion kit for upgrading a KWM-2 to KWM-2A specification. Distinguishing a factory-built KWM-2A from a field conversion matters for collector grading and for understanding what internal modifications may be present. Key indicators:

Factory-Built KWM-2A

Continuous, thick engraved line on the EXCITER TUNING and PA TUNING dials — this is the most reliable single indicator.

Serial number tag is machine-stamped, not hand-written.

Crystal board and associated wiring are factory-fitted with neat, point-to-point routing.

Board-select switch and indicator lamp on front panel are integral to the main escutcheon casting, not retrofitted.

Field-Converted KWM-2

EXCITER TUNING and PA TUNING dials show the original KWM-2 style (dashed or shorter scale lines).

Serial number tag may show handwritten alterations or an affixed conversion sticker.

Crystal board wiring may show evidence of retrofitting — different wire colours, added terminal strips.

Front-panel switch cutout may show slight fitting marks or a non-factory escutcheon.

Rod Blocksome’s Serial Number Survey (CCA) The Collins Collectors Association has an ongoing survey collecting 70K-2 PTO serial numbers to better characterise KWM-2/2A production runs. If you own a unit, consider contributing your PTO serial number and IF can date codes to the CCA survey archive at collinsradio.org. Cross-referencing PTO serials with chassis serials is the most promising avenue for establishing accurate production timelines.

Technical Architecture

The KWM-2/2A is a double-conversion transceiver and exciter sharing all major circuit blocks between transmit and receive paths. This elegant economy of design — the same oscillators, the same Collins mechanical filter, and the same RF amplifier serving both functions — is the characteristic Collins engineering signature that also defines the S-Line. Understanding the shared-circuit architecture is essential for alignment and troubleshooting: a fault that appears to affect only transmit or only receive is very likely in one of the non-shared stages, while a fault present in both modes points squarely at a shared element.

Parameter	Specification
Frequency Coverage	3.4–30.0 MHz in 200 kHz segments (except 5.0–6.5 MHz). Any frequency within range accessible with appropriate crystal.
Modes	USB, LSB, CW (sidetone injection method); RTTY with 516F-2 power supply and external cooling
Power Input	175 W PEP on SSB; 160 W on CW
Nominal Output	100 W (50 Ω, 25–100 Ω usable range)
Frequency Control	70K-2 Permeability Tuned Oscillator (PTO), 2.501–2.701 MHz output range (200 kHz span)
High-Frequency IF	2.955–3.155 MHz (200 kHz bandpass IF — accommodates full VFO span)
Low-Frequency IF	455 kHz
Selectivity	2.1 kHz (−6 dB); 4.2 kHz (−60 dB) — Collins mechanical filter
Receiver Sensitivity	0.5 µV for 10 dB (S+N)/N in amateur bands
AGC Range	Audio output does not change more than 20 dB as input varies from 10 µV to 1 V
Image Rejection	>40 dB; internal spurious below 1 µV equivalent antenna input
CW Sidetone Note	1500 Hz tone oscillator injection; transmitted CW signal is 1500 Hz above dial reading
Dimensions	14¾” W × 7¾” H × 14″ D (37.5 × 19.7 × 35.6 cm)
Weight	18 lbs 3 oz (8.25 kg)
Operating Temperature	0°C to +50°C; 0–90% humidity; 0–10,000 ft altitude

Tube & Semiconductor Complement

The KWM-2/2A uses eighteen active devices in its full complement. The two 6146 finals are the most critically matched pair in the chassis and should be sourced and tested as a matched set. All other tubes are generally interchangeable from good-quality NOS stock without matching.

Type	Qty	Function / Notes
`6146`	2	PA finals — beam tetrode. Should be a matched pair for push-pull balance and to equalise plate dissipation. The 6146B (military spec) is the preferred choice; original 6146 (no suffix) is acceptable. Kenwood-branded 6146B and RCA/GE NOS stock are regarded as the best available.
`12AT7`	2	Balanced modulator / product detector; BFO / carrier oscillator. Low-noise NOS stock preferred.
`6U8A`	4	Used throughout IF and mixer stages. Widely available.
`6AZ8`	3	Mixed-function tubes covering relay amplifier, audio amplifier, and ALC stages.
`6BN8`	3	Noise blanker, VOX, and AGC/ALC rectifier functions.
`6CL6`	1	Driver stage — medium-µ pentode driving the PA grid.
`6DC6`	1	RF amplifier — shared TX/RX. This tube sees the antenna directly and is a common failure point after lightning or static events.
`6EB8`	1	Audio output amplifier — drives speaker or headphones at up to 1 W.
`6AU6 / 7543`	1	PTO oscillator tube (`V301` inside the 70K-2 assembly). The 7543 is the military ruggedised equivalent and is preferred for stability. Use only if plate current and transconductance are within spec — a marginal PTO tube is a common cause of frequency drift.

