vk6ada.com.au • Collins S-Line Technical Reference • IF Filter Guide

Collins S-Line and KWM-2/KWM-2A
Crystal and Mechanical Filter Comprehensive Reference — All Part Numbers, Specifications, BFO Crystals, and Equipment Cross-Reference

A definitive reference for the 75S-1, 75S-2, 75S-3, 75S-3A, 75S-3B, 75S-3C, 32S-1, 32S-2, 32S-3, 32S-3A, KWM-2, and KWM-2A. Covers every Y-case and FA-case mechanical filter, every CW crystal filter, all Z-series sideband selector filters, BFO crystal frequencies, cross-case equivalents, INRAD aftermarket options, and operating notes. Three discrepancies between source documents are explicitly flagged and cited. All filters operate at 455 kHz IF.

  ✍ Mike Peace VK6ADA / r-390a.net Administrator
  📅 April 2026 • Revision 1.0
  ⚙ 455 kHz IF • Mechanical and crystal filters • 12 equipment models • All part number variants

Every Collins S-Line receiver and transmitter, and both KWM-2 transceivers, operates at a 455 kHz intermediate frequency. This is not a coincidence — Collins Radio engineered all equipment in the S-Line family around the 455 kHz IF to allow direct filter interoperability within a station, a single test procedure to verify all equipment, and a common parts inventory for service. The result is a filter ecosystem of unusual technical depth: two distinct filter technologies (mechanical resonator and quartz crystal lattice), multiple physical case styles for different circuit board generations, Z-series asymmetric bandpass filters for sideband generation in the early transmitters, and a comprehensive optional-filter programme that allows operators to customise selectivity from a 200 Hz CW crystal filter through a 6.0 kHz AM broadcast mechanical filter.

This guide documents every filter type, all known Collins part number variants (including the -00 and -000 suffix forms), full electrical specifications, case style cross-references, BFO crystal requirements, INRAD aftermarket equivalents, and the three source discrepancies identified during compilation that the restoration community should be aware of.

⚠ Three Source Discrepancies — Read Before Using This Guide

The following discrepancies were identified between the two primary source documents used for this guide. Each is noted in the relevant section with a citation. The Collins service manuals referenced in Section 14 are the authoritative resolution for all three.

1. X455KO Q200 standard vs. optional in the 75S-3 and 75S-3A. The detailed Collins filter catalog states the X455KO Q200 was standard equipment in every 75S-3 and 75S-3A. A secondary condensed source lists it as optional for those same models. The detailed catalog source is adopted in this guide as the authoritative statement; the 75S-3 instruction book (Rockwell Collins, 4th Edition, July 1963) is the definitive resolution.

2. F455FA-27 shape factor. The detailed catalog states 2.24:1; the condensed source states 2.41:1. Both are plausible for a 2.9 kHz mechanical filter. The 60 dB bandwidth figure (6.5 kHz) appears in both sources consistently; the discrepancy is in the derived shape factor ratio. The 2.24:1 figure from the detailed catalog is used in the specification tables below, but operators should verify against a measured unit.

3. 75S-3B/3C BFO crystal circuit designators. The detailed catalog identifies the USB BFO crystal as Y15 and the LSB crystal as Y16. The condensed source identifies them as Y6 and Y7. The KWM-2/2A BFO crystal designators (Y16 and Y17) are consistent across both sources. The 75S-3B/3C instruction manual (Collins Radio, November 1975) must be consulted to resolve the 75S-3B/3C designator discrepancy.

Section 1 — Filter Technologies: Mechanical Resonator vs. Quartz Crystal Lattice

◆ Mechanical Filter (F-Series)

A series of resonant nickel-alloy metal discs with Q factors of 8,000 to 12,000, coupled by precision wire rods. Magnetostrictive transducers at each end convert between electrical energy and mechanical vibration. These filters provide extremely steep skirt selectivity with shape factors as low as 1.2:1. Frequency drift is only 1.5–2 ppm/°C over the operating range; in accelerated aging tests, maximum deviation was less than 1 ppm over eight months of thermal cycling between 25°C and 90°C.

Designation system: F[centre freq in kHz][case style]-[bandwidth in hundreds of Hz]. Example: F455FA-21 = 455 kHz, FA case, 2.1 kHz bandwidth.

All S-Line and KWM-2 SSB, AM, and CW mechanical IF filters operate at 455 kHz nominal. The 75S-3B/3C replaced earlier crystal CW filters with mechanical FA-case CW filters (F455FA-05 and F455FA-08), which are more rugged and require no BFO crystal change for installation.

◆ Crystal Filter (X-Series and Q-Series)

Piezoelectric quartz resonators in a lattice or half-lattice network. Crystal filters are used exclusively for the narrowest CW selectivity bandwidths — 200 Hz and 500 Hz — where the superior Q of quartz produces bandwidths that the mechanical disc technology of the era could not practically achieve at low cost.

Designation systems: Crystal filters appear under two naming conventions in Collins documentation. The X-prefix (e.g., X455KO Q200) is the older crystal-specific designation used for the 200 Hz filter. The Q-series within the F-prefix system (e.g., F455Q-5) uses an L-type cylindrical case and appears in the filter catalog alongside mechanical types.

BFO crystal dependency: all crystal IF filters require a correctly matched BFO crystal to place the carrier on the correct skirt of the narrow passband. The F455Q-5 500 Hz filter in the 75S-1 requires a dedicated 455.8 kHz BFO crystal (Collins P/N 290-8707-00) not needed for any mechanical filter installation.

