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 Post subject: Tracking error on an HMV 102 with a No 4 soundbox
PostPosted: Sun Nov 12, 2017 5:47 pm 
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Victor Jr
Joined: Fri Mar 17, 2017 2:52 pm
Posts: 16
When I first got my 102, it had a generic soundbox that was riveted together, with "Made in Gt Britain" the only clue as to manufacture. I had a spare brass HMV No 4 soundbox, so I replaced its rubber isolator and brass ring with one intended for an Orthophonic machine. I used that for about a month, until I was able to acquire a 5B soundbox (in almost mint condition), which I've been using ever since. I did this in part because I remember reading somewhere (and I can't remember where now) that the 4 and the 5a/5b were the same diameter and the backs were interchangeable, and because I know that there was a short period in which the 5b was made available with the smaller bore isolator/ring, expressly for fitting on the 101.

However, I did an experiment last week in which I swapped the 4 for the 5b, and noticed that the needle placement is slightly more forward on on the 4 soundbox. This is obviously going to introduce tracking error, but I don't know if it's one that will damage a record.

Here's what I'm looking at (although being able to see it in 3D would be more helpful...)

Attachment:
File comment: No 4 soundbox angle at the start of a record
IMG_0362.png
IMG_0362.png [ 903.86 KiB | Viewed 197 times ]
Attachment:
File comment: HMV 5b soundbox, at the start of a record
IMG_0365.png
IMG_0365.png [ 975.7 KiB | Viewed 197 times ]


This is how the 4 and the 5b sit at the start of a record. Minus the record. (The same needle, a loud tone, is used on both.)

Attachment:
File comment: The 5b next to the spindle
IMG_0366.png
IMG_0366.png [ 1.12 MiB | Viewed 197 times ]


This is where the 5b sits if you put the needle right up to the spindle. It sits next to it, not quite centered, but from what I understand it's best if the needle meets the spindle rather than clearing it. (Note: it looks like the needle angled inward toward the spindle. This is an optical illusion from the normal needle angle. This is where a 3D view would help.)

Attachment:
File comment: No 4 next to the spindle
IMG_0363.png
IMG_0363.png [ 1.02 MiB | Viewed 197 times ]
Attachment:
File comment: HMV No 4 at the spindle, second view
IMG_0364.png
IMG_0364.png [ 1.16 MiB | Viewed 197 times ]


This is the No 4 at the spindle, and as anyone can see, it clears the front of the spindle by a very short distance, but it does clear it.

What I'm wondering, and maybe someone here would know for sure, is whether using the No 4 on my 102 is harmful or not. If the tracking error is enough to cause damage. (I don't worry about using the 5b, since by the time my 102 was built - it's an MOS model from 1945 - the 5b was standard equipment for this machine.)


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 Post subject: Re: Tracking error on an HMV 102 with a No 4 soundbox
PostPosted: Sun Nov 12, 2017 6:22 pm 
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Victor III
Joined: Sun Mar 15, 2009 6:18 am
Posts: 653
I think that the critical parameter to align the soundbox is the angle between the vertical plane that contains the diaphragm and the groove line at the point of contact of the needle to the disc; this angle ideally should be zero, i.e. the diaphragm should be parallel to the groove line. This angle varies during play, ideally around zero degree, and if the needle protrudes beyond the spindle the average angle over the playing distance will not be zero. But having said that, other factors like the condition of the moving parts of the soundbox are more determinant of disc wear, so a priori you cannot say that the 5B will be more lenient to your records than the 4. I would suggest that you try out with a disposable disc and check the amount of residue left at the end of a play utilizing both soundboxes. Hope it helps.


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 Post subject: Re: Tracking error on an HMV 102 with a No 4 soundbox
PostPosted: Sun Nov 12, 2017 7:13 pm 
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Victor Jr
Joined: Fri Mar 17, 2017 2:52 pm
Posts: 16
Carlos, very good point about condition. I should have mentioned that the No 4 was completely rebuilt (by MicaMonster), so it's in top condition. The 5b hasn't been touched, but unless the stylus bar tension is out of whack these don't usually need much attention. The gaskets are felt and don't disintegrate.


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 Post subject: Re: Tracking error on an HMV 102 with a No 4 soundbox
PostPosted: Sun Nov 12, 2017 7:46 pm 
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Victor II
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Joined: Mon Jul 02, 2012 9:57 am
Posts: 488
Location: Hampshire, England.
If you would like to determine the best possible tracking geometry for your machine, and have a little time to spare, I would suggest you might refer to this post which discusses the subject in detail. It is actually written with specific reference to an Expert Senior but every important principle is shown applying to all gramophones.

viewtopic.php?f=11&t=15253&start=10

First measure the offset which, together with the distance from the tonearm bearing centre to the needle point when lying on the record, will enable you to refer to "The table of best overlaps" and read off the ideal overlap (page nine) for your particular gramophone set-up. If you would then like to double-check this setting, acquire or make a simple protractor which will reveal the tracking error from zero at various points across the disc---this could be either positive or negative. It is impossible, by definition, to achieve zero error at all times right across any record played with a swinging tonearm but if you can demonstrate a variation from zero of perhaps three degrees in total, record wear will be kept to an absolute minimum and be of little consequence.