PA Interlock — High Voltage Hazard The KWM-2/2A PA cage is interlocked. The interlock must be closed or bypassed for any transmit testing. When performing PA-cage work, always discharge the high-voltage supply through a 10 kΩ / 10 W bleeder resistor before touching internal nodes. With a 516F-2 supply, plate voltage can exceed 700 V DC under some conditions. This is lethal.

Service Bulletins & Service Information Letters

Collins issued ten numbered Service Bulletins, three lettered sub-revisions of SB 8, and three Service Information Letters specifically covering the KWM-2/2A. The complete set is available from the CCA technical archives at collinsradio.org. A comprehensive printed compendium is available from Electric Radio Magazine (ermag.com). Every restoration should begin by determining which SBs have already been applied to the chassis under work.

Verifying SB Status Check the chassis for handwritten service tags, pencil notes on the chassis underside, or soldering changes referenced in the bulletin text. The KWM-2 community on the CCA groups.io reflector ([email protected] and [email protected]) can assist in identifying whether a modification is factory-applied, SB-applied, or a previous owner’s improvisation.

Bulletin	Description & Restoration Notes
SIL 1-75	Overcome Instability Caused by Aging. Addresses performance degradation in aged units — VFO drift, reduced output, and noise floor issues attributable to component ageing. Review this SIL before condemning any subsystem on an unrestored unit.
SIL 2-75	Extended Operating Range. Addresses marginal performance near the band-edge frequencies at 3.4 MHz and 30.0 MHz. Trimmer adjustments for extreme low and high band coverage.
SIL 2-3-60	VFO Oscillator Drift. Early information letter addressing warm-up drift in the 70K-2 PTO. The definitive long-term solution is a full PTO service including capacitor replacement — see the PTO section below.
SB 2	Add VOX Relay Time-Constant Control. Adds a panel-accessible potentiometer for VOX release time (`R43`). Greatly improves break-in operation for CW and reduces clipping of the first syllable on SSB voice. Highly recommended for any operating unit.
SB 3	Correlation Between Frequencies on Different Bands. Eliminates frequency offset when switching bands at the same PTO/dial setting. Involves trimmer adjustments across all band crystals. Essential if the radio is used for split-band or interband schedules.
SB 4	Improvement of ALC Action; Eliminate ALC Overshoot. Addresses a design limitation that allowed ALC overshoot on loud syllables, producing audio distortion and occasional splatter. Involves component changes in the ALC rectifier and time-constant network.
SB 5 Rev.	Improve Transmit-Receive Exciter Tuning Coincidence. Resolves a frequency offset between the TX and RX tuning peaks, particularly noticeable at higher frequencies. This is a common complaint when operating split. The revised version supersedes the original SB 5.
SB 6	Eliminate Delay When Switching from TX to RX. Addresses a timing issue in the relay sequencing that caused a brief audio dropout at the beginning of received audio after releasing PTT or dropping VOX. The root cause is in the `K2` / `K4` relay drive chain.
SB 7 Reissue	Replace K2 and K4 with Plug-In Type Relays. The original K2 and K4 relays are open-frame types highly susceptible to contact contamination from dust and oxidation. This is one of the most common failure sources in unrestored units — symptoms include intermittent S-meter deflection, varying receiver sensitivity, and sluggish T/R switching. The reissue specifies plastic-encased, plug-in replacement relays that are far more reliable.
SB 8A	Eliminate AGC Overshoot on Noise Pulses; Dual Time-Constant AGC. One of the three most important service bulletins for any KWM-2/2A used on a busy band. Adds a dual-slope AGC time constant so that strong noise pulses trigger fast attack without causing the AGC to suppress subsequent signals. Significantly improves strong-signal handling and band-noise resilience.
SB 8B	Change Source of Delay Bias to AGC Rectifier. A follow-on to SB 8A, this bulletin corrects the bias source for the hang AGC delay circuit, improving stability of the AGC action across varying signal conditions.
SB 8C	Add Delayed-Decay (Hang) AGC to RF Amplifier. Extends hang-AGC action to the RF amplifier stage, providing a more consistent noise floor during the AGC decay period after a strong signal. Applied in conjunction with SB 8A and 8B, these three bulletins transform the AGC behaviour of the KWM-2/2A from acceptable to excellent.
SB 9 Rev. 2	Convert KWM-2 to KWM-2A. The complete field-upgrade kit specification for adding the extended crystal board. Includes a replacement front-panel escutcheon, crystal board assembly, switch, and indicator lamp, plus wiring instructions. Revision 2 is the definitive version. See the KWM-2 vs KWM-2A identification notes above for how to distinguish a field conversion from a factory unit.
SB 10	Eliminate VHF Parasitic Oscillations in Tone Oscillator. The 1500 Hz phase-shift tone oscillator can exhibit parasitic VHF oscillation that modulates onto the transmitted CW signal, producing sidebands well outside the licensed emission bandwidth. A small capacitor addition suppresses this cleanly.