Part number suffix note: -00 and -000 are the same component. Collins part numbers appear in both the two-digit suffix format (e.g., 526-9337-00) and the three-digit suffix format (e.g., 526-9337-000) in different documents. The additional trailing zero was added in later Rockwell Collins documentation to standardise the nine-digit part number format. Both designate the identical component. This guide lists both where they appear in source documents.

Section 2 — Equipment Summary: Standard IF Filter by Model

Standard factory equipment Optional accessory Not applicable to model

Equipment	Type	Years	IF	Standard IF Filter	Circuit Ref	Filter Case
75S-1	Receiver	1958–1962	455 kHz	F455Y-21 (2.1 kHz mechanical)	FL2	Y (solder-in)
75S-2	Receiver	1960–1962	455 kHz	F455Y-21 (2.1 kHz mechanical)	FL2	Y (solder-in)
75S-3	Receiver	1961–1963	455 kHz	F455Y-21 (2.1 kHz mech.) + X455KO Q200 (200 Hz crystal) ^①	FL1 / FL2	Y + crystal
75S-3A	Receiver	1963–1964	455 kHz	F455Y-21 (2.1 kHz mech.) + X455KO Q200 (200 Hz crystal) ^①	FL1 / FL2	Y + crystal
75S-3B	Receiver	1964–1968	455 kHz	F455FA-21 (2.1 kHz mechanical)	FL1	FA (plug-in)
75S-3C	Receiver	1968–1978	455 kHz	F455FA-21 (2.1 kHz mechanical)	FL1	FA (plug-in)
32S-1	Transmitter	1958–1962	455 kHz	F455Y-21 (SSB) + F455Z-4/5 (sideband select)	FL1	Y + Z (solder-in)
32S-2	Transmitter	1960–1962	455 kHz	F455Y-21 (SSB) + F455Z-4/5 (sideband select)	FL1	Y + Z (solder-in)
32S-3	Transmitter	1962–1968	455 kHz	F455FA-21 (2.1 kHz mechanical)	FL1	FA (plug-in)
32S-3A	Transmitter	1968–1978	455 kHz	F455FA-21 (2.1 kHz mechanical)	FL1	FA (plug-in)
KWM-2	Transceiver	1959–1967	455 kHz	F455Y-21 (2.1 kHz mechanical, dual-purpose TX+RX)	FL1	Y (solder-in)
KWM-2A	Transceiver	1966–1978	455 kHz	F455Y-21 (2.1 kHz mechanical, dual-purpose TX+RX)	FL1	Y (solder-in) or FA (late production)

^① The X455KO Q200 200 Hz crystal CW filter was included as standard equipment in every 75S-3 and 75S-3A per the Collins filter catalog (source discrepancy flagged above — see Section 3). Late-production Rockwell Collins KWM-2A units may be found with the F455FA-21 plug-in filter substituted for the F455Y-21 solder-in; this substitution requires installation of a 3-pin transistor socket.

✎ KWM-2/2A Single-Filter Architecture: One Filter, Two Functions The KWM-2 and KWM-2A use a single mechanical filter FL1 for both reception and transmission. During receive operation it provides IF selectivity. During transmit operation it acts as the SSB generation filter, shaping the transmitted audio bandwidth. The BFO crystals Y16 (453.650 kHz, USB) and Y17 (456.350 kHz, LSB) are switched by the emission selector switch to place the carrier approximately 20 dB down on the appropriate skirt of the filter passband. This means the transmit audio bandwidth and the receive IF selectivity are identical and both are determined by the single installed filter — there is no independent transmit filter in the KWM-2/2A design.

Section 3 — CW Crystal Filters: X455KO Q200 and F455Q-5

Crystal (quartz lattice) filters are the narrowest-bandwidth IF filters available for the S-Line and KWM-2 family. Collins used two crystal CW filter types: the 200 Hz X455KO Q200 and the 500 Hz F455Q-5. Both operate at 455 kHz and both require BFO crystal attention for installation. The 75S-3B and 75S-3C replaced crystal CW filters with mechanical FA-case equivalents (F455FA-05 and F455FA-08); these models do not use crystal IF filters.

Filter Type	Collins P/N	Alt P/N	Centre Freq	Bandwidth	Mode	Case Style	Standard In	Optional In	BFO Crystal Required
X455KO Q200	526-7000-021	526-7000-021-000	455 kHz nominal	200 Hz	CW	Special crystal (not Y or FA)	75S-3, 75S-3A ^①	75S-1	Uses existing BFO crystal arrangement; verify with service manual
F455Q-5	526-9367-00	526-9367-000	455 kHz nominal	500 Hz	CW	L (cylindrical, 2⁶⁹⁄₃₂″ long)	—	75S-1, 75S-2	455.800 kHz crystal required Collins P/N 290-8707-00 (for 75S-1 installation)

^① Source discrepancy: the detailed Collins filter catalog states standard in 75S-3 and 75S-3A; a secondary source states optional. See flagged discrepancy notice above. The 75S-3 instruction book (Rockwell Collins, 4th Ed, July 1963) is authoritative.

F455Q-5 BFO crystal requirement in the 75S-1. Installing the F455Q-5 500 Hz crystal filter in the 75S-1 requires an additional dedicated BFO crystal: type 455.800 kHz, Collins part number 290-8707-00. This crystal shifts the BFO frequency to place the CW signal within the 500 Hz passband of the crystal filter. Without this BFO crystal change, the CW signal will not fall within the filter passband regardless of receiver tuning. The installation instructions are documented in Collins document 523-0010-00 (75S-1 CW Filter Installation Instructions, 1 June 1959).

