Checking if a telephone dial is within spec without removing it from the phone.

 

I've previously written a blog post on using a Digital Oscilloscope to adjust the dial in a telephone, but it assumed the dial had been removed for stripping and cleaning prior to adjustment. I've recently been experimenting with some telephones to figure out the tolerances for both the Pulses Per Second and Make/Break ratio of the Grandstream HT8XX ATAs and wanted to check a dial without disassembling the whole phone.

I wasn't quite sure on the best way to go about it so I asked on the Telecommunications Heritage Group mailing list and James Campbell kindly replied with a simple way of checking the dial.

Jame's said the following:

Easy. Just connect the line wires, usually red and white, to a 9v battery and resistor all in series. Suggest 1k for the resistor but not critical. Put your scope across the resistor and you will see a voltage across the resistor when the phone is off hook and then obviously the dial pulses. If the phone has a modern PST plug on it, it’s pins 2 and 5. 

That sounds straightforward enough. Alas I didn't have a 9v battery, or a Master Socket, or a 1K resistor to hand so I improvised a bit ( and ended up taking the phone apart anyway, but next time I'll be more prepared )

GPO 746 with external power and resistor

I ended up using an adjustable power supply set to 9v and a 1.2k resistor secured under the T19 terminal with the positive lead from the power supply and oscilloscope probe connected to the other side of the resistor. The ground for the scope was connect to the straps between T17 and T18, and the negative lead from the PSU was connected to T9.

I tweaked some of the settings on the scope and dialled a digit (in this case number 5) and sure enough it worked, with the scope showing the dial pulses and the on-screen info showing a cycle of 94ms (i.e 9.4 PPS) and a Duty Cycle (I.E make ratio) of 34, which would make the break ratio 66. Well within the original GPO specifications.

In future I'll probably make up a test rig with a 431A socket, 9v battery, and somewhere convenient to attach the test probes.

Many thanks to James for his suggestion