Cleaning a GPO No. 12 Dial

Rotary telephone dials are a thing of beauty. They're entirely mechanical but manage to accurately and consistently "pulse" a telephone line in order to signal the number the user wishes to call. The first time I restored one it happened by accident - I had no intention of fully dismantling it but I removed one too many screws and the spring lost it's tension, so I ended up stripping and re-building it. It's not actually that difficult, so don't be put off. I recently aquired a GPO 332 that needed some love, so I thought I'd document the restoration process. Restoring the dial is a significant section of the process, so I thought I'd split it into a seperate post.

Here's the dial in the condition I aquired it. Visually it's not the worst I've seen, but it wasn't running very well as the speed would vary dramatically and it would stick in places - making it a good candidate for a deep clean. 

 

Dial No. 12 in its uncleaned state.

 

 

 

 

Requirements 

You will need the following:

• Flat head screwdrivers, 
• Needle nose plies
• 3BA or small adjustable spanner
  
• White Spirit
• Light mineral oil (sewing machine oil or you can oil designed for rotary dials on eBay)
• Cloths / Rags
• Metal Polish (Polishing Paste No. 5 or Brasso) 
• Plastic Polish (Novus or similar)
  
• Kitchen Roll
• Cotton Buds
• Something to store the various screws and parts in.
• A bowl or container large enough to submerge the dial in.
• Safety Glasses / Gloves ( Not mandatory might be useful... )
• Replacement dial label / cover
  

Instructions

The following guide isn't really a secret or my own creation, it's a re-hash of the original GPO Engineering Instructions. You can find them on Bob Waters' excellent website and you should definitely give them a read before starting.

 

Start by aquainting yourself with the dial.  Rotate the finger plate and let it return slowly, watching how the various parts operate. Take extra notice of how the pulse and switching springs are manipulated by the dial.

Disassembly

Once you've got a rough idea of how the dial works, you can start dismantling it. Flip the dial over so you're looking at the mechanism, and remove the two screws and finger stop:

 

The highlighted screws secure the finger stop.

 

 

Next, remove the stop screw. Hold the finger plate as you back the screw out, and once its removed let the finger plate slowly rotate as it loses tension:

The highlighted stop screw lifts the stop plate that stops the dial in the correct position

Next remove the two springs that secure the spring-set assembly. Note, don't remove the single screw on the opposite side which is holding the springs together. It's a pain to re-assemble them and generally not required:

The highlighted screws secure the spring-set assembly to the case

 

 

Next remove the trigger lever assembly. Note there's a small washer underneath.

The trigger set assembly (highlighted)

Now it's time to remove the nuts the secure the main spindle. This is where the parts start to multiply rapidly.

 

The spring is hiding underneath the toothed pulse wheel and there is a risk it'll uncoil with some force so you might want to wear some safety glasses. The spring is also made of sharp spring steel so you also may want to wear gloves. The nuts are imperial "3BA" size but you could use an adjustable spanner. 

 

Remove the top locking nut, and then the second nut. Below that is the switching lever, a spacer, the pulse wheel, a thin washer and the spring in its spring box. Remove each of the parts leaving the spring box in situe for now.

The main spindle, pulse wheel, switching lever and sundries.

Now for the exciting part, removing the spring. The spring box and spring can be removed as one piece but if the spring... springs out it doesn't matter as we're going to take it out anyway. To remove it, pull the spring box upwards on the shaft whilst supporting the spring from underneath with a flat bladed screwdriver. Note the screw driver is only to hold the spring in place and shouldn't be used to force anything. The spring box should slip off the spindle with relative ease.

The spring box and spring

 

 

If you've been able to remove the spring box without it launching across the room it should look something like the photo below. Note how the end of spring hooks over the outside of the case through a slot and then spirals inwards in an anti-clockwise direction. I don't have a particular method for removing the spring, I tend to just gently pull the bent section in the middle and un-twist the spring until it eventually bursts out of its own accord.

The spring once it's removed from the spindle

With the spring removed it's time to flip the dial over and remove the finger plate. First of all remove the securing ring from the dial centre as well as the transparent protector and label itself. I tend to use a small flat bladed screw driver to unhook the clip from the hole in the bottom centre of the dial centre. You should be careful not to scratch the protector or finger plate.

