The cracked center gear is a well known issue with most of the SONY DD mechanisms. The cause of this failure is supposed to be a certain plastic composition used by SONY that shrinks in time. SONY supposedly introduced a new improved version of the gear later. But I understand that they are very rare to find and highly sought after. Sony does not manufacture them anymore and collectors who may have a few spare are reluctant to sell them. Since there is no real supply of a replacement gear, the only solution to get these real gems back to near perfect working condition is to somehow mend the broken gear.
I went through various forums (aka tapehead.net and stereo2go) and had a look at how others had attempted to fix the broken gear. My version is a slight variation of what others have attempted. I’ll try to explain the philosophy of my version of the fix in the paras that follow.
Seen below is the (in)famous cracked center gear,
As it can be seen, the outer ring of this gear is made of plastic and the inner disc is made of metal. The “center gear” as it is popularly known, is actually part of the ‘magnetic clutch’ system. There is a magnetized gear which is placed on top of this metal disc, both of which then form the complete ‘clutch’ assembly. The purpose of this clutch assembly is to allow variable speed for the take-up reel. The take-up spindle has to gradually change its speed(reduce RPM) as the reel gets ‘filled up’, to maintain constant linear tape speed. Since towards the end of the tape the circumference is greater, less RPM (on the spindle) is required to take up the same amount of tape. The magnetic clutch performs this by allowing for increased ‘slip’ as the tape progresses towards the end. According to me, this is a very elegant way of doing it and much better than using rubber idlers which tend to wear out in time. Now you may wonder why they didn’t make the whole gear out of metal and get rid of the plastic part. Maybe if they had done that, the metal tooth on the center gear would have been too abrasive for the adjacent plastic gear. And they didn’t make all the gears out of metal since, metal gears are more noisy, more expensive and add more weight. More weight also mean the motor has to do more work and in turn we get less playing time. Just my opinion.
Coming back to the point, we can see how the plastic ring has opened up because of the shrinkage. Because of the shrink, there is no way we can just join the ends and glue it together. Even if you manage to pull the ends together and glue it, there would be enough stress present to again break the bond. The reason why it split in the first place. Someone reduced the diameter of the inner metal disc but that requires special lathe tools. Using a hand file is also very tasking as the disc is probably steel and has some decent thickness. Maintaining a constant diameter on the disc while filing the edges is close to impossible, which may result in increased wow during playback. Others have inserted a small filler teeth from some other donor gear wheel and some have even used epoxy based glue to recreate a teeth in the gap. While both these methods are known to work they seem a bit fragile to me and am not sure how well they will hold up in the long term. Plus, since the outer plastic ring has opened up, the shape of the outer ring is actually oval instead of a perfect circle. This might put extra pressure on the adjacent gear and again affect the wow during playback. Adding or recreating a teeth does not correct this problem.
So trying to keep all the above factors in mind, this is the best that I could come up with,
- Carefully remove the outer plastic gear. It is quite flexible, but still be very careful as you don’t want the gear tearing at random places. You also really do not want to deform it.
- Now approximately opposite the already existing cut, make one more cut. The plastic material is difficult to cut and so use a very sharp/new exacto blade to cut it or you may end up getting a very shabby cut, like I did. Plus, cut it in such a way that the cut is between two tooth(in the trough).
This will help in more than one way. Firstly consider this – With the already existing crack, the plastic ring/gear acts as a ‘spring’ with a tendency to ‘open up’ the ring. Trying to push the ends towards the metal disc and then joining/glue-ing the two ends is akin to working against the internal material generated force. No glue is going to keep it together in the long term. This is because the glue will dry out sooner or later whereas the plastic will retain at least some of its elasticity.
- At this juncture, I kinda ‘tempered’ the two pieces of plastic. I am not sure whether this helped or not but I felt this might be critical. Basically I used a hair dryer to alternately heat and cool the plastic pieces. The idea is to get rid of any volatile material still present in the plastic composition. To be more general, all I did is attempted to hasten the plastic pieces to shrink (if it still wasn’t done shrinking) and make sure it is in some form of equilibrium. I am of the opinion the plastic will not keep on shrinking forever, but will stop at some time, and the aim is to reach that state before any corrective action is performed.
- Now the two pieces of plastic are ready. Arrange both of them around the metal disc so that both the gaps are equal. Refer the following figure. You have to place the gear assembly back in the walkman to complete this step. Here the trick is, instead of filling the gap with an additional teeth, just leave that position vacant. Align the existing teeth to the teeth next to the vacant one. Fortunately because of the size of the adjacent gears, skipping one teeth on the center gear seems to work fine. Keep rotating the center gear with your fingers for multiple revolutions and make sure you don’t feel any discontinuity when the cut engages with the adjacent gear. Keep adjusting the relative position of the two plastic pieces till you can get a smooth engaging of the gears at the cut. Well this is to get rid of the ‘click clack’. Surprisingly this didn’t turn out to be too difficult in my case and it worked pretty well. Make sure the plastic edge gear is fit snugly on the metal disc(all the way in) when you fine tune the positions of the plastic pieces.
- This was just the rehearsal of the final step, where I glue the plastic pieces to the metal disc. I used a combination of glue and epoxy to bond the gear pieces. Just went down to the local hardware store and found myself a glue which claimed to glue plastic as well as metal. Applied the glue on the edge of the metal disc, on both upper and lower side. This glue is relatively thick (more viscous), almost like a gel. Since it is not instant glue, I got some time to fine tune the position of the plastic pieces before it hardens. I put the gear in walkman (carefully) and rotated it by hand to check for any feeling of ‘discontinuity’ at the gaps. Adjusted the plastic pieces iteratively until I got a smooth rotation. After I did this, I used epoxy on the inner edge of the metal and plastic gear. Refer figure. The epoxy is a general purpose 2-tube mix found in hardware store (Be careful and keep the application as narrow as possible on the metal disc. If it extends too much on the inner side, the epoxy will come in contact with the magnetic disc (and raise its height) which is placed on top of the metal disc). The gap between the plastic gear was also filled with epoxy for some added bonding strength and structural integrity.
- The gear was not disturbed for 24 hours to let the glue do its job.
The plastic supply reel pinion had also shrunk and cracked. This gear is supposed to be press fit on the metal spindle/rod attachment. Because of the discontinuity due to the crack, the gear would make lot of clacking noise. The only way of closing the gap was to reduce the thickness of the metal rod. Well that is not an option, so I made the diameter of the inner hole on the pinion larger. I had a phillips driver which was exactly the correct size and used it as a hand drill to make the inner diameter bigger. This would defeat the press fit design, but thats the intention anyways. Just glued the spindle tight to the pinion so that it doesn’t slip during rewind.
Its been around 3 weeks and the it looks like the gear job is holding up pretty well. The click clack is negligible and rewind works. Since now there is substantially less shrink generated stress on the plastic gear, I believe it will hold itself in time. Between, the Walkman sounds unbelievably ‘powerful’. I can easily make out the superior stability of the DD Transport. I have never had anything to match this quality till now. Type 1 cassettes have never sounded so good before. Even old cassettes which I though had souped up long ago sound great on the D3! But metal tapes don’t sound as good as compared to EX Amorphous models. Also the overall build quality is astonishing (as expected from any japanese stuff) and has amazingly detailed finishing. Just love the overall feel of it.
Before I put everything back together, I took a small video of the mechanism at work. I’ll leave you with that. This worked for me and I hope it works for all you DD owners out there. I’ll upload disassembly pics for the WM-D3 in the next post for reference. Long live the Walkman. I won’t let it die.