Pre-Restoration Assessment

Before any soldering iron gets near the KWM-2/2A, a methodical condition assessment will establish what you actually have and prevent chasing faults introduced during a previous owner’s work. Spend a full session on this before proceeding.

Photograph everything before disassembly. Document the top, bottom, rear panel connectors, crystal board, and any non-original modifications. These photographs are invaluable when questions arise mid-restoration.
Read the IF can date codes. Establish the approximate production date. Record it. Compare with any service tags found on the chassis.
Check the front-panel dials for factory vs conversion status. The EXCITER TUNING and PA TUNING dial lines identify factory KWM-2A versus SB-9 conversion.
Inspect the multi-can electrolytic. The chassis-mounted multi-section capacitor can is a common age failure. Check for bulging, electrolyte leakage, or evidence of previous replacement. This single component, if failed, can damage the power transformer on first power application.
Visually inspect K2 and K4 relays. Even relays that look clean can have oxidised contacts. EB5AGV’s documented case of “bright gold” contacts that were intermittent is a well-known cautionary tale in the KWM-2 community. Tapping the relay chassis while monitoring receiver sensitivity is a classic diagnostic.
Audit the PTO. Look for evidence of previous opening (disturbed screws, lacquer on trimmer C308, non-original solder on the PTO wiring), and note whether the dial zero and 200 end-points track correctly against a frequency counter connected to J18.
Check for previous modifications. Unauthorised mods to T1 (the balanced modulator transformer), addition of resistors or capacitors to the ALC chain, or modifications to the PA cage are common findings. Evaluate each one carefully — some are improvements; many are not.
Review the SB application record. Where possible, determine which service bulletins have been applied. If the unit came from a ham shack with service records, this is straightforward. Otherwise, work through the physical checks described in each SB text.
Perform a cold resistance check on the main supply rails. Using a high-range ohmmeter, check between B+ and chassis ground before first power application. A reading below approximately 5 kΩ indicates a shorted filter can or shorted filter capacitor and the supply should not be powered until resolved.
First power-up through a Variac or current-limited supply. Never apply full mains to an unrestored KWM-2/2A. Bring the supply up slowly over 10–15 minutes with a milliammeter in series, watching for smoke and monitoring rail voltages against the manual’s voltage table.

K2 & K4 Relay Service (SB 7)

The two main T/R switching relays — K2 (VOX / PTT relay) and K4 (metering switch relay) — are the single most common failure mechanism in unserviced KWM-2/2A transceivers. The original open-frame relay design allows dust and oxidation to contaminate the contacts over decades. Symptoms are insidious: intermittent S-meter, varying receiver sensitivity (particularly on signal peaks), and sluggish or incomplete T/R switching. Because the contacts look visually clean — even bright gold — the diagnosis is often missed for years.

