Setting Up and Troubleshooting a Small Strowger System

Figure 1 — A Strowger Type 2000 final selector from a 1970s unit automatic exchange, mounted on a test stand for demonstration at the Museum of Technology, Cambridge. Out of its shelf, on a stand, every part … — Figure 1 — A Strowger Type 2000 final selector from a 1970s unit automatic exchange, mounted on a test stand for demonstration at the Museum of Technology, Cambridge. Out of its shelf, on a stand, every part of a switch can be watched, adjusted and tested, which is how a small system should be brought up. — Museum of Technology, Cambridge (museumoftechnology.org.uk)

8.1 Before Anything Else

This volume turns the theory of Volumes 3–5 into practice: how to power a small step-by-step system, bring switches back into service, test them, and find faults when a call fails. It draws on the maintenance chapters of the US Air Force course, which taught exactly this job on Automatic Electric-pattern equipment (CDC 36251, Vols. 2–3, ERIC ED194781); the US Army’s 1950 manual on step-by-step test equipment, TM 11-2111; Automatic Electric’s own bulletins on rotary switches, lubrication and maintenance tools; and a documented hobby installation, the step-by-step demonstrator built for the Telephone Museum of Prince Edward Island in 2006–07 (islandregister.com).

Three cautions apply throughout.

Ringing is dangerous. About 75–100 V RMS at 20 Hz, on top of −48 V DC, reaches roughly 175–190 V peak. It is enough to hurt, and it sits on the same wires as everything else.
The magnets draw real current. A connector’s 57 Ω rotary magnet takes about 0.84 A at 48 V, and about 0.9 A at 52 V, plus its parallel relay: call it 1 A per magnet while it is operated. A stuck magnet left energised will overheat; fuse every shelf, to the ratings on the drawings (the USAF course’s office used 1, 3½ and 5 A indicating fuses).
The USAF course’s first rule: “As long as a switch performs its function, we suggest that you make no attempt to adjust it.” Test first, adjust only what fails, and adjust to the manufacturer’s figures.

8.2 Power

8.2.1 The 48-Volt Supply

The office battery is nominally 48 V with the positive side grounded (Volume 5). The USAF course’s office ran 46–52 V DC from constant-voltage chargers floating a 23-cell lead-acid battery, with a low-voltage alarm below 46 V and a high-voltage alarm at 53 V. The Type 26A switch tester’s own circuit drawing adds an important limit: the tester “is designed to apply proper marginal tests when the circuit under test and the test equipment have an applied voltage of 48.5 to 50 volts. If for some reason the power plant is operated at more than 50 volts, the battery voltage should be reduced” while testing (H-85919-A, note 55). A bench supply set to about 49 V suits both the switches and the tester.

The supply must be rated in amps, not milliamps. The Prince Edward Island demonstrator, with twelve telephones and three paths through line finders, selectors and connectors, ran on a regulated 48 V, 4.2 A switching supply, distributed through a Hendry indicating fuse panel. By contrast, a packaged small-PBX supply such as the Viking PS-48RGA provides 48 V at only 150 mA alongside its ringing output. That is plenty for telephones and far too little for Strowger magnets, so a unit like that is useful only as a ringing source.

8.2.2 Ringing and Tones

Ringing. The P.E.I. demonstrator used a Tellabs ring generator with a separate ring interrupter. Commercial ringing generators of the Charles 8502/8503 type give 100 V AC at 20 Hz ± 2 Hz, the 8503 with a built-in 2-seconds-on, 4-seconds-off cadence. The ringing must pass through an interrupter, and the generator must be connected in series with battery (or the interrupter must switch the ringing lead to battery between rings), so that the connector’s ring-trip relay sees direct current during the silent intervals as well as the rings (Volume 5; Smith and Campbell describe the lead switched alternately between generator and ground). The P.E.I. builder found his ringing “VERY loud.”
Dial tone. The P.E.I. demonstrator used a Lorain dial-tone generator. The 1944 10-line PAX (Volume 6) made its tones with a relay; the USAF course’s office used a 600 Hz tone modulated at 120 Hz.
Busy and ringback. Busy is dial tone interrupted 60 times a minute. Ringback is taken from the ringing itself, through capacitors in the connector.