Crystal filters not available for the KWM-2/2A. The KWM-2 and KWM-2A do not support crystal CW IF filters in the original design. The single-filter architecture (one filter for both TX and RX selectivity) and the BFO switching arrangement are not compatible with the narrowband crystal filter types used in the 75S-1/2/3/3A receivers. CW operation on the KWM-2/2A relies on the 2.1 kHz mechanical filter passband with BFO placement providing adequate selectivity for most CW conditions. The FA-case mechanical CW filters (F455FA-05 at 500 Hz, F455FA-08 at 800 Hz) are available for the 75S-3B/3C only.

Section 4 — Y-Case Mechanical Filters (Solder-In): 75S-1/2/3/3A, 32S-1/2, KWM-2/2A

The Y-case is the original solder-in cylindrical filter used in all early S-Line equipment and the KWM-2/2A throughout its production life. The case is approximately &frac7;16″ in diameter by 2.5″ long with three solder-in pins.

Filter Type	Collins P/N (-00)	Collins P/N (-000)	BW @−6 dB	BW @−60 dB	Shape Factor	Mode	Equipment Applications
F455Y-21	526-9337-00	526-9337-000	2.1 kHz	5.3 kHz	2.52:1	SSB / CW	75S-1/2 (std FL2), 75S-3/3A (std FL2), 32S-1/2 (std FL1), KWM-2/2A (std FL1, dual-purpose TX+RX)
F455Y-31	526-9338-00	526-9338-000	3.1 kHz	6.5 kHz	2.10:1	AM	75S-1/2 (optional AM)
F455Y-40	526-9339-00	526-9339-000	4.0 kHz	8.5 kHz	2.13:1	AM	75S-1/2, 75S-3/3A (optional AM)
F455Y-60	526-9340-00	526-9340-000	6.0 kHz	12.6 kHz	2.10:1	AM broadcast	75S-3/3A (optional AM broadcast)
F455Q-2	526-9309-00	—	2.0 kHz	5.6 kHz	2.80:1	SSB / CW	General catalog Y-case variant at 2.0 kHz — intermediate bandwidth between 1.5 kHz and 2.1 kHz. Note: despite the “Q” designation, this is a mechanical filter in Y case; the Q is a type variant letter, not a crystal-type indicator. Not listed in S-Line equipment use list but may be used as an upgrade substitute.
F455Y-80	526-9341-00	526-9341-000	8.0 kHz	18.5 kHz	—	AM broadcast / wide	General catalog; not standard S-Line equipment

Section 5 — FA-Case Mechanical Filters (Plug-In): 75S-3B/3C, 32S-3/3A

The FA-case is the later phenolic rectangular plug-in filter introduced with the 75S-3B/3C and 32S-3/3A. It fits a standard 3-pin transistor socket allowing tool-free filter swapping. The 75S-3B/3C provides three filter sockets: FL1 for the standard SSB filter, FL2 and FL3 for optional CW filters, and a fourth socket for the optional AM filter.

Filter Type	Collins P/N (-00)	Collins P/N (-000)	BW @−6 dB	BW @−60 dB	Shape Factor	Mode	Equipment Applications
F455FA-05	526-9494-00	526-9494-000	500 Hz	2.5 kHz	5.00:1	CW	75S-3B/3C (optional CW)
F455FA-08	526-9446-00	526-9446-000	800 Hz	4.0 kHz	5.00:1	CW / RTTY	75S-3B/3C (optional CW/RTTY)
F455FA-15	526-9495-00	526-9495-000	1.5 kHz	3.5 kHz	2.33:1	RTTY / narrow SSB	75S-3B/3C (optional RTTY/narrow SSB)
F455FA-21	526-9427-00	526-9427-000	2.1 kHz	5.3 kHz	2.52:1	SSB / CW	75S-3B/3C (std FL1), 32S-3/3A (std FL1)
F455FA-27	526-9500-00	526-9500-000	2.9 kHz	6.5 kHz	2.24:1 ^②	Wide SSB	75S-3B/3C (optional wide SSB)
F455FA-31	526-9496-00	526-9496-000	3.1 kHz	6.5 kHz	2.10:1	AM	75S-3B/3C (optional AM)
F455FA-40	526-9497-00	526-9497-000	4.0 kHz	8.5 kHz	2.13:1	AM	75S-3B/3C (optional AM)
F455FA-60	526-9498-00	526-9498-000	6.0 kHz	12.6 kHz	2.10:1	AM broadcast	75S-3B/3C (optional AM broadcast)

^② Shape factor discrepancy: detailed catalog source states 2.24:1; secondary source states 2.41:1. The 60 dB bandwidth (6.5 kHz) is consistent between both sources. 2.24:1 is used here; verify against a measured unit.

Section 6 — Detailed Electrical Specifications: F455FA-21 (Standard SSB Filter)

The F455FA-21 is the most commonly encountered filter in late S-Line and KWM-2A equipment. The F455Y-21 is electrically equivalent, differing only in physical case style. Both share the same centre frequency, bandwidth, shape factor, transfer impedance, and resonating capacitance.