Service Bulletin 7 Reissue specifies replacement with plastic-encased, plug-in type relays. If full replacement relays are not available, the original relays can be successfully serviced by carefully opening the contact bridge, cleaning each contact surface with paper moistened in Caig DeoxIT, and then reassembling. The procedure is described in detail in DJ7HS’s KWM-2 notes available through the CCAE archive. After cleaning, verify proper make and break action on both K2 and K4 using an ohmmeter before reassembly.

K2/K4 Diagnostic Technique With the transceiver in receive mode and an antenna or signal generator connected, tap K2 sharply with a wooden dowel. If receiver sensitivity momentarily changes — drops, increases, or shows an S-meter flicker — contact contamination is confirmed. Do not attempt to spray contact cleaner into the relays without disassembly; the cleaner will flush contaminants further into the contact bridge and worsen the problem.

Capacitor Replacement

The Multi-Can (Chassis-Mounted Electrolytic)

The most critical capacitor replacement in any KWM-2/2A restoration is the chassis-mounted multi-section electrolytic filter can. This single component contains multiple filter capacitors for the main power supply rails and is the unit’s most vulnerable age point. Failed sections can present as hum, RF instability, reduced output, or catastrophic failure of the power transformer.

A custom-manufactured replacement can — constructed to match the original nickel-plated housing and mounting tabs — is available from KE9PQ.com along with a complete individual capacitor kit. The kit includes a data sheet cross-referencing schematic designators to physical positions, making installation methodical. Many restorers report that even filter caps measuring within tolerance on a capacitance meter showed leakage current that only appeared under ESR and DC leakage testing — substitution typically reveals a performance improvement even when the meter suggested otherwise.

Paper and Mica Capacitors Throughout the Chassis

The KWM-2/2A uses waxed-paper and mica coupling and bypass capacitors throughout the signal chain. All waxed-paper types should be proactively replaced on any unit with original components — they reliably develop leakage after 40+ years, introducing DC offsets that upset bias points and balanced modulator null. Mica types are generally more durable and should be replaced selectively on the basis of measurement, not age alone.

Balanced Modulator Capacitors The balanced modulator circuit (centred on the 12AT7 at V2) is highly sensitive to leakage in its coupling capacitors. Any DC leakage in this network will shift the null point, increasing carrier breakthrough. After replacing capacitors in this area, re-null the balanced modulator according to the manual procedure before final alignment.

Ceramic Trimmer Capacitor Service

The KWM-2/2A uses numerous small ceramic trimmer capacitors distributed across the chassis and band switch assembly. These trimmers — rotating ceramic disc types with a metal clip retainer — are notorious for “freezing” after decades of disuse, making alignment impossible without first freeing the rotating element. Attempting to force a frozen trimmer will shatter the ceramic disc irreparably.

Glen Zook K9STH’s technique, documented in the CCA archive’s “Repairing Frozen Ceramic Trimmers” paper, is the community standard method. The approach involves carefully disassembling the trimmer, separating the rotor from the ceramic disc (using an X-Acto #11 blade at the joint if necessary), cleaning both surfaces in a silver-dip solution such as Tarn-X for approximately one minute, rinsing in clear water, and very carefully drying the ceramic disc — which is fragile and will crack if subjected to thermal shock. Reassembly should be done before cleaning the next trimmer to avoid mixing the small parts.

Band Switch Shaft Removal To access the trimmers around the band switch wafers, the fibre drive shaft must be removed first. The shaft is retained by two Bristol setscrews placed 90° apart. Before loosening the second setscrew, mark the fibre shaft’s position relative to the chassis with a felt-tip pen — correct reinstallation alignment is critical to band switch operation. Detailed photographic documentation of this procedure is in the DJ7HS KWM-2 restoration notes on QSL.net.

70K-2 PTO Service

The 70K-2 Permeability Tuned Oscillator is the heart of the KWM-2/2A — and of the entire S-Line family. It is a masterwork of precision mechanical and RF engineering: a variable permeability inductor driven by a precision lead-screw mechanism, with the oscillator tube and its associated LC tank enclosed in a sealed, temperature-stabilised housing. When a 70K-2 is working correctly, it provides better than 100 Hz/hour drift after a 30-minute warm-up. When it isn’t, the rig is unusable for SSB work.

Symptoms of a Failing PTO

Frequency drift exceeding a few hundred hertz during the first 30 minutes of operation; sudden frequency jumps of 1–30 kHz; frequency sensitivity to power supply voltage variations above 2–3 Hz/V; or a shift of the entire PTO range by a fixed amount (suggesting loss of capacitance in the tuned circuit) all indicate that PTO service is required.

