My Crossbar PBX · Volume 2
Bill of Materials
2.1 How to Read This List
This is everything the four-telephone crossbar exchange in Volume 1 needs, grouped the way it is built: the switch, the relay circuits, power, ringing and tones, the trunk to the Strowger system, telephones, the display, wiring and mounting, and the tools to bring it up. Nothing is owned yet, so the Have column is empty for now.
Two things make this list different from the Strowger build’s:
- The switching network is one part. A single crossbar switch does what the Strowger panel’s four two-motion switches do. Most of the work, and most of the part count, is in the relay circuits around it.
- Much of the common equipment can be shared. The Strowger panel’s 48-volt supply, ringing, interrupters and tones (My Strowger PBX, Volume 4) feed this exchange too: the crossbar system goes on the back of the Strowger panel (decided 26 October 2026), so it shares them (Volume 4).
Relay quantities are estimates until the circuits are drawn in Volume 3. They are worked out from the jobs the 755A’s six circuits did (Bell Laboratories Record, June 1938) and from the relays a Strowger connector uses for the same jobs. They are not copied from a 755A drawing, because none has been found.
2.2 The Crossbar Switch
Table 1 — 1. The Crossbar Switch
| # | Item | Qty | Have | Notes |
|---|---|---|---|---|
| 1.1 | Crossbar switch, 10 verticals × 5 selecting bars (100 crosspoints) | 1 | A Western Electric type-324 100-point switch is about 21 in long, 9 in tall and about 30 lb, “made mostly of steel”, with two magnets per selecting bar and one per vertical: 20 magnets (calling315.com). An Ericsson switch also works (Volume 1). Must be complete: every bar, finger, magnet and contact spring. | |
| 1.2 | A second switch, for spares or later growth | 0–1 | The 755A grew to twenty lines by adding a second switch that acted “as a single switch with twenty vertical bars.” A damaged switch is also a source of fingers and springs. | |
| 1.3 | Mounting brackets or a frame for the switch | 1 | The switch mounts vertically with its bars horizontal, as in the photos in Volume 1. 30 lb needs a proper bracket, not the panel skin alone. | |
| 1.4 | Terminal strips for the switch’s multiples | 2–3 | The verticals (lines) and horizontals (link and trunk ends) each bring out a set of leads; terminate them so the switch can be changed without rewiring. |
What to check on a switch before buying (Volume 1): completeness; the magnet voltage (the 755A ran on 18–25 V, the 756A on 48 V, so a switch may have 24 V or 48 V magnets; Bell System Practice 550-550-110, General Description, 755A PBX (1937)); the coil markings or resistances; any maker’s code on the frame, to find its drawing; and the condition of the contact sets at a few crosspoints. How many contact pairs each crosspoint has matters: a talking connection needs at least the two line wires plus one control lead.
Where to look. No working switch was confirmed for sale on 26 October 2026. The listings found were souvenirs, such as a Western Electric 1XB crossbar set in Lucite as a paperweight at an asking price of US$499.95 (eBay, transient). That is an asking price for a display piece, not a guide to what a working switch costs. The places to watch:
- eBay sold listings for “crossbar switch”, which show what switches actually fetch.
- The collector forums: Classic Rotary Phones has a Western Electric crossbar switch thread, and the Telephone Collectors International (TCI) community.
- Telephone museums and their sales of surplus, and the Lancaster and other collector shows listed in the Strowger build’s sourcing notes.
2.3 The Relay Circuits
The 755A ran its switch with six circuits (“the line circuit; the trunk circuit; the link circuit; the common control and cut-off circuit; the common timing, tone, and ringing circuit; and the ‘link-allotter’ circuit”), largely built from Western Electric “U and Y” type relays. The table estimates each one for four lines and two or three links.