F455FA-21 — Collins P/N 526-9427-00 / 526-9427-000

Centre frequency: 455 kHz nominal
Bandwidth @−6 dB: 2.1 kHz nominal
Bandwidth @−60 dB: 5.3 kHz nominal
Shape factor (60/6 dB): 2.52:1
Passband ripple: 3 dB max (454.3–455.7 kHz)
Transfer impedance: 5,000 Ω ± 2,250 Ω
Resonating capacitance: 130 pF ± 5 pF (includes circuit stray capacitance)
Transmission (insertion) loss: 9.5 dB nominal
Spurious response attenuation: 60 dB minimum (442–468 kHz)
Signal input voltage: 0–2 V RMS maximum
DC voltage max: 300 VDC (coil terminals to ground)
Operating temperature: −40°C to +85°C
Storage temperature: −65°C to +100°C
Case style: FA (phenolic, 3-pin plug-in)
Dimensions: 2.5″ L × ~0.5″ W × ~0.5″ H

F455Z-16 / Spec 526-9605 — Upper Sideband Mechanical Filter — Carrier 455 kHz, Filter Centre 456.45 kHz

Description: Upper sideband mechanical filter. Carrier frequency is 455 kHz; the filter centre is offset to 456.45 kHz to select the upper sideband while rejecting the carrier and lower sideband. Replaces earlier Z-series sideband selectors in later Rockwell Collins commercial and military equipment.
Source document: Rockwell International Corporation, Electronic Systems Group (Dallas TX / Newport Beach CA / Cedar Rapids IA), specification 526-9605, Code Ident 13499, Rev H (dated 5/18/82, released 20 May 1982, 7 sheets). Reprinted in Collins Collectors Association Technical Bulletin TB-2, October 2024. Applicable standards: MIL-STD-202, MIL-F-18327C, QQ-N-290.

25°C Nominal Electrical Characteristics

Centre frequency: 456.45 kHz NOM
Passband ripple: 3 dB MAX
BW F3H−F3L (@−3 dB): 1.7 kHz MIN
BW F6H−F6L (@−6 dB): 2.3 kHz MIN
BW F60H−F60L (@−60 dB): 5.0 kHz MAX
Stop band (F60L to 430 kHz): 60 dB MIN
Stop band (F60H to 470 kHz): 60 dB MIN
F20L: 455.00 kHz MIN
F6L (−6 dB lower skirt): 455.30 kHz MAX
F3L (−3 dB lower skirt): 455.45 kHz MAX
F3H (−3 dB upper skirt): 457.15 kHz MIN
F6H (−6 dB upper skirt): 457.60 kHz MIN
F20H: 458.30 kHz MAX
Transfer impedance: 5.0 kΩ NOM
Insertion loss: 21 dB ± 4 dB (at 25°C)
Resonating capacitance: 130 pF ± 20 pF (includes circuit stray)
Dielectric strength: MIL-STD-202, Method 301, 100 VDC applied between terminal and case

Operating Temperature Range Electrical Characteristics (−20°C to +65°C)

Centre frequency: 456.45 kHz NOM
Passband ripple (over temp.): 4 dB MAX (degraded from 3 dB at 25°C)
BW F4H−F4L (@−4 dB): 1.615 kHz MIN (passband narrows under temperature)
BW F6H−F6L (@−6 dB): 2.185 kHz MIN
BW F60H−F60L (@−60 dB): 5.25 kHz MAX
F4L (−4 dB lower skirt): 455.65 kHz MAX
F4H (−4 dB upper skirt): 456.95 kHz MIN
F6H (@−6 dB upper skirt): 457.40 kHz MIN
F20H: 458.50 kHz MAX
Transfer impedance: 5.0 kΩ NOM
Insertion loss (over temp.): 21 dB ± 5 dB (tolerance widens from ±4 at 25°C)

Test Conditions (from spec sheet Section 3)

Signal input voltage: 3 V RMS MAX (note: different from 2 V max for standard FA-series filters)
Source resistance: 100 kΩ ± 5 kΩ (note: different from 5 kΩ standard filter test impedance)
Load resistance: 100 kΩ ± 5 kΩ
C1, C4: Short circuit
C2, C3: 130 pF ± 20 pF

Environmental & Mechanical

Operating temperature: −20°C to +65°C
Non-operating temperature: −65°C to +100°C
Vibration: MIL-STD-202, Method 201
Shock: MIL-STD-202, Method 202; three 15 G impacts per axis, 18 shocks total
Construction: Epoxy resin sealed; molded phenolic case; smooth plastic finish
Marking: Date code per MIL-STD-1285

Dash Number Variants (Case Style I and II)

526-9605-010 (Case I): Standard marking. Case Style I molded phenolic. Overall length 64.26 mm max (2.53″); body 55.88 ± 0.127 mm (2.200″); width 38.1 ± 0.127 mm (1.500″); height 14.22 mm max (0.56″). Mounting: #112-40 UNC-2A screws, two places. Pin spacing: 5.08 ± 0.127 mm (0.200″) and 2.54 ± 0.127 mm (0.100″).
526-9605-020 (Case II): Case Style II. No additional marking changes vs -010.
526-9605-030 (Case I): Additional marking: 532-1002-001. Date code.

Section 7 — Z-Series Sideband Selector Filters (32S-1 and 32S-2 Transmitters Only)

The 32S-1 and 32S-2 transmitters use Z-series asymmetric mechanical bandpass filters for sideband selection during transmission. These are not IF selectivity filters — they are asymmetric bandpass elements designed to pass either the upper or lower sideband of a double-sideband suppressed-carrier signal centred on 455 kHz, while rejecting the opposite sideband by at least 40 dB and the carrier by at least 20 dB.

32S-3 and 32S-3A operators: Z-series filters do not apply to your transmitter. The 32S-3 and 32S-3A accomplish sideband selection by switching BFO crystals, not by Z-series filters. The KWM-2/2A similarly uses BFO crystal switching for sideband selection. Z-series filters are exclusive to the 32S-1 and 32S-2.

