CHAPTER III.
FOUR-PARTY SET.
(Diagram No. 3.)
Tests of the transmitting and hearing circuits can be made in very much the same manner as in a straight set. On account of the relay coil being bridged between No. 2 and No. 3 terminals of the induction coil, a four-party desk set, if in perfect order, will give two hook clicks, no matter how it is poled.
To make a perfectly sure test for primary and secondary crossed together, it is necessary to disconnect one terminal of the relay winding; then with the receiver on the hook, test from the L2 post to the secondary with your head receiver; if you get current there is a cross.
Reversals of the three wires at the top of the instrument may cause a variety of troubles, just as in the two-party sets.
Ringing Circuit.
=Relay=: Failure of the relay to come up when the operator rings may be caused by:
Condenser open.
Relay short-circuited or open.
Hook contacts, or red and white strands of desk stand cord, or primary and secondary of induction coil crossed together.
Primary open.
Trouble at central office.
Test just as though it was a straight set except when testing for relay short circuited, or for a cross between primary and secondary, (in which case you must open one terminal of the relay winding, as previously directed); and except when testing for relay open, which you can do by bridging your head receiver from L2 post to No. 2 and No. 3 terminals of the induction coil successively; you should get strong battery on No. 2 terminal, if the relay is all right and a strong condenser discharge when you make contact, but a weak battery click when you break contact, on No. 3 terminal.
A ground on the ground side of the line may also prevent the relay from operating, by carrying the generator current off to ground before it reaches the instrument.
If the relay is held up, usually the lamp is kept burning at the central office also, and this means that current is flowing from the central office over the battery side of the line, passing through the relay winding, and returning to the central office over the ground side of the line, or through the ground wire and ground. This condition may be due to:
1st. Winding of relay crossed with frame. Test by disconnecting the “A” or “B” terminals of the ringer from L2 which should release the relay.
2d. As in the case of current through the ringer of a straight set, the trouble may be caused by receiver contact of hook not breaking, white and green strands of desk stand cord crossed together, condenser short-circuited. Test as though it was a straight set.
[Illustration: Fig. 3.]
If the relay vibrates continuously the winding is probably crossed with the frame. To find out whether this is the case, disconnect the ground wire, which should stop the vibrating.
If the relay “jumps” while the operator is ringing, instead of pulling up and staying up as long as the ringing current continues, the relay may be weak, due to a part of the winding being short-circuited out, or to some other cause; or there may be too many bells and relays on the line, so that, owing to the resistance of the line, not enough generator current will flow over it to divide up and give each bell and relay a proper amount of current; or there may be a slight ground on the line, which carries off a part of the ringing current.
To prove that a relay is all right:
1. Short-circuit the condenser; this should hold up the relay, by putting battery current through it.
2. Prove that the spring contact is not crossed with the frame as follows: Disconnect “A” and “B”, then connect the frame of the relay to battery with a test cord, and bridge from the spring contact to ground with your head receiver; if you get no current there is no cross.
3. Prove that the winding is not crossed with the frame as follows: Disconnect “A” and “B” and then test with your head receiver from the frame to the L2 posts; if you get no battery there is no cross.
=Ringer=: If the relay comes up, but the bell does not ring, the trouble may be due to:
Ringer poled wrong.
Bias spring too tight or too loose.
Ringer open.
Ground open.
Ringer short-circuited.
Bridging your head receiver across the ringer while the operator is ringing will show whether or not you are getting generator current on the terminals of the ringer winding; going in series at the “A” and “B” terminal will show whether or not you are getting current through the ringer.
To prove whether a ringer is open or not, connect “A” to battery, hold the relay closed, and then test from “B” to ground with your head receiver; if you get no current the circuit is open; prove that the open is in the ringer itself by testing direct from the two terminals of the winding successively.
The ground can be tested in the same manner as on a two-party set.
To prove that the ringer is short-circuited, connect “A” and “B” across the line, close the relay contact, and bridge across the terminals of the ringer winding with your head receiver; if you get no click the ringer must be short circuited; check this test by putting your head receiver in series with the ringer, while it is connected across the line, and closing the relay contact; this should give you a strong click if the ringer is short-circuited.
=Bell ringing weak may be due to=:
Poor adjustment of armature.
Poor adjustment of bias spring.
Pivot screw too loose or too tight.
Clapper rubbing on guard.
Wiring interfering with clapper stem.
“Whiskers” on magnet head.
Magnet head sticky or gummy.
Polarizing magnet weak.
One or both coils of ringer partially short-circuited.
Too many bells and relays on the line.
Ground on one side of line which takes part of the generator current off to ground before it reaches the instrument.
Trouble at central office.
Earth currents.
To test for earth currents bridge your head receiver from the ground side of the line to the ground; you should get no current; if you do get current you will probably find that it varies continually, being due to trolley cars; report the matter to the chief inspector. In making this test be sure that the line is not in use; if the receiver is off the hook at any station on the line you will get a false test.
Earth currents very often cause bell troubles on suburban stations, particularly those that lie near to trolley lines; but in the city, earth currents are seldom noticeable.
=False rings may be caused by=:
Very strong earth currents.
One coil of ringer short-circuited.
If a biased ringer has one coil short-circuited, then when it is poled for either 1st or 2d stations it will respond to both rings; and likewise when poled for 3d or 4th stations it will ring with either current.
In the bell diagram No. 3 you will see that “B” connects to the spring contact of the relay, the frame of the relay connects to the biased side of the ringer, and the unbiased side of the ringer connects to “A.”
In the older bells, “B” connects direct to the biased side of the ringer; the unbiased side of the ringer connects to the spring contact of the relay, and the frame of the relay connects to “A.”
Therefore, in connecting an extension bell to a set that has no extension bell binding posts, connect one side of the wire to the frame screw of the relay, under the bottom of the set, and the other side to the post at the top of the bell that “A” connects with. Or solder one of the extension bell wires to the spring contact of the relay, and connect the other to the post at the top of the bell that “B” connects with. In every case the auxiliary bell must be connected across the terminals of the ringer without being bridged around the break of the relay. Pole the extension bell by reversing the wires either at the bell or at the set.
=Ringer Adjustments=: In adjusting a biased ringer find out the two points where the bell ceases to ring on account of the bias spring being too tight and too loose, and adjust the spring half way between these points. To locate them, slack off the bias spring until it has no tension at all, and then get the operator to ring while you tighten up the spring, and note the point at which the bell commences to ring; continue to tighten the spring until the bell ceases to ring; by making the adjustment half-way between these two limits, you will have a margin both ways, so that if the generator rises a little above or falls a little below its proper voltage, the bell will still ring.
The little set screw in the yoke at one end of the armature should be adjusted so as to hold the opposite or bias spring end of the armature a little away from the head of the magnet core; 0.02 of an inch, which is a little less than the diameter of a No. 22 cross-connecting wire, “In every test use a little judgment.”