My Strowger PBX · Volume 2
Bill of Materials
2.1 How to Read This List
This is everything the four-telephone exchange in Volume 1 needs, grouped the way the panel is built: switching, power, ringing and tones, telephones, the LED display, wiring, the panel itself, and test equipment. The Have column is filled in as the collection is inventoried. ? means not yet checked against the hardware. Quantities are for phase 1 (two finder-connector links, two-digit numbers); the phase-2 selectors are listed separately.
Strowger parts are not interchangeable just because they look alike. A connector, a selector and a line finder share the same two-motion mechanism but carry different relays, different off-normal springs and different wiring, and each one’s job is set by its circuit drawing (an H- number stamped on the switch; the selector in the collection is H-580059). For every switch below, the drawing number is the thing to record first.
2.2 Switching
Table 1 — 1. Switching
| # | Item | Qty | Have | Notes |
|---|---|---|---|---|
| 1.1 | Line finder, AE two-motion, 100-line, finder circuit | 2 | ? | Hunts up for the marked level, then round for the marked line. Bank wired to the four lines. (Bulletin 821) |
| 1.2 | Connector, AE two-motion, 100-line, local connector circuit | 2 | ? | Returns dial tone, takes both digits, busy-tests, rings, trips ringing, feeds talk battery. A PBX or “terminal per station” connector is ideal; a trunk-hunting connector also works. (Bulletin 807) |
| 1.3 | Banks for each switch (line + private), with bank cable | 4 | ? | Usually part of the switch. Check every bank’s wiring: the selector’s bank cable in the collection has been cut off just past the terminals. |
| 1.4 | Line relay + cut-off relay per line (or one AE line-and-cutoff relay) | 4 sets | ? | E-75159 uses one line-and-cutoff relay per line. Any 48 V telephone-type relays with the right contact springs will do. |
| 1.5 | Start relay | 1 | ? | Common to the group; operated by any line relay. |
| 1.6 | Allotter: two relays, or an AE distributor | 1 set | ? | Picks the idle finder and keeps both finders from hunting the same call. If one of my “distribution switches” is a distributor, it fits here. |
| 1.7 | Switch mounting: shelf brackets or the original shelf rails | 1 shelf | ? | Switches must stand upright (they release by gravity). |
| 1.8 | Relay mounting plates or rack strips | 2 | ? | For the line circuits and allotter. |
| 1.9 | Switch test jacks (optional) | 2–4 | ? | So the Type 26A tester can test each connector in place. |
For the crossbar trunk (the crossbar exchange on the back of the panel; My Crossbar PBX, Volume 3):
Table 2 — For the crossbar trunk (the crossbar exchange on the back of the panel; [My Crossbar PBX, Volume 3](https://telephones.fubsypoly.com/my-crossbar-pbx/vol-3/))
| # | Item | Qty | Have | Notes |
|---|---|---|---|---|
| 1.11 | Line relay + cut-off relay for line 66 | 1 set | ? | The trunk is a fifth line to this exchange. Same relays as 1.4. |
| 1.12 | Bank wiring for 66: finder level 1 contact 5, connector level 6 contact 6, on both finders and both connectors | 1 set | ? | Multipled like the other four lines (Volume 3). |
Phase 2:
Table 3 — Phase 2:
| # | Item | Qty | Have | Notes |
|---|---|---|---|---|
| 1.10 | First selector, AE two-motion | 2 | 1 (H-580059) | One per link, between finder and connector. Numbers become three digits. The one in the collection is a local or incoming, battery-searching selector, rated 45–52 V (H-580059-A); look for a second on the same circuit. (Bulletin 808) |
2.3 Power
Table 4 — 2. Power
| # | Item | Qty | Have | Notes |
|---|---|---|---|---|
| 2.1 | 48 V DC supply, adjustable, positive side to ground | 1 | ? | Set to about 49 V: the Type 26A tester wants 48.5–50 V while testing. Rate it in amps, because every magnet draws about an ampere while it steps. The Prince Edward Island demonstrator ran twelve telephones and three paths on a 48 V, 4.2 A supply; 5 A or more leaves room for phase 2. |
