How It Works — Ae Type 26a Switch Tester

Figure 1 — The Type 26A connected for a connector routine test. The calling cord joins the tester's CALL jacks to the connector's test jack; the connector is dialled to its own bank's test line at terminal 99… — Figure 1 — The Type 26A connected for a connector routine test. The calling cord joins the tester's CALL jacks to the connector's test jack; the connector is dialled to its own bank's test line at terminal 99; the answer and 48 V cord brings that test line, with office battery and ground, back to the tester's ANS and 48V jacks. The remote control set and an optional pulsing test set plug into the two front receptacles. — Original diagram drawn for this dive

2.1 Two Ends of One Call

A connector’s job is to join two telephones: the calling party, who dials it, and the called line, which it tests, rings and feeds. The Type 26A tests a connector by being both telephones at once.

On the calling side, the tester presents a loop to the connector through the calling cord, plugged into the connector’s own test jack. It seizes the switch, dials it, and listens to what comes back: dial-tone-like supervision, busy tone, ringback, the battery reversal that signals an answer.
On the called side, the tester is the called line. Every connector shelf has a test line wired to a reserved position in its bank, terminal 99 in Automatic Electric’s arrangement. The connector, dialled to 99, finds that test line, and the answer cord carries it back to the tester. There the tester can make the test line busy, see and hear ringing arrive on it, and answer it through a chosen resistance.

That arrangement is drawn as Fig. AA, “Typical Exch. Connections,” on sheet 1 of the circuit drawing H-85919-A. Regular connector shelves bring their test terminal out as “CKT. 99.” Trunk-hunting connector shelves bring out “CKT. 90” and “CKT. 99, term ‘C’ only,” and trunk-and-level-hunting shelves add “CKT. 01,” so that a hunting connector can be made to hunt over a known group. Those test terminals are multipled to the exchange’s test circuits (Figs. 4A–7A and 10A on the missing third sheet) and to the office’s automatic routiner, H-85502. The USAF course’s test procedures dial “the test number 199 (299)”: the 1 or 2 takes a first-selector level to the connector group, and 99 steps the connector to the test line (USAF CDC 36251, Vol. 3, ch. 6).

2.2 The Calling Side

2.2.1 Seizure and the Loop

The START turn key “completes a loop in the tester to seize the switch” (USAF course). With the loop closed, the connector’s A relay operates, and the green C lamp shows the C-lead (sleeve) supervision coming back from the switch. The white LOOP lamp shows that the tester’s loop to the switch is complete. RLS opens the loop to release the switch.

2.2.2 Dialling Over a Bad Line, on Purpose

A connector that works on a short, dry line may fail on a long or leaky one, so the Type 26A does all its dialling through an adjustable bad line:

The 300, 600 and 1000 Ω keys put series resistance into the pulsing loop, singly or together (300 to 1900 Ω), imitating a long line.
The LK key bridges a 15,000 Ω leak across the loop, imitating wet or damaged cable.
The dial on the panel, or the remote control set’s dial plugged into the RC-VM jack, or an external pulsing test set (“varying machine”) through the VM receptacle and the VM key, supplies the pulses.

The connector must step correctly over each bad line. The two are tested separately: the USAF course’s LK key “places 15,000 ohm shunt across the pulsing loop which removes all loop resistance.” “If the switch does not step properly, it is considered faulty.”

Figure 2 — The pulsing loop, simplified. The 300, 600 and 1000 Ω keys add series resistance to imitate a long line; the LK key bridges a 15 kΩ leak to imitate a wet one; the dial, the remote set's dial or an … — Figure 2 — The pulsing loop, simplified. The 300, 600 and 1000 Ω keys add series resistance to imitate a long line; the LK key bridges a 15 kΩ leak to imitate a wet one; the dial, the remote set's dial or an external pulse source sends the digits. The connector's A relay must follow the pulses through each in turn. — Original diagram drawn for this dive

2.2.3 Which Kind of Switch

A row of locking keys sets the tester up for the switch on the shelf: TRK HTG CONN / REV CALL CONN, REV CALL / TOLL CONN, and PS / TOLL TRANS (pay-station repeaters and toll transmission selectors). A trunk-hunting connector, a toll connector and a reverting-call switch each behave differently at the busy test, at answer and at release, and these keys change what the tester expects and supplies. Engineering note 51 adds that when the toll features of a combination connector are tested, the switch test plug is inserted “with top down in lower switch jacks.” The CC / CR key and its green CC and red CR lamps belong to the same family of switch-type functions. What exactly they test is in the missing circuit explanation, E-85919-A.