Figure 2 — A bench power arrangement for a small system: a 48 V supply rated in amps, positive side grounded, through a fuse panel to each shelf; a 20 Hz ringing generator through an interrupter to the connec… — Figure 2 — A bench power arrangement for a small system: a 48 V supply rated in amps, positive side grounded, through a fuse panel to each shelf; a 20 Hz ringing generator through an interrupter to the connectors' ringing lead; and a tone source for dial tone and busy. It is a pattern to adapt, not a wiring diagram. — Original diagram drawn for this dive

Figure 3 — A telephone exchange ringing machine: a motor-generator with tone and interrupter cams on one shaft. Machines like this made ringing current, ringing cadence and tones together for a whole office; … — Figure 3 — A telephone exchange ringing machine: a motor-generator with tone and interrupter cams on one shaft. Machines like this made ringing current, ringing cadence and tones together for a whole office; on a bench, a solid-state ring generator and interrupter do the same job. — Richard Ash, via Wikimedia Commons (CC BY-SA 2.0)

8.3 Bringing a Switch Back into Service

8.3.1 Inspect and Clean

Switches that have been stored for decades usually need cleaning more than adjustment. The USAF course cleans relay contacts with “a piece of bond paper” drawn between them, and moving parts with a lint-free cloth. Burnishers are for relay contacts only. It also warns that mechanical faults masquerade as electrical ones: “Helical springs with too much tension will often lead you to believe that either the pulsing relay … or the [holding] relay … is not working properly. The double dog spring with insufficient tension gives a similar response.”

8.3.2 Lubricate, Sparingly

Automatic Electric’s bulletin on its rotary switches is emphatic: “Excessive lubrication should be avoided.” Clean bearing surfaces first. Don’t add graphite oil where graphite already covers the surface; if the graphite is dry, add a little spindle oil; if it is caked or gritty, clean it off and relubricate. Run the switch several times afterwards and wipe excess oil from surfaces that are not bearings. AE even standardised the quantity, the “dip”: “that amount of oil retained in the bristles of a No. 4 artist’s sable rigger brush after it has been dipped into a lubricant to a depth of 3/8 inch and then drawn across the edge of a container” (AE TB 961-473). AE’s lubricants:

Table 1 — Automatic Electric's bulletin on its rotary switches is emphatic: "Excessive lubrication should be avoided." Clean bearing surfaces first. Don't add graphite oil where graphite already covers the surface; if the graphite is dry, add a little spindle oil; if it is caked or gritty, clean it off and relubricate. Run the switch several times afterwards and wipe excess oil from surfaces that are not bearings. AE even standardised the quantity, the "dip": "that amount of oil retained in the bristles of a No. 4 artist's sable rigger brush after it has been dipped into a lubricant to a depth of 3/8 inch and then drawn across the edge of a container" ([AE TB 961-473](https://archive.org/details/how-to-use-rotary-stepping-switches-automatic-electric-1964)). AE's lubricants

Lubricant	AE specification
spindle oil: bearings, bank contacts, wiper tips	5231
graphite oil lubricant, grade C	5232
blended lubricating oil	5684
low-temperature lubricant, substituted for 5232 at −20 °F and below	5563
standard dial lubricant, substituted for 5684 at −20 °F and below	5660

Automatic Electric’s full lubrication instructions for switching equipment are in its Bulletin 505, in the Reference Library.

8.3.3 Adjust Only What Fails

The USAF course’s adjustment figures for the two-motion switch (Vol. 2 §2-1):

Vertical play. “The free vertical play of the shaft should be between 0.002 inch and 0.010 inch.” Above 0.010 in, the vertical magnets must be raised; below about 0.005 in, lowered.
Vertical pawl centred on the ratchet.
Vertical dog clearance against the underside of the ratchet teeth controls vertical overthrow: bend it up if the shaft overthrows.
Double-dog spring: “approximately 250 to 400 grams tension is required to secure the vertical detent.”
Stationary detent must pass freely through the ratchet groove.
Rotary armature: pulled too far, the switch oversteps; not far enough, it does not step; a misset backstop or pawl guide gives erratic stepping.
Release: the armature stroke must kick the double dog clear of both ratchets, and the release link must drop over the dog’s lug without binding.

The tools are an open-end wrench, an offset screwdriver, a T-handle socket wrench, an L-gauge for large clearances, a double-dog bender, a gram gauge, and a fish scale for heavy springs. Relay airgaps are set with thickness gauges between the residual screw and the core (“a .003-inch gauge should fit loosely”), and relays are proved with current-flow tests for operate and non-operate values. Automatic Electric’s maintenance-tools catalogues (Bulletin 540 of 1960 and TB 150-540 of 1963), its Standard Adjustments compilation (more than 900 pages), its wiper-adjustment practice for two-motion switches (GSP 230-005-702, 1963), and the Bell System’s 197-type switch requirements (BSP 030-705-701) are all in the Reference Library.