| 2.2 | Fuse panel, one fuse per switch and per relay group | 1 | ? | The P.E.I. demonstrator used a Hendry indicating fuse panel (alarm-type fuses show which one blew). Any fused distribution block works. |
| 2.3 | Mains inlet with switch and fuse, plus an internal power strip | 1 | ? | Inside the guarded power section. |
| 2.4 | Voltmeter and ammeter for the −48 V bus (panel meters) | 1 each | ? | AE’s P-A-X power shelves carried both; on a display they show the magnets drawing current as the switches step. |
2.4 Ringing and Tones
Table 5 — 3. Ringing and Tones
| # | Item | Qty | Have | Notes |
|---|---|---|---|---|
| 3.1 | Ringing generator, 20 Hz, about 75–90 V AC | 1 | ? | Commercial units of the Charles 8502/8503 type give 100 V AC at 20 Hz; the 8503 has a built-in 2-on, 4-off cadence. It must drive two bells at once (two calls ringing). Ringing must ride on −48 V battery so the connector’s ring-trip relay sees direct current between rings. |
| 3.2 | Interrupter for ring cadence (2 s on, 4 s off) and busy (60 interruptions a minute) | 1 | ? | Not needed for ringing if the generator has its own cadence; still needed for busy tone. A relay timer or a cam interrupter. |
| 3.3 | Tone source: dial tone, and busy (dial tone interrupted) | 1 | ? | The P.E.I. demonstrator used a Lorain dial-tone generator. Ringback comes from the ringing itself, through capacitors in the connector. |
| 3.4 | Tone distribution transformer or coupling capacitors | 1 | ? | As specified by the connector’s circuit drawing. |
A packaged small-PBX supply such as the Viking PS-48RGA gives ringing but only 150 mA of 48 V, far too little for Strowger magnets; it is useful only as a ringing source.
2.5 Telephones
Table 6 — 4. Telephones
| # | Item | Qty | Have | Notes |
|---|---|---|---|---|
| 4.1 | Rotary-dial telephones | 4 | ? | Any standard rotary set (a Western Electric 500 or an AE Type 80 or 90, for example). Each dial must pulse at 10 pulses a second. |
| 4.2 | Line cords and wall or desk jacks | 4 | ? | One per telephone, to the MDF. |
| 4.3 | Stands or a table for the telephones | 1 | ? | The telephones sit in front of the panel, where visitors can reach them. |
Qualify every telephone on the Radio Shack 43-114 before it goes on the exchange: dial speed, dial break, ringer and transmission.
2.6 LED Display
Table 7 — 5. LED Display
| # | Item | Qty | Have | Notes |
|---|---|---|---|---|
| 5.1 | LEDs, panel-mount, in three or four colours | about 40 | ? | Four off-hook, two finder hunting/found, and for each connector: dial tone, pulses, busy, ringing, answered, talk; plus the mimic strip. Volume 5 sets the final count. |
| 5.2 | Opto-isolators, one per sensed signal | about 25 | ? | Each senses a spare relay contact or off-normal spring at −48 V and lights an LED on a separate supply. Nothing in the display touches the talking path. |
| 5.3 | Resistors (opto inputs at 48 V; LED current) | 1 kit | ? | Values in Volume 5. |
| 5.4 | 12 V DC supply for the display | 1 | ? | Separate from the exchange battery. |
| 5.5 | Perfboard or a small PCB for the driver board | 1 | ? |
2.7 Wiring and Termination
Table 8 — 6. Wiring and Termination
| # | Item | Qty | Have | Notes |
|---|---|---|---|---|
| 6.1 | MDF terminal blocks (66-type blocks or screw terminal strips) | 2–3 | ? | Lines in from the telephones, cross-connects to the line circuits, test access. |
| 6.2 | Switchboard cable, 24 AWG tinned solid copper, multi-pair | about 100 ft | ? | Bank multiples, links, line circuits. |
| 6.3 | Cross-connect (jumper) wire | 1 spool | ? | |
| 6.4 | Hook-up wire for battery and ground, 18–20 AWG, red and black | 1 spool each | ? | Heavier, because magnet current is about an ampere per switch. |
| 6.5 | Lacing cord, cable ties, cable trough or raceway | 1 set | ? | Laced cable is part of the look. |
| 6.6 | Solder (rosin-core) and heat-shrink | 1 | ? | Old bank and relay tags are soldered. |
| 6.7 | Labels: switch names, lines, fuses | 1 set | ? | Yellow labels, as on the Waltham band. |
2.8 Panel and Frame
Table 9 — 7. Panel and Frame
| # | Item | Qty | Have | Notes |
|---|---|---|---|---|
