[78-L] Co 113a Alignment issues

[Iñigo Cubillo]

Thanks for your enthusiastic feedback, guys!
I love to tinker with gramophones, and my aim is always to try to make 
modifications to correct design faults WITHOUT SERIOUS MODIFICATION TO THE
ORIGINAL.
With this machine, I have a personal fight... Not being truly exponential, 
its sound is particularly meaty, round and full, and I like it very much. So
it is a constant pain for me to see its bad alignment, due to its short arm 
and bad offset.
Now, answering your questions, for a given distance between the spindle and 
tonearm pivot, there is only an optimum tonearm length, together with an
offset distance, to obtain the minimum possible tracking error across the 
disk. Will not insist on this, for there is ample literature about the issue
on the web. The maths and geometry of this is very simple.

In my photos you may see that there is in fact one only point at the middle 
of the record with zero tracking error (see photos named NULL  ERROR).

Usually, the ideal thing is, for a given spindle-pivot distance:
- a tonearm slightly larger; the surplus distance when you place the needle 
pass the spindle is called OVERHANG, and should be positive. So the needle
must go beyond the spindle when you place the tonearm in line to it.
- a certain inwards angle to the soundbox, so it points backwards (i. e. the 
side opposite to the needle) to a plot at a certain distance to the right of 
the
pivot; this is the OFFSET distance.

With these variables optimized, you can achieve a (-K) tracking error at the 
innermost and the outermost grooves, and the opposite (+K) error at the 
middle of the record. Ideally, there are two points (at roughly 1/3 and 2/3 
of the record) where the error is null.  The longer are these distances, the 
better the adjustment and the smaller the maximum error values. Ideally, the 
tangential solution is a very large tonearm and spindle distance, which go 
to the infinite...

But if you also fix the tonearm length, far from those optimum-aimed values, 
with an "underhang" instead of overhang, which is the case with the 113a,
you have a problem. Still, you can try to modify the inwards angle (or 
offset distance at the back) twisting the vertical plane of the soundbox to
minimize the error. That's what I intended to do with the "wedged" silycone 
joint. This is simple to imagine: I created a rubber joint with variable 
thickness so the diaphragm plane twisted inwards looking to the spindle. 
Adjusted "at the naked eye" I got a fairly good tracking, but the joint was 
so thick at the back, and too compliant, that it caused other problems.

So I prefer to stay with the less bad solution: trying to optimize the 
tracking at the inner grooves, for there is the hardest strain on the record 
grooves.

Of course, when talking about the "tonearm length" one means the ideal line 
going from the pivot to the needle. The inwards angle is the ideal angle 
between the diaphragm plane and this line.

The tonearm length could be modified changing the vertical zenith angle of 
the needle against the record, but this is not advisable, for the ideal 65 
degree angle must be, more or less, respected for the better tracking.

One can also change the position of the soundbox at the tonearm neck by 
adding a purposely made trunnion, with adequate length and curvature to 
provide a certain angle (or offset distance) so to improve things. Playing 
freely with this trunnion you can modify the tonearm length, etc. This is 
still to be tested at home...

My pain is, for instance, when seeing a portable like HMV-101, which has a 
fairly good adjustment (Long tonearm for the pivot-spindle distance, well 
adjusted offset). So it could be done; why Columbia engineers did not make a 
good design too?
HMV 102 has a short tonearm and a good set of parameters, but the 
maximum/minimum error values are well noticeable. This has been well 
optimized, but for a set of values (spindle-pivot distance and tonearm 
length) very small, to make the case shorter, I suppose, due to the larger 
size of the horn mouth. Compromise...

Hope to entertain you,

Inigo Cubillo