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 Post subject: Re: Tracking error on an HMV 102 with a No 4 soundbox
PostPosted: Sun Nov 12, 2017 9:41 pm 
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Victor Jr
Joined: Fri Mar 17, 2017 2:52 pm
Posts: 16
I have the Wilson-Webb book, although I never attempted to copy its protractor (I did see the other, larger version in the thread you linked). However, there are a pair of gauges on the Graham-ophones site, http://www.graham-ophones.co.uk/keeping-on-track/4590882209 that I printed out and used. One is a tracking assessor, the other a tracking protractor.

Using these, I determined that with an HMV No 4 soundbox, the 102 has tracking errors anywhere from 10 to 15 degrees, which seems extreme. With a 5b, it's much less - anywhere from around 2 to maybe 8. This doesn't seem logical, but part of the calculations of the protractor involves the distance between the stylus point and the backplate, which is slightly but measurably less on the 4 than the 5b. If you mesh the reeding together, with the pivot bars next to each other, the pivot bar on one won't line up with the other.


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 Post subject: Re: Tracking error on an HMV 102 with a No 4 soundbox
PostPosted: Mon Nov 13, 2017 7:52 am 
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Victor II
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Joined: Mon Jul 02, 2012 9:57 am
Posts: 488
Location: Hampshire, England.
shoshani wrote:
If you mesh the reeding together, with the pivot bars next to each other, the pivot bar on one won't line up with the other.


Many thanks for the link to the "graham-ophones" which I have not seen before. His explanation is excellent and I was amused as my name is also Graham !

I wonder if you would mind expanding on your statement above as I am not quite sure what you mean by "reeding" ?


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 Post subject: Re: Tracking error on an HMV 102 with a No 4 soundbox
PostPosted: Mon Nov 13, 2017 8:57 am 
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Victor III
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F. Depero, "Grammofono", 1923.
Joined: Thu Feb 24, 2011 4:19 am
Posts: 546
Location: Italy
What you refer to as "clearing the front of the spindle" is technically named overhang in literature.

In general, while ingeneering a tonearm, a centimeter or two of overhang helps in keeping the tracking error as low and as constant as possible. However, the tracking error depends not only on overhang, but also on many other parmeters, like the length of the tonearm, the position of the tonearm hinge, the angle of the transducer, etc. As a result, there are many examples of excellent tonearms which overhang is zero or negligible.

Again in general, the 102 (I also own one) is a very well engineered gramophone, one of the few in which the tracking error has been taken in serious consideration. In my opinion, repeating the calculations is quite useless, as there isn't any tracking error adjustment point anywhere.

I'd rather check if the 5B sits indeed well in the tonearm bayonet. Although it's a bit hard to assess by your pictures, in image 0365 it looks as if the needle is almost vertical and would read the record at an alarming angle. By the way, this would also explain the overhang discrepancies.


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 Post subject: Re: Tracking error on an HMV 102 with a No 4 soundbox
PostPosted: Mon Nov 13, 2017 10:49 am 
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Victor II
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Joined: Mon Jul 02, 2012 9:57 am
Posts: 488
Location: Hampshire, England.
Marco Gilardetti wrote:
In my opinion, repeating the calculations is quite useless, as there isn't any tracking error adjustment point anywhere.



I would have to disagree with that statement as Shoshani's original query was "This is obviously going to introduce tracking error, but I don't know if it's one that will damage a record".

I take your point about the bayonet attachment not allowing any alteration of overlap in its final fixed position but by taking two simple measurements and referring to the chart it is quickly possible---and I should have thought rather informative and helpful---to confirm the tracking accuracy or otherwise. Do not forget that needle extension also affects tracking.


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 Post subject: Re: Tracking error on an HMV 102 with a No 4 soundbox
PostPosted: Mon Nov 13, 2017 11:40 am 
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Victor Jr
Joined: Fri Mar 17, 2017 2:52 pm
Posts: 16
emgcr wrote:
shoshani wrote:
If you mesh the reeding together, with the pivot bars next to each other, the pivot bar on one won't line up with the other.


Many thanks for the link to the "graham-ophones" which I have not seen before. His explanation is excellent and I was amused as my name is also Graham !

I wonder if you would mind expanding on your statement above as I am not quite sure what you mean by "reeding" ?


Perhaps "milling" would have been a better word. Reeding is mostly used to describe the edges of coins; the outer edge of the backplate on both the 4 and the 5b has a similar edge.


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 Post subject: Re: Tracking error on an HMV 102 with a No 4 soundbox
PostPosted: Mon Nov 13, 2017 11:44 am 
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Victor Jr
Joined: Fri Mar 17, 2017 2:52 pm
Posts: 16
Marco Gilardetti wrote:
I'd rather check if the 5B sits indeed well in the tonearm bayonet. Although it's a bit hard to assess by your pictures, in image 0365 it looks as if the needle is almost vertical and would read the record at an alarming angle. By the way, this would also explain the overhang discrepancies.


Hm. I don't see that in the image, but both soundboxes sit correctly, with the stylus at a 60 degree angle. The bayonet is strong both in the isolator/ring assembly and in the arm itself. I can turn the soundbox back, after fitting it on, to a positive stop, and the "grip" of the rubber bush on the 5b is very good at keeping it in place.


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