 

Unhook the clip from the highlighted side.

Once the label has been removed, remove the two screws that secure the label holder, and the label holder itself:

The two screws (highlighted) secure the label holder to the finger plate.

Now unscrew the finger plate securing screw in the centre of the dial and remove the finger plate and spacer:

The finger plate securing screw is in the centre of the plate

 

 

Remove the clip that secures the number ring. Again I usually use a small flat bladed screw driver to hook it out of the groove. You need to be incredibly careful not to slip and scratch the number plate! 

 

Once the clip is removed, STOP! The number plates are delicate so make sure nothing is in range that might scratch it. Older dials will have an enameled thin metal plate with the letters painted on the front. Newer dials will have a plastic number plate with the letters applied from behind and then painted over with a white paint. This may be an emulsion based paint which easily absorbs oil and other contaminants, so you want to ensure it stays as clean as possible. Wash your hands, carefully remove the number plate, and place it out of the way of the other parts. Possibly even in a Ziplock bag. If its an enameled dial plate it'll have a rubber spacer behind, remove it and keep it for later.

The two ends of the securing clip should be in the highligted region, but aren't always

 

 With the finger plate removed you can now see the insides of the dial, comprising of the stop plate, governor and gears. Remove the main spindle in the middle (Note: There's no need to unscrew the two screws in the main spindle):

 

Number plate and spindle removed, showing the inside of the case

 

 

Next unscrew the two screws that secure the stop plate. With the screws removed, take out the the stop plate and and governor gear bracket and gear:

Stop plate and governor bracket screws (highlighted)

Next unscrew the two screws that secure the governor cup bracket (Note: There's also two washers) and remove the cup and governor:

 

Governor cup screws (highlighted)

Next remove the governor pivot bearing. There's another small washer underneath:

Governor pivor bearing and washer (highlighted)

 

Once finished you should have a pile of parts that looks something like this:

Fully strippped dial components

Cleaning

With the dial stripped, we can now get to work cleaning it, removing decades worth of dust, gunk, and and oils or grease.

Place the metal components ( Thats everything including the governor gear but excluding the number label protector and number plate) in a container filled with enough white spirit to submerge them. Let them soak for a while and then give them a scrub with an old tooth brush. We're not trying to get the spotless, just clean, but if you really want to you could go over them with Polishing Paste No. 5 or Brasso and then rinse them in White Spirit.  Give some extra attention to the governor cup and weights, using a cotton bud to give the inside of the cup a good clean.

Components being cleaned in a bath of white spirit

Once cleaned, put them on a bit of kitchen roll out of the way and let them air dry for a while. An hour or two is probably fine but at this point I usually could do with a break so I leave them overnight.

Next up, polish the finger plate, label holder, finger stop and the edge of the case assembly (The bit thats visible when the dial is mounted) with Polishing Paste No. 5 or Brasso and buff them until they shine.

We now need to clean and/or polish the number plate. As previously mentioned they can be pretty delicate, so you need to clean your hands and work area to make sure there's no oils or contaminants present. If its an enamel version you want to use a damp cloth and minimal friction. I'd avoid soaps or chemicals if you can.  Wash the rubber spacer with warm soapy water and let it dry.

For the plastic versions you can get away with using a little plastic polish (Like the Novus range) to attempt to remove any scratches or dirt on the plastic. Avoid getting any polish on the rear of the plate and buff any remaining polish off the plate.

 

Cleaned and polished parts

 

Reassembly

 Assembly is the reverse of disassembly ;)

 

It can be a little to get the governor cap screws in so I tend to do this part first whilst there's a bit of room to maneuver. Put it in posistion and insert the screws and washers and tighten them a couple of turns - just enough so they won't fall out. We'll need to adjust the position of the cap later.

Dial case with governor cap fitted

Next apply a tiny drop of oil to each end of the governor gear spindle and the clutch - thats the gap between the smaller and larger gears. It really only needs to be a tiny amount. Insert the gear assembly and bracket into the case, and place the stop plate over that. Insert the screws and tighten them a little so we can adjust it later. Note, there's no washers here.

Dial after the governor gear assembly and stop plate have been fitted

Next apply a tiny drop of oil to each end of the governor spindle and insert it so the governor weights slot into the cup. Screw in the governor pivot bearing to the side of the casing, remembering to add the tiny washer. Adjust the governor spindle so it sits in both the bearing and the cups.