F455Z-4 — Upper Sideband Selector (32S-1/2)

F455Z-4 Electrical Specifications

Carrier frequency: 455 kHz
Sideband selected: Upper (USB)
Bandwidth @−6 dB: 3.3 kHz ± 300 Hz
Bandwidth @−60 dB: 6.0 kHz max
Peak-to-valley ratio: 3 dB max
Terminal impedance: 27,000 Ω
Transfer impedance: 8,500 Ω ± 45%
Resonating capacitance: 130 pF
Transmission loss: 10 dB
Spurious response attenuation: 60 dB minimum
Attenuation at 454.60 kHz: 40 dB minimum (opposite sideband)
Attenuation at 455.00 kHz: 20 dB minimum (carrier)
Attenuation at 455.30 kHz: 6 dB max (passband edge)
Attenuation at 455.45 kHz: 3 dB max (passband edge)
Attenuation at 458.15 kHz: 3 dB max (passband edge)
Attenuation at 458.30 kHz: 6 dB max (passband edge)

The F455Z-5 (lower sideband selector) is the mirror image of the F455Z-4, with the passband shifted to the lower side of the 455 kHz carrier. Electrical specifications are equivalent with the passband symmetrically reversed.

Z-Series Complete Part Number Variants

Filter Type	Collins P/N	Sideband	BW @−3 dB	BW @−6 dB	Case	Notes
F455Z-4	526-9164-00	Upper (USB)	2.7 kHz	—	F (metal can)	Older catalog listing
F455Z-4	526-9364-00	Upper (USB)	—	3.3 kHz	Y (long)	Standard 32S-1/2
F455Z-4	526-9364-011	Upper (USB)	2.7 kHz	—	Y (long)	Variant suffix -011
F455Z-4	526-9364-021	Upper (USB)	2.6 kHz	—	Y (long)	Variant suffix -021
F455Z-4	526-9364-031	Upper (USB)	3.4 kHz	—	Y (long)	Variant suffix -031
F455Z-4	526-9364-041	Upper (USB)	2.7 kHz	—	E (metal can)	Metal can variant
F455Z-5	526-9365-00	Lower (LSB)	2.7 kHz	—	Y	Standard 32S-1/2; also 32MS-1A, 32RS-1C/1H
F455Z-5	526-9365-011	Lower (LSB)	2.7 kHz	—	F (metal can)	Variant suffix -011
F455Z-5	526-9365-021	Lower (LSB)	3.4 kHz	—	Y (long)	Variant suffix -021
F455Z-5	526-9365-031	Lower (LSB)	3.1 kHz	—	Y (long)	Variant suffix -031
F455Z-5	526-9365-041	Lower (LSB)	2.7 kHz	—	E (metal can)	Metal can variant
F455Z-1	526-9095-00	Upper (USB)	2.7 kHz	—	H (metal can)	H-case upper sideband variant
F455Z-2	526-9096-00	Lower (LSB)	2.7 kHz	—	H (metal can)	H-case lower sideband variant
F455Z-6	526-9368-00	Upper (USB)	1.7 kHz	—	Y (long)	Narrow 1.7 kHz USB; same bandwidth as 526-9605 spec filter
F455Z-7	526-9395-00	Lower (LSB)	1.7 kHz	—	Y (long)	Narrow 1.7 kHz LSB companion to Z-6
F455Z-10	526-9445-00	Upper (USB)	3.3 kHz	—	Y	Wide USB; same bandwidth as standard 526-9364-00
F455Z-12	526-9486-00	Upper (USB)	1.2 kHz	—	Y	Very narrow 1.2 kHz USB; narrowest Z-series in catalog
F455Z-14	526-9527-001	Upper (USB)	5.46 kHz	—	F (metal can)	Wide 5.46 kHz USB; later commercial/military use
F455Z-15	526-9528-001	Lower (LSB)	5.46 kHz	—	F (metal can)	Wide 5.46 kHz LSB companion to Z-14

Section 8 — BFO Crystal Frequencies by Equipment

The BFO frequency is placed approximately 20 dB down on the skirt of the mechanical filter response curve, ensuring the desired sideband falls within the passband while the unwanted sideband is attenuated by the filter. Sideband inversion in the 75S-3B/3C receiver means the BFO crystal assignment is reversed from the KWM-2/2A: USB reception requires the lower BFO frequency in the 75S-3B/3C, which is the opposite of the KWM-2/2A arrangement.

Equipment	Crystal Designator	Frequency	Sideband Selected	Notes
KWM-2 / KWM-2A	Y16	453.650 kHz	USB	Unused crystal shorted by switch S9H during mode change
KWM-2 / KWM-2A	Y17	456.350 kHz	LSB	Consistent across both source documents
75S-3B / 75S-3C	Y15 ^③	453.650 kHz	USB	Sideband inversion in 1st mixer means lower BFO freq = USB (opposite of KWM-2/2A)
75S-3B / 75S-3C	Y16 ^③	456.350 kHz	LSB	Designator discrepancy noted below
75S-1 (with F455Q-5 CW option)	Optional add	455.800 kHz	CW	Collins P/N 290-8707-00. Required only when F455Q-5 is installed. See Collins document 523-0010-00.

^③ BFO crystal designator discrepancy: the detailed Collins filter catalog lists Y15 (USB) and Y16 (LSB) for the 75S-3B/3C; a secondary condensed source lists Y6 (USB) and Y7 (LSB) for the same models. The KWM-2/2A designators Y16/Y17 are consistent across both sources. Consult the 75S-3B/3C instruction manual (Collins Radio, November 1975) to resolve.