| 7.1 | Panel, 48 × 36 in: ¾ in plywood or ½ in clear acrylic | 1 | ? | Clear acrylic shows the back wiring; plywood is stiffer and easier to fix things to. Weigh the switches before choosing: several switches on one shelf load the panel heavily. |
| 7.2 | Rolling frame: two uprights on a wide base, rails for the panel, four locking casters | 1 | ? | Keep the base wide; a tall panel with heavy switches near the top tips easily. |
| 7.3 | Clear covers for each switch, Waltham-style | 4 (6 in phase 2) | ? | Clear cylinders with metal end bands. Look for original switch covers from collectors first; otherwise fabricate from clear tube or bent polycarbonate. Design still to come. |
| 7.4 | Clear guard over the power, ringing and tone section | 1 | ? | Polycarbonate, on standoffs, screwed down. |
| 7.5 | Steel angle for the switch shelf, standoffs, fasteners | 1 set | ? |
2.9 Test Equipment and Maintenance
Table 10 — 8. Test Equipment and Maintenance
| # | Item | Qty | Have | Notes |
|---|---|---|---|---|
| 8.1 | AE Type 26A switch tester | 1 | ✓ | Needs its cords (see the Type 26A dive). |
| 8.2 | Radio Shack 43-114 telephone tester | 1 | ✓ | For the telephones. |
| 8.3 | Digital multimeter | 1 | ? | |
| 8.4 | Switch lubricants and cleaning supplies | 1 set | ? | Per AE Bulletin 505: sparingly. |
| 8.5 | Relay and spring adjusting tools, feeler gauges | 1 set | ? | Adjust only what fails (Standard Adjustments). |
2.10 Where to Find the Parts
Most of this list is ordinary catalogue stock: power supplies, fuses, meters, terminal blocks, cable, LEDs, plastics and extrusion. The switching gear is not. It has not been made for decades, and it rarely appears in shops or on auction sites. In late 2026 a search of the usual antique-telephone dealers and auction listings turned up only one Automatic Electric two-motion switch, and it was a test distributor, the wrong circuit for this exchange. The real market is the collectors’ own:
- The Telephone Collectors International mailing list (singingwires on groups.io), where collectors offer line finders, connectors, shelves, relays and occasionally whole demonstration systems.
- The TCI show in Lancaster, Pennsylvania, where heavy switches change hands without the cost and risk of crating and freight.
Collectors there agree on what is scarce: not the switches but wired banks, shelf jacks and shelf iron. That is why a small demonstration kit with its banks already cabled is worth more than its switches alone. Western Electric switches are offered more often than Automatic Electric ones. The two makes can share an exchange at the circuit level, but they do not drop into each other’s shelves.
Three things to check before paying for any switch:
- The circuit number (H- for AE, ED- or SD- for Western Electric).
- That its coils are 48 V. AE’s 10-line PAX of 1944 was a 24-volt design, so not every small-exchange part is 48 V.
- That its banks are present and their cables not cut.
Covers come with switches, and the originals are metal. Clear covers like the Waltham band’s are custom work, so they are listed here as something to make.
No published source gives the size or weight of an AE two-motion switch. A collector’s portable frame with four Western Electric switches reports that “the four step switches add about 70 pounds” (Alderdice). AE’s linefinder shelf works out to roughly five inches of shelf per switch (Bulletin 821). Measure a real switch before cutting the panel.
2.11 In the Reference Library
- AE Training Bulletin 812: Power and Supervisory Equipment: AE’s own power plants, ringing and tone equipment.
- Technical Bulletin 565: General Maintenance of P-A-X’s.
- The switch bulletins: 821 Linefinder, 807 Connector, 808 Selector.
Sources
- Power, ringing and tone figures (the P.E.I. demonstrator’s 48 V 4.2 A supply, Hendry fuse panel and Lorain tone generator; the Charles 8502/8503; the Viking PS-48RGA; the Type 26A’s 48.5–50 V): Telephone Switching History, Volume 8, which cites each one.
- Ring trip, ringback and busy tone: Telephone Switching History, Volume 5.
- Line-and-cutoff relays and the two-relay allotter: AE circuit explanation E-75159 (1944), Reference Library.