Adjusting a GPO No. 12 Dial with a Digital Oscilloscope

In order for an exchange to detect the "pulses" a rotary dial generates, the timing needs to be within an acceptable range. There's two main parameters, the pulse speed, which should be 10 ± 0.5 pulses per second, and the break ratio, which should be between 63 - 67%. These will drift over time and will likely change after the dial has been cleaned, so adjusting the dial is best practise during restoration.

There are specialist pieces of equipment for testing dials, but you can get away with using a cheap digital oscilloscope that calculates the duty cycle and cycle time. In theory you could calculate both of these using an analog scope, but I won't go into that here.

I use a "DS213" oscilloscope which is a tiny handheld thing, but you should be able to use almost anything - we're only trying to measure 10Hz. If you don't have an oscilloscope, consider buying a cheap one or asking your local electronics / radio enthusiast, a Hackspace or an Amateur Radio Club. It might also be worth checking if your multimeter has basic oscilloscope functionality.

Before adjusting the dial you should give it a through clean, I've documented the process in another blog post.

DS213 oscilloscope


Before you can start to adjust the dial you'll need to get hold of a couple of things:
  • Digital Oscilloscope 
  • Cables for the oscilloscope and jumper wires
  • Power supply, only needs to be 1 - 5 volts, preferably current limited
  • Needle nose pliers. Engineer PS-03's are perfect for this

Connecting the Dial

To measure the properties of the dial we apply a voltage to the pulse contacts and measure the output. In the resting position the contacts are closed and the oscilloscope should display whatever voltage you're feed in.

Connect the positive feed of the power supply to the first terminal from the left with the finger stop at the 9 O clock posistion (The purple wire in the photo below). Connect another wire to the second terminal from the left - the top one (The grey wire in the photo below), and connect it to the tip of your oscilloscope. Connect the ground of your oscilloscope to the negative feed of the power supply.

Dial wired up

My setup looks something like the photo below. I'm using a USB battery bank and a USB CC / CV power supply but you can use almost anything as a voltage source, even a battery. I'm using an oscilloscope lead with "dupont" style connectors connected to jumper wires, but you should be able to rig something up with a traditional or crocodile clip style probe.

Setup the oscilloscope

The next step is to get the oscilloscope displaying the required info.

  • Set your voltage per division to a sensible value - I'm putting 1V in so I set it to 1V per division
  • Set your time base to a sensible value, we're measuring 10Hz so something in the order of 100-200ms should be fine
  • Set the trigger to falling edge so we capture when the contacts are open.
  • Set the mode to single, so the oscilloscope captures the signal then holds.
  • Configure the oscilloscope to show Duty Cycle and Cycle Time.

Testing the dial

With the oscilloscope connected, we can now test the dial. Dial a Zero and observe the oscilloscope. You should get something like the photo below. It's worth dialling a few times to check the results are reproducable. A dirty dial might give inconsistent results.

Waveform from the dial contacts

You can see 10 pulses in the waveform above. The cycle time is 93.3ms which is 10.72 pulses per second (1000 / 93.3) - a little high. The duty cycle is 34% making the break ratio 66% - within the defined range.

Adjusting the dial

To get the dials parameters into the correct range you'll have to adjust and re-test several times until you get the right values.


To adjust the PPS of the dial it's necessary to bend the "wings" of the governor. Bending them inwards increases the speed. Bending them outwards decreases the speed. Try to keep them even and avoid kinks, any bend should be a smooth curve rather than a sharp angle.

The wings can be accessed from the back of the dial with a pair of needle nose pliers. I tend to adjust to around 10.5 PPS (95ms cycle time). A dial is likely to slow over time rather than speed up, so this should extend the time required between adjustments.

Dial with the governor wings highlighted.

Break Ratio

The break ratio is adjusted by carefully bending the thicker of the two pulse springs - situated third from the right of the dial if the finger stop is at the 9 O clock position. It should only require tiny adjustments. You're aiming for a break ratio of 63 - 67% - so a duty cycle between 33 - 37 %. Moving the spring towards the outside of the dial will reduce the break period. Moving it closer to the centre of the dial with increase it.

Dial with the pulse springs highlighted.


Finishing Off

You may have been wondering what the coloured stickers are for. I add a blue sticker to indicate a dial has been greened, and a green sticker to indicate the dial has been adjusted. It wouldn't hurt to add the date too, so you know when it was adjusted last.

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