Section 9 — Filter Case Styles Reference

Case Style	Description	Mounting	Dimensions (approx.)	S-Line / KWM-2 Use
Y	Cylindrical, long body; solder-in	Solder-in (3 pins)	From catalog drawing: 7⁄16″ approx dia × 2 3⁄16″ body; Y Long length: 2.5″. Lead spacing 1⁄8″ ± 1⁄32″. Y-series mounting accessories: flat surface mount 591-0613-002; feed-through 591-0615-002 (recommended); tube socket adapter 591-0617-002.	75S-1/2, 75S-3/3A, 32S-1/2, KWM-2/2A (standard); Z-series sideband selectors (Z-4-00, Z-5-00 and variants)
FA	Rectangular phenolic, compact; plug-in	Plug-in (3-pin transistor socket; 7⁄32″ pin dia)	From catalog drawing: 2.500″ L × 0.600″ H × 0.528″ W. Body: 1.500″ wide × 0.300″ H. Pin spacing: 0.200″ (outer), 0.100″ (inner). Ground terminal at centre. Note: F455FA-120 stop BW is 30.0 kHz (wider than other FA bandwidth entries at that passband).	75S-3B/3C, 32S-3/3A (standard); late-production KWM-2A (substitution)
C	Large rectangular metal can	Solder-in	From catalog drawing: 3.5″ L overall × 1.531″ body × 3⁄8″ H; pin spacing 1.531″ to 3.000″; 4.40″ total footprint. Largest metal can case in catalog.	250 kHz Z-series (F250Z-4, F250Z-5); not used in S-Line / KWM-2 equipment
E	Rectangular metal can	Solder-in	From catalog drawing: 2 13⁄16″ (2.375″) W × 15⁄16″ H; body 1.531″ wide; pin spacing 0.486″	Z-series metal can variants (Z-4-041, Z-5-041); available in all standard 455 kHz IF bandwidths
FC	Flat-pack phenolic; low-profile	Solder-in (flat surface)	From catalog drawing: 2.500″ L × 0.600″ H × 0.218″ body height. Input/output lead spacing 0.150″ from ground terminals; pin dia 0.031″. Note: F455FC-60 (526-9522-001) has passband of 6.0 kHz @−6 dB but stop bandwidth of 25.9 kHz @−60 dB — significantly wider stop band than all other case styles at the same passband bandwidth.	F455FC-60 (526-9522-001); commercial/military applications; not standard in S-Line equipment
H	Cylindrical with flanged mounting	Solder-in; flanged mount	From catalog drawing: 2 3⁄4″ L × 0.775″ dia body; 1.125″ flange width; 7⁄32″ pin diameter	Z-series H-case (F455Z-1, F455Z-2 at 2.7 kHz); available in all standard IF bandwidths
J	Cylindrical; 5-pin circular	Solder-in; 5-pin circular connector	From catalog drawing: 2 3⁄4″ L × 0.775″ dia. 5-pin circular pattern.	Collins 75A-4, KWM-1 (not amateur S-Line)
K	Cylindrical; flanged with mounting bracket	Solder-in; flanged bracket mount	From catalog drawing: 2 7⁄8″ L × 0.775″ dia; larger flanged mount than J-case; 7⁄32″ pin spacing	Not standard in amateur S-Line; available in all standard IF bandwidths
L	Large cylindrical	Solder-in (2 pins, end-mounted)	From catalog drawing: 6″ body length (6 7⁄16″ overall) × 1.000″ dia. F455Q-5 crystal filter uses a shorter variant (2 29⁄32″ body).	F455Q-5 crystal CW filter (2 29⁄32″ variant); also used for multiple sideband filters at lower carrier frequencies (60–108 kHz range)
N	Rectangular; larger body	Solder-in	Dimensions not confirmed in available drawings. Note: F455N-160 (526-9162-00) has stop bandwidth of 38.0 kHz @−60 dB, significantly wider than the 27.5 kHz stop bandwidth of other case styles at the same 16.0 kHz passband. This anomaly in the catalog is confirmed from the TB-2 source and should be verified before substitution.	Available for 2.1 kHz (526-9163-00, F455N-20), 4.0 kHz (526-9160-00, F455N-40), 8.0 kHz (526-9161-00, F455N-80), 16.0 kHz (526-9162-00, F455N-160). Not standard in S-Line equipment.

Y-case and FA-case filters are NOT directly interchangeable without modification. The Y-case requires soldering directly to the circuit board; the FA-case plugs into a 3-pin transistor socket. Converting a Y-case position to accept an FA-case filter requires installation of the appropriate socket. Collins offered this modification for upgrading 75S-3/3A receivers to accept FA-case filters. Electrically, Y-case and FA-case filters of the same bandwidth specification are equivalent once the correct mounting arrangement is in place.

Section 10 — Cross-Case Equivalent Part Numbers by Bandwidth

Collins produced the same nominal bandwidth in multiple physical case styles for different circuit board generations and commercial/military applications. This table shows all confirmed equivalent part numbers across case styles for the 455 kHz filter family.

Bandwidth	Y-Case P/N	FA-Case P/N	J-Case P/N	K-Case P/N	E-Case P/N	N-Case P/N
500 Hz	—	526-9494-00	526-9154-00	526-9228-00	526-9321-00	—
800 Hz	—	526-9446-00	526-9090-00	—	—	—
1.5 kHz	—	526-9495-00	526-9155-00	526-9168-00	526-9370-00	—
2.1 kHz	526-9337-00	526-9427-00	526-9156-00	526-9317-00	526-9322-00	526-9163-00 (F455N-20)
2.9 kHz	—	526-9500-00	—	—	—	—
3.1 kHz	526-9338-00	526-9496-00	526-9089-00	—	526-9074-00	—
4.0 kHz	526-9339-00	526-9497-00	526-9327-00	526-9303-00	526-9324-00	526-9160-00 (F455N-40)
6.0 kHz	526-9340-00	526-9498-00	526-9091-00	—	526-9084-00	—
8.0 kHz	526-9341-00	—	526-9329-00	526-9328-00	526-9332-00	526-9161-00 (F455N-80)
16.0 kHz	526-9343-00	—	526-9334-00	526-9315-00	526-9320-00	526-9162-00 (F455N-160) — 38.0 kHz stop BW (wider than other cases)

Section 11 — Complete Filter Cross-Reference by Equipment

Compiled from the Rockwell Collins 2000 Equipment Filter Use List (WA3KEY) and original Collins service manuals. Bandwidth values in the equipment use list occasionally differ slightly from nominal catalog values due to measured versus nominal specifications.