Table 2 — 2. The Relay Circuits
| # | Circuit | Per | Qty | Relays (estimate) | Job |
|---|---|---|---|---|---|
| 2.1 | Line circuit | telephone | 4 | 2 each (line and cut-off), so 8 | Sees the handset lift, tells the common control, and later operates the line’s hold magnet. The same pair the Strowger panel uses. |
| 2.2 | Link circuit | link | 2–3 | about 8–10 each, so 16–30 | Everything a Strowger connector’s relays do except stepping: battery feed and supervision, dial-pulse following, busy test, busy tone, ringing, ring trip, ringback, release. |
| 2.3 | Register, part of each link | link | 2–3 | see the table below | Records the dialled number. |
| 2.4 | Common control and cut-off | exchange | 1 | about 4–8 | Operates the right select magnet, then the right hold magnet, then lets go. Only one select magnet may operate at a time, and it must stay operated until the hold magnet has pulled in (telephonetribute.com, ch. 6, via the sourcing notes). |
| 2.5 | Link allotter | exchange | 1 | 2–3 | Picks the idle link for the next call. |
| 2.6 | Trunk circuit, on vertical 9 | trunk | 1 | about 4–6 | To the Strowger P-A-X, looks like a telephone line: seizes it, passes the dial pulses. Incoming, it detects the P-A-X’s ringing, answers, and asks for a link to one chosen crossbar telephone, set by a selector switch (Volume 3). |
| 2.7 | Common timing, tone and ringing | exchange | 0 | 0 | Shared with the Strowger panel (section 4). |
The register depends on the numbering. The numbers are 61, 62, 63 and 64: two digits, like the Strowger panel’s 22–55, and never clashing with them. So each link’s register must record two digits, which can be counted three ways (the third, single-digit numbers, is shown for comparison):
Table 3 — The register depends on the numbering. The numbers are 61, 62, 63 and 64: two digits, like the Strowger panel's 22–55, and never clashing with them. So each link's register must record two digits, which can be counted three ways (the third, single-digit numbers, is shown for comparison)
| Register | Per link | For | Against |
|---|---|---|---|
| Relay counting chains, as the 755A’s “pulsing and register relays” | about 10–12 relays per digit | All relays, period-correct; the visitor sees each pulse counted | The largest relay count; two digits double it |
| A small rotary stepping switch per digit | 1 stepping switch per digit, plus 1–2 relays | Few parts; each pulse visibly steps a wiper, a link back to the Strowger panel | Not how Bell did it |
| Single-digit numbers (1–4, and 9 for the trunk) | one digit’s worth of either of the above | Halves the register | Numbering differs from the Strowger panel’s two digits |
Estimated total: with two-digit numbers, about 60–90 relays if the registers are relay counting chains, or about 35–60 if each digit is counted by a small stepping switch. The register choice is now the biggest single lever on the part count.
Which relays. Two routes, which can be mixed:
Table 4 — Which relays. Two routes, which can be mixed
| # | Item | Qty | Have | Notes |
|---|---|---|---|---|
| 2.8 | Period telephone relays: Western Electric U/Y-type or wire-spring, 48 V coils | per the circuits | Correct for a Bell crossbar exchange, and they turn up on the collector market (eBay: Western Electric relays, transient). Coil and spring combinations vary: buy against the circuit design, not by the pound. | |
| 2.9 | Modern plug-in relays, 48 V DC coil, four changeover contacts (for example the Omron MY4 family) | per the circuits | About US$15–19 each from distributors (search snippets, 26 October 2026; confirm the coil voltage on each listing). Easy to source and replace; four changeover contacts are enough for most jobs here. | |
| 2.10 | Sockets for 2.9 (for example Omron PYF14A) | one per relay | About US$3–10 each (search snippets). Lets a relay be swapped in seconds during bring-up. | |
| 2.11 | Relay mounting: rack strips, DIN rail or plates | 3–6 | One plate or rail per circuit, labelled, so each circuit can be watched and fault-found on its own. | |
| 2.12 | Spark suppression for every magnet and relay coil | per coil | The 1944 Strowger PAX used capacitors with series resistors across its magnet circuits (E-75159). Each select and hold magnet is an inductive load that throws a spike when released. | |
| 2.13 | Link cutout keys, one per link | 3 | As on the 755A (CO1–CO3): take a link out of service so calls are forced onto the others, for testing (Volume 6). |
2.4 Power
Table 5 — 3. Power
| # | Item | Qty | Have | Notes |
|---|---|---|---|---|
| 3.1 | 48 V DC supply, positive side to ground | 0 | Shared: the Strowger panel’s supply (a 10.5 A unit, My Strowger PBX, Volume 4); check its steady load with the hold magnets (Volume 4). The 756A ran on 48 V (BLR, December 1957), so a Bell switch is expected to have 48 V magnets, but this must be confirmed on the switch bought. | |
| 3.2 | Fuses for the crossbar exchange | 7 | X1–X7 (Volume 4). The Strowger fuse panel’s ten positions are already assigned, so it becomes a 20-position panel, or gains a second panel. |
Current. Unlike a Strowger switch, a crossbar switch draws current for the whole call: each held crosspoint needs its vertical’s hold magnet operated (US 3,400,225, via the sourcing notes). Two calls hold four verticals at once, on top of the link relays. The hold-magnet current is not known until a switch is measured, so the supply rating is checked then.
