My Crossbar PBX · Volume 6
Troubleshooting
6.1 What This Volume Covers
This is the troubleshooting manual for the crossbar exchange on the back of the panel. Its method is the same as the Strowger side’s (My Strowger PBX, Volume 6): first ask who is affected, then make one call and watch which light was the last to come on. What is different is where faults live. A Strowger switch’s faults are mostly in its stepping mechanism; a crossbar switch has almost no motion, so its faults are mostly in contacts (the crosspoints, the off-normal springs) and in the sequence of the common control.
Everything here is drawn from the circuits of Volume 3, the 755A’s own practices and the Strowger side’s manual, until the exchange is built.
6.2 First: Who Is Affected?
Table 1 — First: Who Is Affected?
| Class | Who is affected | Where the fault must be |
|---|---|---|
| 1 | one telephone only | its telephone, its wiring, its line circuit, or its vertical (hold magnet, crosspoints, off-normal springs) |
| 2 | calls on one link only | that link circuit, or its bar (select magnets, fingers, off-normal springs, its two rows of crosspoints) |
| 3 | everyone, some of the time | something used part of the time: one link, the link allotter, the register, the trunk |
| 4 | everyone, all the time | the common control, the fuses X1–X7, the shared supply, ringing or tones |
To test one link, busy the others: hold a call up on them, or (as the 755A provided) switch them out. The 755A had link cutout keys CO1–CO3 for exactly this; its link test begins by operating all three and checking that “busy tone is heard” (BSP 550-550-590, §4.01–4.02). A key per link on this exchange makes every link testable on its own.
6.3 Then: Which Light Stopped?
Table 2 — Then: Which Light Stopped?
| Last light lit | What did not happen | Look at |
|---|---|---|
| nothing | the line relay did not operate | the telephone; fuse X3; L through CO’s break springs |
| OFF-HOOK, but no common control BUSY | the start | ST and LS wiring; the common control’s start relay; fuse X7 |
| common control BUSY, but no bar light | no select magnet | the link allotter (is any link free?); fuse X1; the select magnet itself |
| bar light, but no mimic LED | the hold magnet did not close the crosspoint | the bar’s off-normal springs (the common control waits for them); fuse X2; the hold magnet; a bent or stuck finger |
| mimic LED flashes and goes out | the link’s hold ground did not take over | the link’s B relay; the row’s S lead; the S contact pair at that crosspoint |
| SEIZED, but no DIAL TONE | the link’s A relay or the DT lead | A; the shared dial-tone supply |
| register shows the wrong digits | pulse counting | the dial (test it on the RS 43-114); the register’s slow-release digit timing |
| register right, but BUSY on an idle line | the busy test saw ground on BY | that vertical’s off-normal springs stuck operated |
| completing mimic LED never lights | completion | as for the caller’s crosspoint, on the completing row: bar down, then the called vertical |
| RINGING, but no TALKING on answer | ring trip | relay F; ringing must ride on battery (shared, My Strowger PBX, Volume 4) |
| TIME-OUT lamp | a step never reported | whichever off-normal spring the sequence was waiting for |
6.4 Symptoms and Causes
6.4.1 The switch
Table 3 — The switch
| Symptom | Look at |
|---|---|
| A bar will not rock | Its select magnet, its fuse (X1), the magnet’s armature and return spring |
| A bar rocks but the crosspoint never makes | A finger bent, broken or missing at that crosspoint; the actuating spring; the contact pair itself |
| A crosspoint makes, but the call is noisy or one-way | One contact pair of the crosspoint dirty or open: T or R. “Dirty contacts are electrically open contacts” (TB 565) |
| Two crosspoints close on one vertical | Two select magnets operated at once: the common control’s one-at-a-time chain has failed. The 755A’s call allotter prevented “double connections” this way (BSP 550-550-110, §6.09) |
| A vertical stays in after the call | Its hold magnet still has ground on S: the link has not released, or S is shorted to ground; or the armature is sticking mechanically |
| A crosspoint opens by itself during a call | The S contact pair at that crosspoint is dirty, so the hold magnet loses its ground |
6.4.2 The relay circuits
Table 4 — The relay circuits
| Symptom | Look at |
|---|---|
| A line is always busy | Its vertical’s off-normal springs stuck operated (BY grounded), or its cut-off relay stuck |
| Every call takes the same link | The link allotter is not advancing; the 755A’s allotter rotates “successive links on successive calls” (BSP 550-550-110, §3.02) |
| Busy tone on lifting the handset | All links busy or cut out; the link allotter; the link cutout keys left off |
| The common control hangs until reset | A missing off-normal report, caught by the time-out (Volume 4): find which step it stopped on from the lights |
| Dialling extra pulses or a dropped digit causes a wrong number | The register’s digit timing; the 755A’s links were tested to make “dialing of preliminary or extra pulses … ineffective” (BSP 550-550-590, §1.03) |
6.4.3 The trunk to the Strowger side
Table 5 — The trunk to the Strowger side
| Symptom | Look at |
|---|---|
| Dial 9: no Strowger dial tone | The cut-through relay CT; the trunk’s supervisory relay; then line 66 on the Strowger side, as a class 1 fault there (My Strowger PBX, Volume 6) |
| Strowger dial tone, but 22–55 never ring | The pulses are not reaching the Strowger connector: a series element in the loop (CT not fully operated, a resistor left in by the supervisory relay) |
| Dialling 66 from the Strowger side rings nothing here | The ring detector; the trunk circuit’s fuse; the selector switch setting |
| The trunk stays busy after a call | The supervisory relay holding: the link has not seen the release |
6.5 Tools and Routine Tests
- The bring-up controller (Volume 4) is also the best test set for the switch: it can operate any bar and vertical by hand, and read every off-normal spring.
- A multimeter across each crosspoint’s contact pairs, with the crosspoint closed by the controller.
- A hand test set on a line’s terminals, as the 755A’s practices used (“connect the dial hand test set to the T and R terminals associated with an idle key station”, BSP 550-550-590).
- The link cutout keys, to force calls onto one link.
The 755A’s equipment test list (BSP 550-001-011) scheduled its alarm tests and its link and link-allotter tests. Adapted for a display exchange:
Table 6 — The 755A's equipment test list ([BSP 550-001-011](https://telephones.fubsypoly.com/library/bsp-550-001-011-755a-equipment-test-list-1972/)) scheduled its alarm tests and its link and link-allotter tests. Adapted for a display exchange
| When | Test |
|---|---|
| Each time it is shown | Every line calls every other; two calls at once; a call each way to the Strowger side |
| Monthly | With the cutout keys, each link alone: dial tone, busy tone with the others cut out, a call through it (BSP 550-550-590) |
| Every two months | With the controller: every crosspoint in use makes and breaks on all its pairs; every off-normal spring reports |
| Yearly | Clean the switch: dust out the fingers and contacts; burnish only contacts that test dirty |
6.6 In the Reference Library
- BSP 550-550-590: 755A Link and Link Allotter Circuits, Operation Tests.
- BSP 550-550-230: 755A Tests and Inspections at Installation.
- BSP 550-550-500: 755A Alarms, Operation Tests.
- BSP 550-001-011: 755A Equipment Test List.
- BSP 551-100-101: 756A PBX Repair: trouble-shooting a real crossbar PBX.
Sources
- Bell System Practices 550-550-110 (1937), 550-550-590 (1952) and 550-001-011 (1972), for the 755A PBX, Reference Library.
- Automatic Electric, Technical Bulletin 565 (1961), Reference Library.
- My Strowger PBX, Volume 6, and My Crossbar PBX, Volumes 3–5, on this site.