The XCB-32: Building the Rarest S-Line Transmitter

A reversible Extended Crystal Board kit that converts a Collins 32S-1 into a genuine-specification 32S-2

The Transmitter Art Collins Un-Named

Ask a room full of Collins collectors to name every S-Line transmitter and most will get the 32S-1 and the 32S-3/3A without hesitation. The 32S-2 is the one that trips people up — and for good reason. After the 32S-1 and 75S-1 reached the market, Collins began fielding requests from MARS operators, embassy stations, and commercial users who wanted the same equipment with convenient coverage outside the amateur bands. The factory’s answer was simple: build stock 32S-1 and 75S-1 units on the same production line with the optional second crystal board installed, and give them the 32S-2 and 75S-2 designations.[1]

Art Collins reportedly put a stop to the separate model number almost immediately, ruling that the change was too small to justify one.[1] When the 32S-3 generation arrived, the extended-crystal variant was demoted to a suffix — the 32S-3A — and the same convention held across the family: KWM-2/KWM-2A, 75S-3/75S-3A, 75S-3B/75S-3C. In every pairing, the second model is the first model with the extra crystal board fitted, and nothing else. There is no other circuit difference between units built in the same period.[2]

That short production window is why genuine 32S-2s are seldom seen — and it is also why the conversion is one of the most honest “upgrades” you can perform on an S-Line piece. You are not modifying the design. You are completing a factory option, sixty-odd years late.

What the XCB-32 is: a second bank of fourteen HC-6/U injection-crystal positions, wired in parallel with the original crystal deck and selected by a return-side bank switch — the same architecture Collins used at the factory. What it is not: a performance modification. It adds band coverage, nothing more.

Design Philosophy: Reversibility Above All

Like the KRM-2A relay modernization kit, the XCB-32 is designed around one non-negotiable rule: the radio must be restorable to bone-stock 32S-1 condition in under thirty minutes, with no evidence the kit was ever fitted. In practice that means:

  • No holes drilled. The board mounts on existing chassis hardware points adjacent to the original crystal deck, and the bank-select toggle rides a bracket captured under an existing screw.
  • Exactly one desoldered joint on the radio: the original crystal deck’s return is lifted from its chassis ground point and re-routed through the bank switch. Photograph and tag that joint before you touch it.
  • Every pigtail lands on an existing terminal or socket pin with a soldered connection that can be removed cleanly. Original harness lacing is teased open, never cut.
  • Anything removed goes into a labeled bag that stays with the radio.

How It Works

Each of the fourteen XCB-32 crystal positions connects to the hot pin of the corresponding original crystal socket, so the transmitter’s bandswitch selects position-for-position on either bank. Bank selection is done on the return side: the toggle grounds either the XCB-32 return bus or the original deck’s return. The unselected bank’s crystals sit open-circuit on the return end — exactly the compromise Collins accepted — contributing only a small fixed stray capacitance to the bandswitch node.

XCB-32 BANK-SELECT ARCHITECTUREHF injection crystal oscillator(hot pin — verify against Instruction Book)Bandswitch crystal waferselects position 1–14 on BOTH bankshot pins paralleled position-for-position — pigtails < 75 mmBANK AOriginal crystal deckpositions 1–14 (stock, untouched)BANK BXCB-32 boardY1–Y14 + series trimmers C1–C14ORIG_RETXCB_RETthrow B (original)throw A (XCB-32)chassis groundSW1 — bank select(C&K 7201, pole 2 spare)original deck return LIFTED from chassis groundand re-routed to SW1 — the one desoldered joint
Figure 1 — Bank-selection architecture. The bandswitch drives both crystal banks in parallel; SW1 grounds the return of one bank at a time. Shown selected to Bank B (XCB-32). The unselected bank sits open-circuit on the return side, per Collins factory practice.

Every position carries a series ceramic trimmer (5.2–30 pF Murata TZ03)[3] for per-channel netting, following Collins deck practice. This matters more than it looks: the high-band injection crystals operate in overtone mode, where added stray capacitance from the second board and its wiring will pull frequency and, in the worst case, impair oscillator starting. The layout discipline that follows from this is simple and strict — every pigtail stays under 75 mm, the board sits as close to the original deck as the chassis allows, and the acceptance test for any installation is a before-and-after measurement of injection level on the highest band.

ONE CHANNEL (×14 IDENTICAL)PBnpigtail from originalsocket hot pin (BANDnn)Yn — HC-6/UCn — TZ03 5.2–30 pFseries netting trimmerXCB_RET busto SW1 throw A — see Fig. 1
Figure 2 — Channel schematic. Each crystal drives the common XCB_RET bus through its own series netting trimmer, so every channel can be zeroed against the receiver or a counter after installation.

Crystal frequencies are ordered per the crystal chart in the 32S-1/32S-2 Instruction Book[4] — reproduce the factory chart rather than deriving frequencies yourself, because the fundamental-versus-overtone cut changes by band. New-manufacture HC-6/U crystals cut to the Collins chart are available from Krystaly (Hradec Králové, Czech Republic)[5], and readers who prefer the traditional path are directed to my quartz crystal grinding reference elsewhere on this site.

The Board

The Rev A board is a two-layer 1.6 mm FR-4 design, ENIG finish, drafted in KiCad. Fourteen HC-6/U positions are arranged in two rows of seven at 21 mm pitch, each socket built from a pair of Mill-Max precision receptacles[6] rather than a stamped socket — the machined shell and beryllium-copper contact clip give a gas-tight grip on the crystal pins and survive repeated crystal swaps. Trimmers sit directly below their crystals for shortest lead length. Pigtail pads run along the lower edge, the back copper is a stitched ground pour, and all four mounting positions are grounded pads that bond the pour to the chassis through brass standoffs and star washers.

XCB-32 REV A — BOARD PLAN (TOP VIEW)OUTLINE PLACEHOLDER — FINALIZE EDGE.CUTS AGAINST CHASSIS SURVEYY1C1Y2C2Y3C3Y4C4Y5C5Y6C6Y7C7Y8C8Y9C9Y10C10Y11C11Y12C12Y13C13Y14C14B01B02B03B04B05B06B07B08B09B10B11B12B13B14RETGNDXCB-32 Rev A • 32S-1 ▸ 32S-2 • VK6ADA21.0 mm pitch160.0 mm (placeholder)62.0 mm (placeholder)● 4× grounded mounting holes, Ø3.4 mm, on EXISTING chassis hardware points — no new holes.Pigtail pads B01–B14, RET, GND along lower edge. Back copper: stitched GND_CHASSIS pour bonded through standoffs.
Figure 3 — Rev A board plan. Two rows of seven HC-6/U positions at 21 mm pitch with each series trimmer (red) directly below its crystal; pigtail pads along the lower edge nearest the original deck.
HC-6/U SOCKET DETAILHC-6/Ucrystal12.34 mm (0.486″) pin spacingø .050″ nominal pinBoard cross-sectionMill-Max 3490-0-15-15-02-27-10-0FR-4, 1.6 mmdrill 1.7 mm PLACEHOLDER — verify vs. Mill-Max drawing;trial-fit a receptacle on a real crystal pin firstMachined shell + BeCu contact clip: gas-tight grip, survives repeated crystal swaps.
Figure 4 — Socket construction. Each HC-6/U position is a pair of Mill-Max precision receptacles at 12.34 mm spacing — the crystal plugs straight in, no stamped socket.

The published outline of 160 × 62 mm is a drafting placeholder. Chassis clearances vary with production era and installed accessories, so the Edge.Cuts layer and mounting-hole pattern must be finalized against measurements of the actual radio, and the oscillator attachment points confirmed against the Instruction Book and the combined 32S-1/2 service bulletin held in the Collins Collectors Association archives[4]. The silkscreen carries that warning, and so will the instruction manual.

Full Parts List

Quantities are per kit. Distributor availability was checked at time of writing; hardware items are specified generically because final lengths depend on chassis measurement.

Item Description Vendor / Part Number Qty
PCB XCB-32 Rev A, 2-layer FR-4, 1.6 mm, ENIG, silkscreened band legend Fab from KiCad files (JLCPCB or OSH Park) 1
Crystal receptacles Mill-Max pin receptacle, accepts .040–.050 in pins (HC-6/U pin is nominal .050 in — trial-fit before panel drilling); 2 per crystal position Mill-Max 3490-0-15-15-02-27-10-0 (Mouser / DigiKey) 28
Netting trimmers Ceramic trimmer, 5.2–30 pF, 100 V, through-hole, top adjust Murata TZ03R300E169B00 (DigiKey / Mouser) 14
Bank-select switch Sealed miniature DPDT toggle, on-on, solder lug (one pole used, one spare) C&K 7201SYZQE (Mouser / DigiKey) 1
Coax pigtails RG-316/U, MIL-DTL-17, pre-cut and tinned; hot-lead runs kept under 75 mm Any MIL-DTL-17 RG-316/U, 2 m (Mouser, Pasternack) 2 m
Standoffs Brass hex standoff, 4-40, length finalized after chassis measurement; with 4-40 screws and internal-tooth star washers for ground bonding Keystone Electronics brass hex series (Mouser); hardware per McMaster-Carr 4
Switch bracket 0.062 in 5052 aluminum angle, captured under existing chassis screw — no new holes Custom fabrication (drawing in kit docs) 1
Dressing Waxed polyester lacing tape to match original harness dress; assorted heat-shrink Breyden waxed lacing tape; generic polyolefin heat-shrink 1 set
Crystals HC-6/U, custom cut per the Collins crystal chart for the operator’s chosen coverage — supplied by the builder, not the kit Krystaly Hradec Králové (custom order) up to 14
Documentation Printed installation manual with photographic attachment-point guide, crystal ordering chart reproduced from the Instruction Book, netting procedure, and restoration (removal) procedure Included 1