2.3 The Called Side

The answer cord brings the connector’s test line to ANS JK3, and the USAF course has it carrying “battery and ground and —, +, and C-leads.” On that test line the tester can:

Make the line busy. The BUSY key “places ground on C-lead bank contact of the test terminal to mark the test line busy” (USAF course). A connector dialled to a busy line must return busy tone. The red BUSY lamp lights on first plugging in if the switch is already in use.
Watch ringing arrive. When the connector sends ringing to the test line, the white RING lamp lights, and with the BUZ switch on, a buzzer sounds with it. +RING / −RING presumably select which side of the line the ringing is expected on (an inference from the names; the explanation is missing), and the drawing’s straps B and S set the test line up for battery-connected generator, superimposed ringing, or ground-connected generator with a 25-watt protective lamp (note 53).
Answer, at a measured resistance. The FALSE TRIP / ANS key bridges the test line with a standard resistance during the silent part of the ringing cycle. FALSE TRIP applies a resistance high enough that a good ring-trip relay must not trip; ANS applies one low enough that it must. The READJUST turn key shifts both values for re-tensioning a relay that fails (Volume 3).
Talk. RING / TALK and the operator’s telephone let the tester listen to and speak over the completed connection, proving the talking path through the connector’s capacitors.

The white SUPY lamp is “loop supervision” in the USAF course’s list. Its exact conditions are in the missing explanation; on a connector the obvious candidate is the battery reversal the connector applies when the called line answers (history dive, Volume 5).

Figure 3 — The Type 26A's controls as laid out on the drawing: twelve lever keys with their upper and lower functions and whether each locks, the START and READJUST turn keys, and the seven lamps. A buzzer so… — Figure 3 — The Type 26A's controls as laid out on the drawing: twelve lever keys with their upper and lower functions and whether each locks, the START and READJUST turn keys, and the seven lamps. A buzzer sounds with the RING lamp. — Original diagram drawn for this dive from drawing H-85919-A

Figure 4 — The lever-key layout as drawn on sheet 1 of H-85919-A, "Lever Keys (Front)": FALSE TRIP/ANS, RING/TALK, RLS/BUSY, +RING/−RING, REV CALL/TOLL CONN, PS/TOLL TRANS, CC/CR, LK/VM, 300, 600, 1000, TRK H… — Figure 4 — The lever-key layout as drawn on sheet 1 of H-85919-A, "Lever Keys (Front)": FALSE TRIP/ANS, RING/TALK, RLS/BUSY, +RING/−RING, REV CALL/TOLL CONN, PS/TOLL TRANS, CC/CR, LK/VM, 300, 600, 1000, TRK HTG CONN/REV CALL CONN, each marked L (locking) or NL (non-locking). — Automatic Electric, drawing H-85919-A, sheet 1 (TCI Library, contributed by Paul Wills)

2.4 The Operator’s Telephone

The tester has its own talking circuit, so the maintainer can listen to tones and speak over a completed call. The REC and TRANS twin jacks take an operator’s headset (the drawing’s Fig. 17A, which note 66 says is provided with the test set), and the HANDSET jack takes a hand test telephone instead. In Fig. 1A the transmitter and receiver work through an induction coil, marked IND, with windings marked 9.4, 4.30 and 9.4, and 2 µF capacitors C1, C2 and C3. A varistor, VR1, sits across the receiver circuit, presumably the usual click suppressor. Sheet 1’s note 67 records that, from issue 12, a “hand test tel adapter cord” was shown on the drawing and supplied with the set.

2.5 Power

The Type 26A draws its power from the office, not from batteries. The answer cord’s second plug, into 48V JK4, brings office battery and ground. Note 55 sets the limits: “This test equipment 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 and maintained within the required limits while the tests are being made.” The trip and false-trip values are only right at that voltage. The front panel also carries GEN JK5 and 10V JK6 twin jacks, and the top panel two binding posts marked +L and −L. Their exact use is in the missing explanation.