Equipment	Collins P/N	Filter Type	Bandwidth	Status	Application
32S-1/2	526-9337-000	F455Y-21	2.1 kHz @−6 dB	Standard	SSB generation (solder-in)
32S-1/2	526-9364-00	F455Z-4	2.7 kHz @−3 dB	Standard	Upper sideband selector (transmit)
32S-1/2	526-9365-00	F455Z-5	2.7 kHz @−3 dB	Standard	Lower sideband selector (transmit)
32S-3/3A	526-9427-000	F455FA-21	2.1 kHz @−6 dB	Standard	SSB generation (plug-in)
75S-1/2	526-9337-000	F455Y-21	2.1 kHz @−6 dB	Standard	SSB IF selectivity (FL2)
75S-1/2	526-9367-000	F455Q-5	500 Hz @−6 dB	Optional	CW IF selectivity; requires BFO crystal 290-8707-00
75S-1/2	526-9338-000	F455Y-31	3.1 kHz @−6 dB	Optional	AM IF selectivity
75S-1/2	526-9339-000	F455Y-40	4.0 kHz @−6 dB	Optional	AM IF selectivity
75S-3/3A	526-9337-000	F455Y-21	2.1 kHz @−6 dB	Standard	SSB IF selectivity (FL2)
75S-3/3A	526-7000-021	X455KO Q200	200 Hz CW	Standard ^①	CW IF selectivity (FL1)
75S-3/3A	526-9339-000	F455Y-40	4.0 kHz @−6 dB	Optional	AM IF selectivity
75S-3/3A	526-9340-000	F455Y-60	6.0 kHz @−6 dB	Optional	AM broadcast IF selectivity
75S-3B/3C	526-9427-000	F455FA-21	2.1 kHz @−6 dB	Standard	SSB IF selectivity (FL1)
75S-3B/3C	526-9494-000	F455FA-05	500 Hz @−6 dB	Optional	CW IF selectivity (FL2 or FL3)
75S-3B/3C	526-9446-000	F455FA-08	800 Hz @−6 dB	Optional	CW/RTTY IF selectivity (FL2 or FL3)
75S-3B/3C	526-9495-000	F455FA-15	1.5 kHz @−6 dB	Optional	RTTY / narrow SSB
75S-3B/3C	526-9500-000	F455FA-27	2.9 kHz @−6 dB	Optional	Wide SSB
75S-3B/3C	526-9496-000	F455FA-31	3.1 kHz @−6 dB	Optional	AM IF selectivity
75S-3B/3C	526-9497-000	F455FA-40	4.0 kHz @−6 dB	Optional	AM IF selectivity
75S-3B/3C	526-9498-000	F455FA-60	6.0 kHz @−6 dB	Optional	AM broadcast
KWM-2 / KWM-2A	526-9337-000	F455Y-21	2.1 kHz @−6 dB	Standard	SSB/CW (dual-purpose TX+RX)

Section 12 — Aftermarket Replacement Filters: INRAD

International Radio Corporation (INRAD) produces the most widely used aftermarket mechanical filter replacements for Collins S-Line equipment. INRAD filters use a 2,000 Ω source and load impedance rather than the original Collins 5,000 Ω specification. An INRAD adapter board with impedance-matching networks is mandatory for installation in original Collins equipment; installing INRAD filters without the adapter board produces excessive insertion loss and degraded passband response.

INRAD Model	BW @−3 dB	Collins Equivalent	Notes
INRAD #712-S	300 Hz	— (narrower than any original Collins filter)	CW only; no Collins original equivalent
INRAD #705-S	500 Hz	F455FA-05 (526-9494-00)	Direct functional replacement for 75S-3B/3C CW filter
INRAD #720-S	2,000 Hz	F455FA-21 (526-9427-00)	Slightly narrower than original 2.1 kHz
INRAD #706-S	2,500 Hz	F455FA-21 (526-9427-00)	Slightly wider than original 2.1 kHz
INRAD #726-S	2,900 Hz	F455FA-27 (526-9500-00)	Matches original wide SSB option
INRAD #724-S	3,800 Hz	F455FA-40 (526-9497-00)	AM replacement
INRAD #707-S	5,800 Hz	F455FA-60 (526-9498-00)	AM broadcast replacement

INRAD impedance adapter board is mandatory for all S-Line installations. INRAD filters are designed for a 2,000 Ω source/load impedance. The original Collins circuit uses 5,000 Ω. Installing any INRAD filter without the INRAD adapter board will result in excessive insertion loss (potentially 6–10 dB above normal) and significantly degraded passband flatness. The adapter board is available from INRAD and is described in their installation instructions for the Collins 75S-3B/3C (INRAD document revised 10 April 2008).

Section 13 — Operating Notes and Best Practices

Shunt feed is mandatory — no DC through the transducer coils.

Direct current flowing through the magnetostrictive transducer coils of any mechanical filter will alter the filter’s electrical characteristics, causing bandwidth shifts and increased insertion loss. All original Collins circuits use shunt feed to block DC from the filter terminals. This must be verified whenever using non-original circuit boards, after modifications, or when troubleshooting unexpected filter behaviour. This is the most common cause of filter performance degradation in modified equipment.

Signal level limit: 2 V RMS maximum at FA-series filter inputs.