Figure 4 — A Japanese Type A (Strowger) selector out of its shelf at the NTT History Center of Technologies. With the switch out, the relays, magnets, ratchets, wipers and the plug-in shelf jack at the base a… — Figure 4 — A Japanese Type A (Strowger) selector out of its shelf at the NTT History Center of Technologies. With the switch out, the relays, magnets, ratchets, wipers and the plug-in shelf jack at the base are all within reach of a gauge. — Darklanlan, via Wikimedia Commons (CC BY 4.0)

8.4 Test Equipment

A step-by-step office kept a standard kit of test sets, listed in the US Army’s TM 11-2111 (1950):

The test handset, “probably the most versatile piece of test equipment in your exchange” (USAF course). Plugged into a switch’s test jack, it lets the maintainer seize the switch, listen for dial tone, dial a test level, and watch the switch work. The handset test plug goes into “the two top selector test jack springs,” or the two bottom springs for line-finder group jacks.
The stepping-switch test set (“also called impulse testing machine, varying machine, interrupter machine, dial pulse generator”), which sends accurate pulses over a loop that can be made deliberately bad: in the USAF course’s version, a loop key inserts 1000 Ω and a shunt key adds a 15,000 Ω leak. It “must warm up for 30 minutes.”
The dial speed and pulse-ratio test set, for checking telephones’ dials against 10 pulses a second and about 61 per cent break.
The current-flow test set, for relay operate and non-operate margins.
The connector routine test set, which tests regular and PBX connectors for stepping over high-resistance and leaky loops, busy tone, ringing and ringback, talking, and release “when the last party hangs up.”

Automatic Electric’s portable connector routine test set was the Type 26 Switch Tester, later the Type 26A: a self-contained case with its own dial, loop-resistance and leak keys, a calling cord to the switch’s test jack, and the tests for ring trip and false trip that no handset can make. My own Type 26A, and how it works, has a dive of its own. For the telephones themselves, a set tester such as the Radio Shack 43-114 (a bench console that simulates a central office and grades a telephone set) checks each instrument’s dial, ringer and loop before it goes on the switch (RS 43-114 dive).

8.5 A Bring-Up Sequence

Step-by-step’s strength for a builder is that it can be brought up one stage at a time, each stage tested before the next is connected. This order follows the switch train:

Power alone. Supply at about 49 V, positive grounded; every fuse at its drawing value; no switches plugged in. Check the voltage at each shelf’s fuse.
Telephones. Check each set’s dial (speed and ratio), ringer and loop resistance on a set tester before it is connected.
Line circuits. With a telephone off-hook, the line relay should operate partially (first step) and mark the line; with a test ground applied to the sleeve, the cut-off step should operate fully and release the line relay.
Line finder. Plug in the finder (and its distributor or start relays). Lift a handset: the finder should step up to the marked level, round to the line, stop, and operate the cut-off. Hang up: it should release cleanly to normal.
First selector, if fitted. With a test handset on its test jack: dial tone; dial a level; watch the vertical steps and the hunt along the level; check that a busy trunk is passed over; check the 11th step and all-trunks-busy tone on a level with every trunk busied.
Connector. Dial a line: tens up, units round; busy tone on a busy line; ringing and ringback on an idle line; ring trip on answer; talk both ways; release.
The whole train. Calls from every telephone to every other, with the call-progress display watching.

8.6 Fault-Finding by Symptom

The USAF course teaches fault-finding from the symptom back to the stage and then to the contacts. The table below follows its worked troubles and answer key.