Add a drop of oil to the main spindle shaft and insert it into the middle of the dial. I tend to insert, remove, and twist it to ensure the oil is well distributed. Adjust it until it mates with the smaller governor gear - at this point it might not spin freely.

It's now time to adjust the various components so they spin freely. There's no particular science or method, but you can eyeball the governor spindle to make sure its sitting straight in the bearing and cups, and check there's a sufficent gap between the worm and gear. I tend to get things close, tighten the screws almost all the way down, make any final adjustments then tighten the screws all the way. You'll know when you've got it right because everything will rotate freely.

Once everything is in posistion, apply a tiny drop of oil to the worm gear and the interface between the main and governor gears.

Dial with governor spindle and main spindle fitted

 

Next we need to lubcricate the spring and wind it back into the spring box. The official instructions suggest dropping the spring in a bath of oil, removing it and letting the excess drip off. That sounds a little messy to me, as well as requiring a large amount of oil, so I tend to apply several drops to a piece of kitchen roll and wipe it over the spring several times, ensuring even coverage all along the spring, on both sides.

Spring removed and the spring box

The smaller hook of the spring hooks through the slot on the side of the spring box, and then the spring is coiled anti-clockwise inside itself. Note the first loop of the coil passes through a a small clip. Once wound it should look like this:

The spring re-wound into its box

Next we need to slide the spring onto the main spindle, making sure the larger hooked section of the spring slots into the small groove in the casing. To do this I tend to remove the spindle, get the spring slot into the groove and in posistion, and then re-insert the spindle whilst holding the spring box in place. You may need to rotate the spindle slightly so the key fits in the slot in the shaft.

The spring box fitted to the main spindle

 

Next we need to add the washer, pulse wheel, spacer and switching lever to the main spindle, in that order. Not the orientation of the switching lever isn't important now. Once they're in position, screw the first nut onto the threaded section - just finger tight for now.

The pulse wheel, trigger lever and nuts fitted to the spindle

 

Next add a tiny drop of oil to the trigger lever screw and screw it into posistion. Don't forget the washer:

Trigger lever screw fitted to the dial

Now install the spring set assembly and tighten the two screws. Note how the outter spring at the top of the photo applies pressure to the trigger lever:

Springset assembly in position

 

Next assemble the finger plate by screwing the label holder into the spacer behind the plate:

Fingerplate and number label holder reassembled

 

Next insert the number plate into the casing. Remember to wash your hands / work area and any other excess oil or dirt on the dial before handling the plate. Once inserted posistion it to the slot lines up with the finger stop posistion, and fit the securing clip:

Number plate fitted and secured

 

Next place the finger plate on the spindle and align it so it sits flush and then tighten the screw in the middle until its secure. At this stage the finger plate and numbers behind won't align, dont panic:

Fingerplate re-attached and secured to the dial

 

Flip the dial over and insert the stop screw (Remember the two washers). Give it a couple of turns so it stays in position. It's now time to tension the mechanism. Rotate the finger plate clockwise several turns until you start to feel it tighten and place your thumb on the side and then let it rotate back one full turn, then tighten up the stop screw before releasing the dial. The holes in the finger plate and numbers should now be aligned. If not, loosen the screw in the centre of the finger plate and adjust it until everythings aligned.

Stop screw (highlighted)

 

Next the finger stop to the dial case and tighten the two screws on the rear until it's secure:

The dial once the finger stop has been attached

 

The next step is to adjust the switching lever so the switching springs are just open when the dial is in the resting position. As per the official instructions - "The switching lever should be adjusted to test at the top of the ’V‘ bend in the first switching lever spring"

Approximate position of the switching lever

 

Close up of the contacts with the dial in the resting position

 

Now you can finish off reassembly the dial by adding the number label, protective cover, and securing clip. If I'm struggling to keep the label centred I add a little "Pritt Stick" glue. Its tacky enough to secure it but easily removed in future.

The end result

Congratulations, you should now have a shiny dial that looks like it came straight out of the factory. With the dial cleaned and re-assembled it'll probably need re-adjusting to ensure the Pulses per Second and Make / Break ratios are within specifictation. For information on adjusting a dial please see this blog post.