Fine Tuning Victor Exhibition Reproducers

[vichighmathguy]

In the course of rebuilding Victor exhibition reproducers/sound boxes, I have always wondered what guidelines those who do this might use to expedite the process. Specifically, in addition to adjusting the front and rear adjusting screws (A and B in the attached diagram) so that the needle arm just touches the diaphragm, how much pressure do you place on the springs while doing this? Those arm springs are notoriously stiff. The Victor instructions that I have seen apparently avoid this aspect of adjustment, referring only to the positioning of the needle arm relative to the diaphragm with a phrase such as “when the screws are properly adjusted”.
I am hoping for answers along the line of “only adjust the springs as tightly as is necessary to remove any free play between needle assembly and fulcrums”, or perhaps “adjust screws so that there is no contact looseness between the needle assembly and the fulcrums, then increase the pressure on the arm springs by screwing in the arm spring screws one quarter turn (insert your version here) more”.

This question comes from knowing that the needle arm must not push or pull on the diaphragm, but that various pressures on the springs achieving that goal will have various impacts on the ability of the needle arm to move in compliance with the vibrations in the record groove. Some balance must be reached between the record blasting and the record sounding “muddy” with loss of sensitivity. I am looking for a “rule of thumb” here based on empirical experience which will achieve a good sounding reproducer that does not blast but which optimizes good frequency response.

I would assume any answer might also apply to Victor No. 2 or No. 4 reproducers and the like. Looking to achieve the best “first approximation” with the least effort here.

So, any rebuilders out there who might like to weight in on this? Please feel free to comment on the efficacy of sharpening fulcrums (knife edges) or other fine tuning techniques.

 

(Question first posted to The Talking Machine Forum in 2021 without resolution)

Thanks,

Don

[Henry]

AFAIK, it isn’t possible to adjust the tension of the springs separately from their function to position the needle arm. Furthermore, it appears to me that the path of vibration from the needle to the diaphragm does not involve the springs. The absence of any mention of this topic in the available literature would seem to support this conjecture.

[Jerry]

7 hours ago, Henry said:

AFAIK, it isn’t possible to adjust the tension of the springs separately from their function to position the needle arm. Furthermore, it appears to me that the path of vibration from the needle to the diaphragm does not involve the springs. The absence of any mention of this topic in the available literature would seem to support this conjecture.

 

Yes, it's possible to vary the position of the needle arm relative to the diaphragm by adjusting spring tension. And yes, the springs do have an influence on the ability of the needle bar to vibrate. 

Edited Friday at 12:30 AM by Jerry

[Henry]

Yes, of course the needle arm position is adjusted by turning the front and back adjusting screws, and those screws bear on the springs. My point is that you cannot adjust the springs apart from adjusting the needle arm position by turning the screws. If you know a way to do this, please tell us!

[PhonoDon]

It’s a given that the springs must bring the end of the needle arm to gently touch the centre of the diaphragm, but beyond that, it is possible to do that with either very light spring tension on both springs or very heavy tension on both springs. The question is how much tension to apply on the springs to optimize the sound of the reproducer. Empirically, one could increment the spring tensions slightly higher in stages and evaluate the sound at each stage, but I thought a little time could be saved if somebody has discovered this “golden tension“ on those springs. As yet, I’ve not heard from anybody who can offer their experience in that regard.