Installation Outline

  1. Pull the transmitter from its cabinet, discharge the power supply, and photograph the crystal deck area and its return ground point from several angles.
  2. Verify the oscillator hot-pin attachment points and the deck return location against the Instruction Book and the 32S-1/2 service bulletin.[4] Do not proceed on memory or on this article alone — production-era wiring differences exist.
  3. Mount the XCB-32 on its standoffs at the existing hardware points; confirm ground continuity from the pour to chassis of well under an ohm.
  4. Fit the toggle bracket, then lift the original deck return (the one desoldered joint), extend it to the switch, and wire the switch common to chassis ground and the second throw to the XCB-32 return pad.
  5. Land the fourteen band pigtails on the original socket hot pins, dressing them into the harness with lacing tape. Shortest practical runs, always under 75 mm.
  6. Install crystals, then net each channel with its trimmer against the station receiver or a counter, per the manual procedure.
  7. Acceptance test: measure injection level on the highest band with the bank switch in each position and compare against the pre-installation reading. Equal readings mean the installation passes.
INSTALLATION — CHASSIS TOP VIEW (SIMPLIFIED, NOT TO SCALE)32S-1 chassisOriginalcrystal deck(Bank A — untouched)return ground pointXCB-32(Bank B)4× brass standoffs + star washerson EXISTING hardware points — no new holes14 laced pigtails< 75 mmSW1 on bracket(existing screw)lifted return (ORIG_RET) — the one desoldered jointXCB_RETPhotograph and tag the return ground point BEFORE lifting it.Everything removed goes in a labeled bag that stays with the radio.
Figure 5 — Installation pictorial. The board mounts beside the original deck on existing hardware points; the only desoldered joint on the radio is the deck return, re-routed to SW1.

A word on honesty: a converted 32S-1 becomes a 32S-2 in capability, not in provenance. If the radio is ever sold, describe it as a 32S-1 with the XCB-32 extended crystal option fitted. The kit’s full reversibility exists precisely so the collector market never has to guess.

The KiCad schematic and draft board layout are available for download alongside this article. Rev A is a working draft — the outline, mounting pattern, and attachment points are finalized only after measurement of a specific chassis, and I will publish the Rev B production files together with the prototype build log and injection-level measurements in a follow-up article.

Notes & Sources

  1. Collins Collectors Association historical archives, “32S-2 & 75S-2, Driver for the 204H-1” by K0CXX — the origin of the S-2 designations as production-line 32S-1/75S-1 units with additional band crystal positions, and Art Collins’s decision to retire the separate model number. collinsradio.org/32s-3-75s-3c-winged ↩
  2. Dr. Gerald N. Johnson, K0CQ (Technical Advisor to the CRA), Collins reflector post, July 2005 — the model pairings KWM-2/KWM-2A, 75S-1/75S-2, 75S-3/75S-3A, 75S-3B/75S-3C, 32S-1/32S-2, 32S-3/32S-3A, in each case the second model being the first with the optional second crystal board kit installed and otherwise no circuit difference between contemporaneous units. Archived at qth.net/pipermail/collins ↩
  3. Murata TZ03R300E169B00 ceramic trimmer, 5.2–30 pF, 100 V, through-hole, top adjust — distributor listing at DigiKey. Lead spacing per the Murata T13 trimmer catalog. ↩
  4. Combined 32S-1/32S-2 Service Bulletin, Collins Radio Company — scanned copy in the CCA manual archive: collinsradio.org/archives/manuals/32S-1_2SB_2.pdf. Consult together with the 32S-1/32S-2 Instruction Book for the crystal ordering chart and production-era wiring details. ↩
  5. Krystaly, Hradec Králové, a.s. (Czech Republic) — current manufacturer of custom-cut HC-6/U quartz crystals: krystaly.cz ↩
  6. Mill-Max Mfg. Corp. — precision-machined pin receptacles with beryllium-copper contact clips; construction and pin-acceptance details in the Mill-Max engineering notebook “The Contact Clip” and the pin receptacle product pages. Confirm the 3490-series drill and pin-acceptance range against the current Mill-Max drawing before board fabrication. ↩

Acknowledgements & Credits

The historical record drawn on here exists because the Collins Collectors Association has kept the factory manuals, service bulletins, and equipment histories scanned, organized, and freely available — support them. Particular credit to K0CXX for documenting the S-2 story and to Dr. Gerald N. Johnson, K0CQ, whose decades of reflector posts remain the definitive technical commentary on S-Line production practice. Thanks also to the Antique Radio Forums Collins community for keeping this equipment on the air and the questions honest.

All five figures are original drawings by the author, prepared for this article; they may be reproduced for personal, non-commercial restoration use with attribution to vk6ada.com.au. The XCB-32 is an independent hobby project by the author. It is not affiliated with, endorsed by, or connected to Collins Aerospace or any successor to the Collins Radio Company; Collins names and marks are the property of their respective owners and are used here solely for historical identification of the equipment discussed.

Mike Peace VK6ADA / r-390a.net Administrator