"The phone is a remarkably complex, simple device, and very rarely ever needs repairs, once you fix them." - Dan/Panther
Started by royalbox, January 09, 2017, 10:32:17 AM
Quote from: royalbox on January 09, 2017, 04:32:08 PMHello Andy,Here's what I've found here: The white (T18) is negative and the red (T8) is positive. If I swap those 2 wires then when you speak or blow into the transmitter, you no longer hear yourself in the receiver. All the phones I've checked including modern ones send some of the transmitter to the receiver, otherwise it feels like the phone is not connected.There is a reason for all this, honest! I just don't want to get over-complicated explaining why straight away or it'll get confusing.I'll keep searching online to see if I can find any more on this.Thanks for your help,Barry.
Quote from: royalbox on January 09, 2017, 04:32:08 PMI just don't want to get over-complicated explaining why straight away or it'll get confusing.
Quote from: unbeldi on January 09, 2017, 06:11:37 PMDoes the phone have a diode (=rectifier) installed ?I recall that this was often practiced.
Quote from: unbeldi on January 09, 2017, 06:11:37 PMI recall that this was often practiced.
Quote from: twocvbloke on January 09, 2017, 06:40:46 PMI believe you're thinking of the Rectifier 205A, which wasn't related to the line polarity... The Rectifier No. 205A was placed across the receiver connections inside the phone to reduce the loud pop produced when pressing the hookswitch for a follow on call to reduce the "Acoustic shock" issue that can occur (and acoustic shock is not a nice thing, having done it to myself a few times!) with the user still holding the handset to their ear. It's just two parallel diodes wired in opposite polarity to each other, and can be made using cheaply available Schottky diodes (such as the 1n400x types)...
Quote from: Jack Ryan on January 09, 2017, 06:42:41 PMBut that doesn't affect the polarity of the phone.Jack
Quote from: twocvbloke on January 09, 2017, 07:23:12 PMHence why I said that it didn't affect line polarity, in the first line....... There is also the regulator in there, which is a series of diodes wired in a full wave rectifier manner, but, again, this doesn't affect the polarity (in essence it actually makes it unaffected!!)...
Quote from: Jack Ryan on January 09, 2017, 06:21:49 PMT18 is the A line.T8 is the B lineA = Ground = 0 voltsB = Battery = -50 volts nominal
QuoteAs a matter of curiosity, why isn't the bridge rectifier and the zener part of the pulse to tone convertor? It would simplify installation.
QuoteWhen Rotatone was being developed, a decision was made not to incorporate a polarity correction circuit within Rotatone module. This decision was made for a number of reasons, the chief of which is that the polarity protection was best applied to the entire phone, and not just to the Rotatone module.
QuoteVoltages up to 35 volts are acceptable, as the module is fitted with its own voltage regulator. Voltages however must not exceed 40 volts, so do not connect the unit on the exchange side of the hook switch where it would be exposed to ringing voltages (it would cause the line to go 'off hook' but this can't berelied on to protect the circuit). For additional surge protection, a 24v Zener diode can be installed across the line after the switchook as shown in the generic diagrams below.