2.6 Inside: What Fig. 1A Shows

Sheet 2 of the drawing is the full schematic, Fig. 1A. The surviving scan is clear enough to identify the main parts, but not to trace every circuit with confidence, and the circuit explanation that would describe each relay’s job (E-85919-A) has not been found. What can be read:

Relays, named across the top of the drawing: RET I, IND (the induction coil), RET II, LL, B (a polarized relay), T, R, F (Type 58A), G (shunt-field), H (Type 58A), J, S, A, D, C (an AC relay), E and M. The symbol key on sheet 1 shows the shunt-field relay G, the AC relay C and the polarized relay B in detail.
Manufacturing note 2: “Adjust residual screw of relay R to provide 190–220 milliseconds release time.” R is a timing relay.
Precision resistors, mostly 1 per cent, 2 W: two sets of 500, 300 and 150 Ω (R3–R8) and further 300 and 150 Ω units in the loop and trip networks; 1600 Ω (10 W) and 200 Ω units in the trip networks; 2700 and 3300 Ω; a 1000 Ω, 10 W resistor; and R19, which was 1500 Ω at issues 7–8 and 500 Ω from issue 9 (note 65).
Diodes CR1 and CR2 (FD-1029-DG); a component marked TR1, apparently a thermistor, in series with a 3400 Ω relay; and the buzzer BUZ.
Receptacles: the 10-pin VM RECP at the top left, wired “to Fig. 2A,” and the 15-pin REMOTE RECP facing the remote control set, Fig. 3A.

Figure 5 — Sheet 2 of H-85919-A: Fig. 1A, the complete Type 26A circuit, and Fig. 3A, the remote control set, at the left. The relays are named across the top; the CALL, ANS and 48V twin jacks are at the righ… — Figure 5 — Sheet 2 of H-85919-A: Fig. 1A, the complete Type 26A circuit, and Fig. 3A, the remote control set, at the left. The relays are named across the top; the CALL, ANS and 48V twin jacks are at the right; the operator's telephone circuit and the dial are at the lower left and centre. — Automatic Electric, drawing H-85919-A, sheet 2 (TCI Library, contributed by Paul Wills)

2.7 The Remote Control Set

A connector shelf is tall, and the switch under test may be at head height while the tester sits on a stand. The remote control set, D-322542-A, let the testman “perform a connector inspection within 16 feet of the test set. It has a dial and keys corresponding to those on the test set” (USAF course). Fig. 3A shows its dial and keys LOOP, LEAK, RLS, RING, BUSY, FALSE TRIP & TALK and ANS, wired through a 15-conductor cord to a plug that mates with the tester’s REMOTE receptacle, and its dial wired separately “to Fig. 1A RC-VM jack.” Volume 4 tabulates the pins.

Figure 6 — Fig. 3A of H-85919-A: the remote control set D-322542-A. Its dial (top) goes to the RC-VM jack; its keys go through a 15-conductor cord, plug pins 1–15, to the REMOTE receptacle. The VM receptacle'… — Figure 6 — Fig. 3A of H-85919-A: the remote control set D-322542-A. Its dial (top) goes to the RC-VM jack; its keys go through a 15-conductor cord, plug pins 1–15, to the REMOTE receptacle. The VM receptacle's ten pins are at the top right. — Automatic Electric, drawing H-85919-A, sheet 2 (TCI Library, contributed by Paul Wills)

2.8 Pulsing Test Sets and Routiners

The VM receptacle connects the Type 26A to an external pulse source. Fig. 19A on sheet 1 draws the adapter cords: a 4-conductor cord from the VM receptacle (pins 1, 2, 9 and 10) to the “CONTR” and “TP” jacks of Automatic Electric’s Type 28 semi-automatic routiner, H-850046; a 2-conductor cord from the RC-VM jack to the Type 28’s “SW” jack; and a dummy plug for the RC-VM jack. Note 60 adds that with a Western Electric pulsing test set J-34717-A, or the AE Type 28, “use connecting cords specified on circuit H-850220-A.” With issue 13 the Fig. 19A cords became standard equipment.

2.9 In the Reference Library

Sources

Automatic Electric, drawing H-85919-A, Type 26A Switch Tester, sheets 1–2 of 3 (1962–c.1970), TCI Library (contributed by Paul Wills).
USAF Extension Course Institute, CDC 36251, Telephone Switching Equipment Repairman (Electromechanical), Vol. 3, ch. 6, pp. 72–74 and answers 436-3/4. https://archive.org/details/ERIC_ED194781