PTO Internal Capacitor Replacement

The primary cause of 70K-2 long-term drift and instability is degradation of the small mica and polystyrene capacitors in the oscillator tank circuit. The key components and their schematic designators in the 9th edition manual are:

Ref.	Original Value	Notes
`C301`	Small mica	Handle with extreme care — this is the most physically fragile capacitor in the PTO. A cracked C301 causes a large, sudden frequency shift. Cannot be obtained commercially; must be preserved from the original assembly.
`C302`	999 pF mica	Replace with dipped mica type — not polystyrene. A degraded C302 that develops voltage-dependent capacitance causes frequency instability correlated with RF signal level in the PTO, a subtle and easily misdiagnosed symptom.
`C303`	3051 pF mica	Replace with new mica type. Note that each replacement will shift the idle frequency slightly — expect cumulative shifts of several kHz that require L302 span re-trimming after all capacitors are replaced.
`C304`	207 pF mica	9th edition Change 20B specifies replacement with 50 pF mica (effectively 55 pF measured). This reduces the V301 grid-to-ground contribution to the tank and lowers warm-up drift. The frequency will rise by approximately 900 Hz after this change.
`C308`	Ceramic trimmer	Sets USB/LSB frequency shift (~2.7 kHz). The trimmer’s ceramic disc is prone to freezing. Clean using the K9STH method. Lacquer applied to lock this trimmer (a previous owner’s “fix”) is a common finding and must be carefully removed.
`CR301`	1N34A germanium	Switching diode. Replace with 1N4148 silicon. The lower junction capacitance of the silicon diode reduces frequency jumps. Expect a small idle frequency rise (~300 Hz) after replacement.

PTO Lubrication

After capacitor replacement, clean the lead screw spindle thoroughly of all original grease — which will have oxidised and thickened over decades — and apply fresh lithium grease sparingly. The u-shaped indexing member on the variable inductor core is a precision part; handle it with care and do not attempt to force it if it feels stiff. Clean rather than force.

PTO Span Alignment After Service

The PTO output range must span exactly 200 kHz (2.501–2.701 MHz) with the mechanical end stops set correctly so that 0 and 200 on the frequency dial correspond to the electrical end points. After any capacitor replacement, re-establish the span using L302, with a frequency counter connected to J18 at the rear of the chassis. Set the mechanical end stops after the electrical span is correct — not before. The CCAE-Collins groups.io archive contains detailed bench test jig designs for off-radio PTO alignment that simplify this process considerably.

CCA PTO Resource Library The CCA maintains an extensive PTO service library at collinsradio.org/pto-specification-data/, including W5HTW’s three-part tutorial on 70K-2 assembly, the Blocksome tracking and accuracy guide, assembly and lubrication notes, and annotated schematics for the 9th edition PTO circuit. This is the definitive reference for any 70K-2 work.

PA Neutralization

The KWM-2/2A’s push-pull 6146 PA stage requires periodic neutralization — the process of cancelling the feedback from plate to grid through the tube’s internal capacitance that would otherwise cause oscillation. Neutralization is particularly important after tube replacement and is also a maintenance item on any unit where the tubes have been changed at any point in its history. Failure to neutralize after tube changes is a common cause of the rig oscillating on certain bands, producing a characteristic “squeal” or unexplained RF output when the exciter is off.

The neutralizing capacitor C184 is accessible from below the chassis with the PA shield cover removed. The Collins manual procedure (Section 4 of the 9th edition) covers transmitter neutralization, driver neutralization, and feedback neutralization as three sequential steps. Use a non-metallic alignment tool for C184 adjustment. DJ7HS’s QSL.net page “Collins KWM-2: Installing New Parts for PA Neutralization” provides an excellent photographic step-by-step for this procedure including the correct post-neutralization verification sequence.

Never Use a Metal Tool for C184 Adjustment C184 is the PA neutralizing capacitor and its body is at significant RF and DC potential during the neutralization procedure. Always use a non-conductive plastic or fibreglass alignment tool. A metal screwdriver will introduce stray capacitance that invalidates the adjustment and could contact live HV circuitry.

Alignment Overview

Full KWM-2/2A alignment requires a signal generator, a calibrated frequency counter, a wideband RF voltmeter or oscilloscope, a dummy load, and a reliable 50 Ω source. The manual’s Section 4 (Alignment Procedures) covers the procedure in detail across five major steps:

PTO calibration and span verification. With a counter on J18, set the zero and 200 end points using L302, then set the mechanical end stops. All subsequent alignment depends on the PTO being correct first.
Band-pass IF alignment (2.955–3.155 MHz). Align the bandpass IF transformers for flat response across the 200 kHz span. This is the alignment that most directly affects receiver sensitivity and transmitted spectral purity.
RF amplifier and crystal oscillator alignment. Peak each band’s RF amplifier trimmer and verify crystal oscillator frequency against a counter. SB 3 (band frequency correlation) is applied here.
Transmitter circuit alignment. Includes balanced modulator null (carrier suppression), ALC threshold, and transmit IF alignment. The balanced modulator null should be adjusted for maximum carrier suppression — typically better than 40 dB below PEP output.
PA neutralization and final output check. Perform PA neutralization as described above, then verify output power and ALC action against the manual’s specification tables.

Band Switch Shaft Reinstallation Alignment The CCA RX page at collinsradio.org/rx/ includes specific guidance on “Band Switch Shaft/Shield Can Removal and Cleaning” — an article that covers the critical alignment marks required when reinstalling the fibre band switch shaft after cleaning. Incorrect shaft reinstallation is a common source of band-switching problems after service.

The KWM-2/2A Station Ecosystem

The KWM-2/2A was designed as the centrepiece of a complete Collins travelling station or fixed shack installation. Understanding the companion equipment and their interconnections is important both for restoration completeness and for understanding how faults in one unit can manifest as symptoms in another.

Equipment	Role & Connection Notes
516F-2 Power Supply	The standard companion power supply providing +275 V DC (receive), +375 V DC (transmit), 275 V filament, and 12 V / 24 V mobile supplies. The 516F-2’s multi-section filter can is another known age failure point — a failed supply will produce hum in audio, low output, or erratic receiver behaviour in the KWM-2/2A that can be misdiagnosed as a transceiver fault. See the 516F-2 restoration guide on vk6ada.com.au for full detail.
30L-1 Linear Amplifier	Four-tube (811A) linear amplifier providing approximately 1 kW input / 500 W output. Connects to the KWM-2/2A via J10 (50 Ω antenna output). The 30L-1 includes ALC output that feeds back to the KWM-2/2A’s ALC input at J14, constraining exciter output to keep the linear in its linear operating region. Four Collins SBs cover the 30L-1 (SB 1 through SB 4).
30S-1 Linear Amplifier	Higher-power companion using a single 4CX1000A ceramic tetrode — the premium station amplifier. Same ALC interconnection arrangement as the 30L-1.
312B-4 / 312B-5 Speaker / VFO Console	The station console providing an external speaker, S-meter, VFO, phone patch, and monitoring facilities. The 312B-5 includes a second 70K-2 PTO that must be independently serviced if used as an external VFO. The PTO in the 312B-5 uses the same capacitors and lubrication requirements as the KWM-2/2A’s internal PTO.
PM-2 Mobile Power Supply	12 V / 24 V DC mobile supply for vehicle or field operation. SB 1 Revision covers input voltage design specification extension. The PM-2’s chassis-mounted multi-can capacitor should be replaced as part of any PM-2 restoration.

Military Configuration: AN/FRC-93 The KWM-2A was also deployed as the primary transceiver in the US Army and Air Force AN/FRC-93 fixed-station radio set. This configuration paired the KWM-2A with the 30L-1 (AM-3979/FRC-93), 516F-2, 312B-4 and 312B-5, and the 180S-1 antenna coupler. Military examples of these sets can carry Air Force T.O. 31R2-4-183-3 maintenance stickers or Army TM 11-5820-554-12 reference cards and were often maintained to a higher standard than commercial amateur units of the same period.

Community Resources

The KWM-2/2A is one of the best-documented amateur transceivers ever built, with an active international restoration community. These are the primary resources you should have bookmarked before beginning any restoration work.