Overdriving a mechanical filter causes intermodulation products within the resonator assembly, degrading adjacent channel rejection. In transmitter applications where the filter is driven by a balanced modulator, the drive level must be confirmed to be within specification. High-level signals at the filter input are particularly destructive in the Collins design because the filter provides both selectivity and operates directly in the signal chain between the balanced modulator and the carrier amplifier.

Resonating capacitance: 130 pF nominal, including circuit strays.

The specified 130 pF (±5 pF for the FA-21; ±20 pF for the Spec 526-9605 filter) includes circuit stray capacitance. The input and output capacitors must be correctly resonated to obtain the best passband response. Incorrect resonating capacitance shifts the effective centre frequency and degrades passband flatness. This is relevant during board restoration: if capacitors adjacent to the filter are replaced with different physical styles (different lead lengths, different stray capacitance), the 130 pF resonating capacitance specification must be re-verified.

Shielding: essential for rated 60 dB stopband attenuation.

Effective shielding between the filter input and output, short lead lengths between the filter terminals and the termination circuitry, and a common ground connection for the filter input shield and output are all required to achieve the rated 60 dB stopband attenuation. Lead lengths longer than a few millimetres between the filter terminal and the circuit board can create a bypass path that reduces effective stopband attenuation well below the rated figure. This applies to both Y-case and FA-case installations.

Temperature stability: 1.5–2 ppm/°C; bandwidth variation ±5% max.

Mechanical filters exhibit a frequency shift of 1.5 to 2 parts per million per degree Celsius over the operating range. Over the full operating range of −40°C to +85°C, the centre frequency may shift by up to approximately 250 Hz in the worst case, which is negligible for amateur radio SSB and CW operation. Bandwidth variation over temperature is ±5% maximum. In accelerated aging tests, maximum frequency deviation was less than 1 ppm over eight months of thermal cycling between 25°C and 90°C. These characteristics are superior to LC filters and require no operator compensation in normal amateur use.

Foam deterioration: inspect and remove during any restoration. The foam rubber filler blocks inside the filter covers of Y-case and FA-case mechanical filters deteriorate over time, turning into a sticky brown or black residue that can contaminate the filter terminals and surrounding circuit board traces. These foam blocks serve no electrical function and their removal does not affect filter performance. During any restoration work, the filter cover should be inspected for foam deterioration and the residue cleaned with isopropyl alcohol if present. Filters showing heavy foam contamination on the terminal pins should be cleaned before electrical testing.

Sources and References

Collins Radio Company, Collins Mechanical Filters General Catalog, Bulletin 1031A, Newport Beach, California. Primary source for filter type designations, electrical specifications, and case style descriptions throughout this guide.
Rockwell Collins, 2000 Rockwell/Collins Equipment Filter Use List, provided by WA3KEY. wa3key.com/filters.html (last revised 20 March 2005). Primary source for the complete equipment cross-reference table in Section 11.
Collins Radio Company, Collins Mechanical Filters Catalog, General Catalog (Peakbagging Archive), Newport Beach, California. Supplementary catalog source.
Collins Radio Company, 75S-1 CW Filter Installation Instructions, Document 523-0010-00, 1 June 1959. Primary source for F455Q-5 installation procedure and BFO crystal requirement (P/N 290-8707-00) in Section 3.
Rockwell Collins, 75S-3 and 75S-3A Receivers Instruction Book, 4th Edition, July 1963. Authoritative source for X455KO Q200 standard-vs-optional status in 75S-3/3A (source discrepancy flagged in introduction).
Collins Radio Company, KWM-2/2A Instruction Manual, 9th Edition, January 1978. collinsradio.org. Primary source for KWM-2/2A BFO crystal designators Y16/Y17 and switch S9H shortout arrangement.
Collins Radio Company, 75S-3B and 75S-3C Instruction Manual, November 1975. collinsradio.org. Authoritative source for 75S-3B/3C BFO crystal circuit designators (source discrepancy flagged in introduction and Section 8).
Rockwell International Corporation, Mechanical Filter Specification 526-9605, Electronic Systems Group (Dallas TX / Newport Beach CA / Cedar Rapids IA), Code Ident 13499, Rev H, 7 sheets, released 20 May 1982. Reprinted in Collins Collectors Association Technical Bulletin TB-2, October 2024. collinsradio.org. TB-2 also includes the Collins mechanical filter general catalog page (455 kHz full listing) and the Collins case styles catalog drawing sheets — primary source for: 526-9605 test conditions (3 V max input, 100 kΩ source/load), operating temperature electrical performance, dash numbers (-010/-020/-030), Case Style I physical dimensions, additional Z-series part numbers (Z-1, Z-2, Z-6, Z-7, Z-10, Z-12, Z-14, Z-15), F455Q-2 2.0 kHz Y-case mechanical filter (526-9309-00), N-case part numbers, FC-case stop bandwidth anomaly (F455FC-60: 25.9 kHz), and all case style drawings with confirmed dimensions.
International Radio Corporation (INRAD), Collins 75S-3B/C: Installing Inrad Filters, revised 10 April 2008. inrad.net. Source for INRAD model numbers, bandwidths, and impedance adapter board requirement in Section 12.
WA3KEY, Rockwell/Collins Mechanical Filters, wa3key.com/filters.html, last revised 20 March 2005. Cross-case equivalent part number table source for Section 10.

✍ Mike Peace VK6ADA / r-390a.net Administrator • April 2026 vk6ada.com.au — Collins Radio Technical Resource

Collins S-Line and KWM-2/KWM-2ACrystal and Mechanical Filter Comprehensive Reference — All Part Numbers, Specifications, BFO Crystals, and Equipment Cross-Reference