Table 2 — Fault-Finding by Symptom

Symptom	Where to look first
Every idle finder steps to one line and stays there	That line’s line-and-cutoff relay: dirty or shorted contacts, maladjusted springs, or the relay not operating fully. “When only one subscriber’s telephone is affected, the most probable trouble is the L relay.”
A whole finder shelf won’t step	A shorted vertical or rotary interrupter in one finder. The interrupter leads of all the finders are multipled to the group relays, so one fault locks the pulsing relay and “no vertical stepping takes place in that shelf.”
No dial tone anywhere in a group	All finders busy or out of service; no shelf power; the dial-tone supply.
No dial tone on one line	The line relay not operating: an open line, a high-resistance loop, a cross-connection error.
Dial tone doesn’t stop when dialling	The selector’s A relay not following the pulses: dial speed or ratio outside limits (8–14 pps, about 38½ per cent make), a high-resistance or leaky loop, or a sluggish A relay.
The selector is seized but releases at once	Its B relay not holding the finder: B’s ground to the sleeve wiper.
The selector steps up but never hunts	The rotary-magnet path through B and E; or E failing because of off-normal or C-relay contacts.
The selector hunts, then releases	The cut-through relay’s holding path: cam springs, rotary interrupter, off-normal springs, a dirty sleeve wiper or bank contact, or the next switch’s B relay.
Wipers stuck at the 11th step, release magnet not operating	A mechanical fault in the cam springs or adjustment, given in the course as “not an electrical problem”.
Wrong numbers, double stepping	An overstepping rotary armature, vertical overthrow, excess vertical play; or the dial.
The connector steps up but not round	The rotary path through H, G, C, the off-normal springs, B and A.
The called line never rings	The ringing supply or interrupter (affects everything); otherwise H’s ringing contacts or F’s break contacts.
Rings, but no ringback	H and F contacts, or the ringing source if every line is affected.
Ringing doesn’t stop on answer, or stops by itself	The ring-trip relay out of adjustment. Test with a Type 26A false-trip test (2300 Ω must not trip, for a battery-connected generator; 2200 Ω for superimposed ringing, 2500 Ω for a ground-connected generator) and trip test (1600 Ω must trip), and re-tension the relay’s springs.
Answered, but no speech	The two 2 µF capacitors; the D relay’s windings and contacts.
A switch won’t release	”Look first at the mechanical devices”: release armature pin, release link, helical spring; then the off-normal, A, B and F contacts in the release path.

Figure 5 — Fault-finding by symptom. Start from what the caller hears, go to the stage it points to, then make the first checks, as taught in the USAF course's worked troubles. — Original diagram drawn for this dive

8.7 Watching a Call

Real offices already had lamps that showed call states: shelf alarm and supervisory lamps, indicating fuses, and connector supervisory leads that lit a lamp when the called or calling party hung up (USAF course). A demonstration system can go much further and show every stage of a call as it happens. The places to sense each state follow directly from the circuits in Volume 5. The rule for all of them is isolation, because every circuit sits at −48 V with ringing peaks of 175–190 V.

Figure 6 — Where a call-progress LED can watch each stage: off-hook and line busy at the line circuit; hunting and found at the finder; dial tone, digits, hunting and all-busy at the selector; stepping, busy,… — Figure 6 — Where a call-progress LED can watch each stage: off-hook and line busy at the line circuit; hunting and found at the finder; dial tone, digits, hunting and all-busy at the selector; stepping, busy, ringing, talking and hang-up at the connector. A design proposal derived from the circuit descriptions, with the isolation rules every sensing circuit must follow. — Original diagram drawn for this dive

That is the idea behind the display on the system this project is building, a three-telephone step-by-step exchange whose lights let a visitor follow a call from the lifted handset to the ringing bell. Its design, schematics and build will be documented in the project’s Mini PBX build dive.

8.8 In the Reference Library

Sources

USAF Extension Course Institute, CDC 36251, Telephone Switching Equipment Repairman (Electromechanical), Vols. 2–3, ERIC ED194781. https://archive.org/details/ERIC_ED194781
US Army TM 11-2111, Tools, Testing Equipment and Common Supplies, Step-by-Step Dial Central Office Equipment, May 1950. https://www.radionerds.com/images/9/97/TM_11-2111.pdf
Automatic Electric, Technical Bulletin 961-473, Rotary Stepping Switches (1961). https://archive.org/details/how-to-use-rotary-stepping-switches-automatic-electric-1964
Automatic Electric, circuit drawing H-85919-A, Type 26A Switch Tester (1962–c.1970), note 55 (Reference Library).
D. Hunter, Strowger demonstration switch for the Telephone Museum of Prince Edward Island (2006–07). https://www.islandregister.com/phones/demo.html
Automatic Electric Bulletin 505 (lubrication), Bulletin 540 and TB 150-540 (maintenance tools), Standard Adjustments, GSP 230-005-702 (wiper adjustment); Bell System Practices 030-705-701/703 (197- and 198-type switches) (Reference Library).