2.5 Ringing and Tones
Table 6 — 4. Ringing and Tones
| # | Item | Qty | Have | Notes |
|---|---|---|---|---|
| 4.1 | Ringing generator, interrupters, dial and busy tone | 0 | Shared with the Strowger panel’s common equipment: the crossbar links take RG, DT and BT from the same leads as the Strowger connectors (My Strowger PBX, Volume 4). The generator must ring up to four bells at once. |
2.6 The Trunk to the Strowger System
Table 7 — 5. The Trunk to the Strowger System
| # | Item | Qty | Have | Notes |
|---|---|---|---|---|
| 5.1 | Trunk circuit relays | in 2.6 | ||
| 5.2 | A fifth line on the Strowger P-A-X: line and cut-off relays | 1 set | The trunk is a telephone line to the P-A-X. Its number goes on a free connector bank position (for example 66), with its line equipment beside the other four. | |
| 5.3 | Bank wiring for the new number, on both finders and both connectors | 1 set | The Strowger panel’s bank map leaves room: every position except the four lines is unwired (My Strowger PBX, Volume 3). | |
| 5.4 | Cable between the two systems | 1 | One pair plus a spare, through the panel to line 66’s equipment on the Strowger side. |
2.7 Telephones
Table 8 — 6. Telephones
| # | Item | Qty | Have | Notes |
|---|---|---|---|---|
| 6.1 | Rotary-dial telephones | 4 | A Western Electric 500 suits a Bell crossbar exchange. Qualify each one on the Radio Shack 43-114 before it goes on the exchange: dial speed, ringer, transmission. | |
| 6.2 | Line cords and jacks | 4 |
2.8 The Display
Table 9 — 7. The Display
| # | Item | Qty | Have | Notes |
|---|---|---|---|---|
| 7.1 | Clear cover for the switch | 1 | So visitors see the bars rock and the verticals snap in without reaching the contacts. | |
| 7.2 | LEDs: a 30-LED crosspoint mimic (lines × link rows), plus lights for the bars, lines, links, trunk and common control | about 60 | The mimic LEDs are switched by a spare contact pair at each crosspoint (the 755A’s switch had six pairs per crosspoint; the call uses three), so they need no opto-isolators. The rest watch relays through opto-isolators, as on the Strowger panel (Volume 5). | |
| 7.3 | Opto-isolators | about 30 | The 12 V display supply is shared with the Strowger panel. Nothing in the display touches the talking path. |
2.9 Wiring and Mounting
Table 10 — 8. Wiring and Mounting
| # | Item | Qty | Have | Notes |
|---|---|---|---|---|
| 8.1 | Switchboard cable and hook-up wire | as needed | As for the Strowger panel. | |
| 8.2 | Terminal blocks: telephones, trunk, power | 3 | ||
| 8.3 | Board for the crossbar system | 0 | None: it goes on the back of the Strowger panel, leaving the right-hand third of the back free for text and photographs (Volume 4). | |
| 8.4 | Labels: switch rows and verticals, circuits, fuses | 1 set | Yellow labels, matching the Strowger panel. |
2.10 Bring-Up and Test
Table 11 — 9. Bring-Up and Test
| # | Item | Qty | Have | Notes |
|---|---|---|---|---|
| 9.1 | Microcontroller board and 20 magnet drivers (transistor or relay outputs, each with a flyback diode) | 1 set | Route C in Volume 1: prove the switch’s 20 magnets and 100 crosspoints before any relay circuit is built. Each crosspoint is closed by operating a select magnet, then a hold magnet, then releasing the select magnet (calling315.com). | |
| 9.2 | Continuity tester or multimeter | 1 | To check every crosspoint’s contacts close and open. | |
| 9.3 | Contact burnisher, relay adjusting pliers, feeler gauges | 1 set | Shared with the Strowger panel’s tool kit. |
2.11 Shared with the Strowger Panel
The crossbar system goes on the back of the Strowger panel (decided 26 October 2026). The table shows what that choice shares, beside the alternative it replaced:
Table 12 — The crossbar system goes on the back of the Strowger panel (decided 26 October 2026). The table shows what that choice shares, beside the alternative it replaced
| If the crossbar system goes… | Shared | Extra |
|---|---|---|
| on the back of the Strowger panel (chosen) | 48 V supply, fuse panel, ringing, interrupters, tones, display supply, tools | access to both faces; wiring through the panel |
| on its own board, beside it | tools; the trunk cable joins the two | its own 48 V supply, fuses, ringing and tones, display supply |
2.12 In the Reference Library
- Bell Laboratories Record, June 1938: the 755 PBX’s six circuits.
- Bell Laboratories Record, July 1937: the switch itself.
- BSP 551-100-100 and 551-100-101: installing, testing and repairing a real small crossbar PBX.
- Crossbar Systems, Peter Walker.
Sources
- Bell System Practice 550-550-110, General Description, 755A PBX, issue 1 (1937), Reference Library.
- J. N. Reynolds and C. R. Gray, Bell Laboratories Record, June 1938, Reference Library.
- O. H. Williford, “A Modern Crossbar PBX”, Bell Laboratories Record, December 1957, Reference Library.
- Western Electric type-324 switch size, weight, magnets and drive sequence: calling315.com.
- Driving rules and hold-magnet current: the sourcing notes (
02-inputs/research/crossbar_options.md), citing telephonetribute.com and US 3,400,225. - Relay and socket prices: distributor search snippets, 26 October 2026; check live before ordering.
- My Strowger PBX, Volumes 2–4, on this site.