CCA Manual Archive — collinsradio.org Complete KWM-2/2A manuals (1st through 9th edition), service survey documents, PTO service library, and CCA Technical Bulletins. The definitive primary source for all factory documentation.
CCA Service Bulletins — collinsradio.org Digitised factory Service Bulletins and Service Information Letters, hosted by the Collins Collectors Association. Some bulletins require CCA membership to access.
CCA RX Library — collinsradio.org/rx/ Extensive article library covering KWM-2 specific topics: voltage data, RIT modification, audio howl reduction, band switch cleaning, S-meter zero correction, and more.
CCA PTO Resource Library — collinsradio.org Definitive 70K-2 PTO service reference including W5HTW’s three-part tutorial, Blocksome tracking guide, annotated schematics, and lubrication notes.
CCA Reflector — [email protected] The primary English-language Collins community discussion list. Searchable archive covers decades of KWM-2/2A restoration discussions. Essential for unusual fault diagnosis.
CCAE Collins Reflector — [email protected] European Collins community list with a strong technical focus. Particularly valuable for 70K-2 PTO work — several definitive PTO repair narratives originated here.
CCAE Tools Archive — ccae.tm6cca.com European Collins Collectors Association restoration archive. Contains DJ7HS’s KWM-2 restoration notes (PDF), PTO repair guides, relay modification guides, and multilingual restoration reports from F6CER, IV3UVW, IK0IXI, and others.
DJ7HS 70K-2 PTO Repair — qsl.net/dj7hs Ernst Schroeder’s detailed photographic PTO repair accounts — two separate repair case studies covering the most common PTO failure modes. Indispensable reading before opening a 70K-2.
SB-8 Installation Guide — jessystems.com Step-by-step procedure specifically for installing Service Bulletin 8 (AGC modification) in the KWM-2/2A. Includes component layout photographs.
WA3KEY Service Bulletin Index — wa3key.com Quick-reference index of all Collins Service Bulletins and SILs for the KWM-1/2/2A and S-Line series. Useful for cross-checking bulletin descriptions before retrieving full documents.
Electric Radio SB Compendium — ermag.com 260-page printed binder containing all known service bulletins and factory modifications for the S-Line, KWM-1, KWM-2, and KWM-2A. The definitive printed reference for bench work.
RadioNerds KWM-2A Military Page — radionerds.com Military documentation for the AN/FRC-93 radio set including the KWM-2A. Covers T.O. and TM references for the Army and Air Force configurations.
r-390a.net Administered by VK6ADA — while the primary focus is the R-390A/URR military HF receiver, the Pearls of Wisdom and restoration methodology articles are broadly applicable to all military-grade HF equipment of the KWM-2/2A era.

VK6ADA Notes — A Personal Perspective

The KWM-2/2A is a different restoration challenge to the S-Line separates. Where the 32S-3 transmitter and 75S-3 receiver each inhabit their own chassis with room to work comfortably, the KWM-2/2A packs a complete double-conversion SSB transceiver into a chassis not much larger than a modern tabletop radio. The component density is remarkable and demands patience. Every connection matters. Every SB matters. And the 70K-2 PTO — once you’ve opened one and understood the mechanism — is a genuine marvel of 1950s precision engineering.

The community around these radios is extraordinary. The CCA reflector has been active for decades, and the institutional memory held in its archives — the Hollow State Newsletter threads, the Electric Radio articles, the DJ7HS repair logs — represents a depth of collective expertise that simply does not exist for modern commercial transceivers. If you’re attempting a KWM-2/2A restoration, you are not alone.

My recommendation: apply all SBs. Replace the filter can and all waxed-paper capacitors proactively. Service the relays before you do anything else. Then open the PTO only if you have a legitimate drift complaint — a 70K-2 that’s working well is best left alone. And if you do open it, work from the DJ7HS and W5HTW references, replace only C302, C303, C304 and CR301, and do not touch C301 unless the frequency has shifted by more than 20 kHz.

The reward for a correctly restored KWM-2/2A is a transceiver that still astonishes. On 20 metres SSB, through a good antenna and into a quiet band, the combination of Collins’ mechanical filter selectivity, the clean double-conversion architecture, and the silky-smooth 70K-2 PTO produces an operating experience that no amount of digital signal processing has ever quite replicated.

73 de VK6ADA. Good luck with your restoration.

Collins KWM-2 / KWM-2ARestoration Deep-Dive