The Pioneer SX-5580 I am renovating was not looked after very well at some point, surprising considering how prestigious a piece of equipment it is and how much it would have cost. One of the problems was some bent volume control shafts and switch shafts. The 10KHz Treble switch in particular was very bent indeed. The others were not too much of a problem as it was only a case of bending the two split parts of the aluminium shaft into alignment. But the 10KHz switch was a whole different game, just looking at it made me feel a little ill as I knew how hard finding a solution was going to be. The shaft was broken right at the edge of its casing (as you’d expect) so did not protrude at all.
This picture shows the board this switch came from, the switch is already out having been de-soldered – you can see the gap it’s left in the assembly hanging at the bottom (the white card is polystyrene foam from a pizza box and used to insulate and protect the board that’s hanging in the middle):
This problem (broken potentiometer / switch shaft) is quite common and is now solved so I am going to put everything down here in case others find it useful.
First of all, I looked at the web forums and the consensus was buy a new potentiometer / switch. That was not a solution for me as this switch is dedicated to the SX-5580 and finding a different switch to replace it would cost too much (approx. £50). Plus I am renovating this unit to sell in order to finance Liquiphonics and buyers (understandably) want as much originality as possible. So onto the fix…
All materials (as far as I’m aware) work harden, thats how a strong piece of metal can be broken, just keep bending it at the same point; eventually it will give and break in two. I’m no scientist and in some ways understanding what happens to metal crystals when they stretch through work hardening is irrelevant; the fact is, if you keep bending a piece of aluminium rod, it goes hard and will at some point snap instead of bending. Aluminium does this fairly soon in the number of bends and since this shaft was bent so much, I felt very nervous about forcing it too much and too far. I used a piece of steel tube with a good fit and put it over the end of the shaft and pulled, I did this in situ so the casing took the load and not the PCB. It bent a little but not enough so I did it again and looked at the results. I kept doing this until it was very nearly inline – and then it broke. There’s no point in me telling you what I called it as you can probably make a pretty good guess.
When taking a switch apart, it’s good practice to put it in a position that will help you put it back together. That way all the parts will be in correct alignment so say -10dB on the scale gives -10dB and not +20dB. I turned the switch so that it was completely clockwise and then took lots of photos of it to show me where the insides were positioned:
When metal (or any other material) breaks the slight imperfections in its structure cause greater or lesser stretching of the material and this is like a finger-print. In other words, the two pieces will fit closely together in one position only. This ‘fingerprint’ is useful as it helps to align the two elements. Nothing is perfect and there may be subtle differences between the two parts but in my case they were close enough for this to not be a problem. You can see the shape of the metal in this picture:
Putting the two together showed the join to be very tight fitting and the shaft to be almost straight. I then superglued it in this position.
I haven’t measured the angle but looking at it against a right angle it looks some fraction of a degree.
Superglue is nowhere near strong enough for this application and even if it were stronger, I would feel uncomfortable about relying on it. What I really wanted was the two parts to be mechanically joined and the best way I could think of was to use a split pin rod, the split rod can be seen in a picture further down.
To hold the two parts rigid together while I drilled it, I put three layers of heat-shrink sleeving around the shaft:
The picture below shows the drill in position. Actually I took the photo after all the work was done but took it just to be complete – I know how scary this sort of thing can be even for experienced workers in this field so every picture counts. Plus I’m a bit anal when it comes to this sort of thing. Down with this sort of thing (that’s an in-joke for lovers of ‘Father Ted’):
When holding a hand-drill it must be kept perpendicular to the work, the last thing you want is for the bit to exit out the side of the shaft. That’s unlikely as it’s not that hard to keep the drill vertical but even a little miss-alignment may mean the shaft has a kink in it at the joint which would totally defeat the whole point of doing this. Fortunately with this shaft, it has a split down the middle (as many do) and that meant the drill bit was kept in correct alignment in the z axis (away and towards you). That means only the x axis has to be judged by the person drilling. So keep the slot running left to right (along the x axis) and it is then easier to see that the drill bit is vertical.
Drilling the hole broke the superglue hold but I had put enough of a hole in the lower section for that to not be a problem. I simply pulled out the upper section and carried on drilling:
Once drilled I removed the heat-shrink sleeving. The hardened steel split pin can be seen to the left of the shaft:
Here is something important to note. The split pin I used was 2mm in width and 12mm long (the drill bit I used was 2mm though measured 1.96mm with a vernier gauge). I wanted about 7mm in the lower section of the shaft leaving about 5mm in the upper section. Since I have a shite memory I totally forgot to watch how far I drilled in, most of my brain cells were watching for alignment (the rest were off napping somewhere). I could have put a piece of PVC tape around the drill bit to show me when to stop. The result was I drilled about 2mm too deep into the lower section. I cut a piece of 14 SWG (2.032mm) tinned copper wire and dropped that into the hole. In case you don’t know (but you probably will), wire cutters leave a right-angle cut or a sloped cut depending on which end of the cut you look at – I used the cutters to give right-angles at both ends. This piece of wire stopped the split pin from going in too far when I tapped it into the lower section with a small hammer.
One thing I did note from looking at the forums (a list of which is below), they recommend JB Weld so I bought some and used that to glue the too parts in addition to the pin. Before glueing, since I would not be able to see the join of the two parts, I put an indelible mark on both sections of the shaft to make it easier to align once the JB Weld was added. The marks are only for rough alignment as your fingers will feel it drop into place when exactly aligned:
Having put the glue on and pushed the upper shaft section onto the split pin using the small hammer and watching the alignment marks, I smeared the glue out and over the join to help reinforce the junction and fill the gap. I then left the whole thing for the glue to set over night and cut off the excess with a sharp knife (scalpel) leaving a smooth finish. To clean the shaft I used a fibreglass cleaning stick. I’m mentioning this here as I find this stick to be incomparably good at cleaning things such as this and especially corroded switch or connector contacts. This stick is a rod of long glass fibres held together with plastic (presumably heat-shrunk on). The ends of the fibres do the cleaning and being made of glass they are hard and abrasive but very fine indeed plus the stiffness (and therefore abrasion) can be regulated by the length of the fibres showing by cutting back the plastic – so one end can be long, the other short. I have had one for decades now, use it all the time and having started out at average pencil length, it is still three inches long. So well worth any cost involved.
Here is the finished shaft join:
One last thing and my apologies if you already know this but if not it could prove useful. Drill bits should not be put too far into the drill’s chuck. Drill bits are hardened at the cutting end but softer at the chuck end. This is so the bit does not bend (or break) when pushing forces are applied. If you find your bits are breaking or bending you are putting it in too far. I generally put them in no further than 1/2″ (1.5cm).
Forums I used, many thanks to those people: