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 Post subject: Re: Need help restoring a Meltrope III soundbox
PostPosted: Thu Jul 05, 2018 12:53 pm 
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Victor III
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Hi Inigo, yes, seeing the photo of your front gasket, there is a gap at the join. You need to cut the gaskets longer - slightly over-length in fact - so that they seal 100% airtight. This may be your problem, or part of it. Expect almost to "fight" a little to get them in.
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 Post subject: Re: Need help restoring a Meltrope III soundbox
PostPosted: Thu Jul 05, 2018 1:17 pm 
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Victor III
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Your cover-plate upturn to retain the top ball bearings does seem rather pronounced compared to others I have seen and the stylus fulcrum bar, which is sandwiched between the ball-bearings, often rests with its top surface only very slightly above the surrounding casework. However, pressure can be varied by bending the upturn, so this aspect should not unduly adversely affect performance but the pressure does not want to be excessive--just sufficient to preclude rattling on highly modulated records.

Where you definitely do have a problem is with the gaskets. As Orchorsol suggests, it is imperative that both have airtight compression joints where the cut tubing butts together or you will never obtain satisfactory results. A gap allows air to escape thus dramatically reducing motive energy into the tonearm. A good source of tubing is from bicycle valve rubber which is the appropriate diameter---0.125” or 3.18 mm. This material is very compliant and gives top results.


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 Post subject: Re: Need help restoring a Meltrope III soundbox
PostPosted: Mon Jul 09, 2018 2:22 am 
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Victor II
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So I can conclude that, 1) the gaps in the gasket must be zero, using longer gaskets; 2) use bicycle valve tubing; 3) the large ball bearings are too large, the fulcrum plate must rest only slightly above the soundbox body.
I will amend all that and try again.
Nevertheless, there its something odd about the feeling of the diaphragm. When you scrub a 5a/5b diaphragm slightly with the finger tip, the sound coming out is a very light scratch, in which you can feel in a certain way the sound of it. All the treble of the soundbox is there, it is a 'gramophonic' sound. It seems that the bass is obtained later, by the pumping action, the soundbox chamber and the sound conduit up to the horn mouth. But all the sound its in there, in the sound of this tiny scratching. I don't know if you have ever experimented that...
The sound of the Meltrope diaphragm (in my case) when scrubbed or touched is much more 'dead', as if the diaphragm was too thick... It lacks the treble I find when touching the 5a or 5b. And it its a small replica of what I find when playing a record with the soundbox. My Meltrope has a poor upper register... the sound its not 'open' at all. I feel the bad thing is my diaphragm...

Thanks to all you for your help. Any more suggestions are welcome!
Inigo.
Inigo


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 Post subject: Re: Need help restoring a Meltrope III soundbox
PostPosted: Mon Jul 09, 2018 3:49 am 
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Victor III
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I don't think you should change the size of the ball bearings. Here is a link to one of soundgen's auctions - the photo shows the relative size clearly: https://www.ebay.co.uk/itm/Meltrope-111 ... SwnEtbM1SF Instead, as emgcr says, you could bend the "wings" of the cover plate gently/slightly, to relieve the pressure a little.

I also think (as mentioned before) that part of your perceived problem with treble response could be a design impedance mismatch between the Meltrope soundbox and the Orthophonic horn. (Does anyone else have experience and a clear impression of how a Meltrope III performs on a 102?) Once you've corrected and optimized the Meltrope rebuild, try it on another machine!
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 Post subject: Re: Need help restoring a Meltrope III soundbox
PostPosted: Tue Jul 10, 2018 2:35 pm 
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Victor II
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Yes, Orchorsol. Could be the impedance mismatch. This afternoon I've rebuilt the Meltrope III again, using a new front gasket, cut to fit tightly with no gaps. I've laso changed the large balls for others a bit smaller (not as small as the tiny ones) and it looks frakly better. The sound has improved in the bass, but it continues to sound 'boxy' on the HMV-194. Compared to the 5B, the sound of the latter is much more round and clear, more natural and equilibrated. I've nevertheless noticed a slight deviation on the needle from the vertical, and this could be that the diaphragm should be shaped more convex so the needlebar goes further away from the soundbox back. The larger balls under the fulcrum plate would aggravate this problem. The direction of tilting is such like if the diaphragm was 'too deep' into the soundbox (needle pointing outside of the diapragm plane). A larger bore back gasket would also improve the angle. But I'm using 3.8mm tubing inside the groove of the back... ??? :?:
Inigo


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 Post subject: Re: Need help restoring a Meltrope III soundbox
PostPosted: Wed Jul 11, 2018 3:31 am 
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Victor III
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I think you may find all will be well when you use the correct diameter/compliance tubing and that the geometry will come right. The diaphragm should never be dished in any way and should lie completely flat. You are currently using a diameter of tubing nearly 20% too large so check that the fulcrum pivot bar is not touching the rubber. There should be an air gap between the two. Check also that the stylus is a tight fit in the diaphragm. It looks good judging by the photo but a drop of beeswax (easier on the inside) never goes amiss.

If you would like to send me a private message (pm) with your address, I shall send you enough tubing to do the job.


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 Post subject: Re: Need help restoring a Meltrope III soundbox
PostPosted: Mon Jul 16, 2018 9:05 am 
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Victor II
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Pm sent
Inigo


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 Post subject: Re: Need help restoring a Meltrope III soundbox
PostPosted: Thu Jan 03, 2019 9:13 pm 
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Victor II
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These days I’ve been chatting by e-mail with emgcr, who generously has sent me a piece of soft rubber tubing, adequate for the gaskets of the Meltrope soundboxes. It is bicycle tyre valves rubber tubing (3.8 mm diameter and thin wall). After the long conversation, we’ve decided that a summary report about the repair work done and the soundbox trials would be interesting for the forum. So I’m editing the e-mails, and here is the result.
My own writing is in black, Graham’s comments in red.


I – Meltrope repair and trial – by Inigo (Dec. 23rd)

I put hands on the Meltrope remaking it with the rubber tubing generously presented by emgcr. I have to say that I went for all it, for the diaphragm looked suspicious of needing a good edge flattening, and a thorough cleaning. I suspected that the initial bad sound could be caused by inadequacy of the gaskets, but also that the diaphragm seemed a bit thick, and sounded dead when touched with the fingers. The cleaning method (Jeff Lutton, Bob Waltrip et al.) for the aluminium diaphragms gives good results, so I decided to detach the diaphragm from the Meltrope stylus bar (machine-stamped) and do a thorough work on the diaphragm to see if the sound improves.

The trick about how to detach the diaphragm from the stylus bar end was solved using a small rotative drilling tool (Dremel) with a grinder attachment. I ground out the machine-stamped end of the stylus bar that holds the diaphragm in place. A delicate and slow work, but went well as expected, without damage to the diaphragm. But the stylus bar stamped head was naturally destroyed in the process, and will need a replacement.

Once the diaphragm removed, I bathed it in alcohol and cleaned out all that copper-brown painting, lacquer, etc. The diaphragm, attached to the stylus bar by its central dome was indeed sandwiched between two other small domes of the same material, located at either side; kind of reinforcement for that area. I also removed them.

I thoroughly cleaned the diaphragm, and straightened the wide flat edge area all around, pressing softly against a hard surface with a cotton pad. I also did a thorough scrubbing with metal cleaner (magic cotton) and polished it using fine steel wool. The diaphragm resulted pretty shiny, reasonably straightened and overall good looking.

I reattached the stylus bar to the diaphragm using a drop of tin solder; a bit difficult to get it hot enough to solder, but it finally worked. Nevertheless, I couldn’t replace the two small dome reinforcements at either side, as I found it greatly difficult to hold all it in place while soldering the stylus bar end to the diaphragm through that complicated arrangement.

I don’t know if these little dome reinforcements have something to do with the sound, or they are a mere structural reinforcement. I’ve read in Meltrope adverts (Gramophone) that these soundboxes had a patented spider. I’ve not found any spider. I don’t think this could refer to the little dome reinforcements; they are very small and only cover the small central dome of the diaphragm. An orthophonic spider carries the needlebar vibration to an intermediate ridge in the diaphragm (at ½ of radius). The Bettini diaphragms had an arrangement of stiff bars carrying the vibrations to several points around the centre of the diaphragm. This is the same principle of the orthophonic spider, but a small dome in the centre...?

Anyway, I reassembled the diaphragm on the soundbox, well centred, etc., using the new rubber tubing, adding some vaseline to make it all slippery and easier to adjust. I added the small bearing balls and adjusted the cover.

At first I noticed that the two ‘pressing’ screws nearest to the fulcrum plate exerted too much pressure on the balls and the stylus was less free to move, and the sound was not good. I adjusted them first at mid-depth, exerting just a small pressure on the fulcrum. Some investigation and a later adequate bending of the cover legs that hold the balls in place allowed to install the screws at full depth but exerting only a slight pressure on the balls. Just to avoid rattle. The other way looked no good at all with the screws at mid-depth, and needed fixing them with glue. The cover never rested flat on the soundbox body.

I’ve been trying the soundbox again, and thanks to the new rubber gaskets and the thorough cleaning and straightening of the diaphragm, the sound of the Meltrope has improved notably over past experiments.

Anyway, being a pretty loud sound with a full bass, I can’t get rid of that ‘tunnel’ sensation. The mid frequencies sound exaggerated over the high treble register, which sounds poor, resulting a sound somewhat similar to that through a poor horn. I’ve tried soft steel needles, bamboo needles, etc., and compared with the 5A and 5B I find the sound of the HMVs much more open clear and pleasant to the ears than that of the Meltrope. I recall many colleagues saying that the sound of a Meltrope III is almost as good as an EMG/Expert soundbox. It intrigues me very much, so I will continue trying to improve the Meltrope until I get rid of that ‘tunnel’ sound. Another colleague told that maybe the Meltrope didn’t match very well with the HMV194 horn. It could be, but this horn with a 5A/B gives a marvellous sound, full and rich, and very well equilibrated.

Maybe it is that my Meltrope diaphragm was not original, or is too thick. Meltropes were cheap soundboxes, although a happy circumstance made them marvellously well designed for the sound they are reported to produce. I’ve always had that feeling of lack of treble when slightly scratch the diaphragm with the fingertip. The sound of 5A/B diaphragms (standing alone out of the soundbox cupped in the hand) is much more crispy, and that small scratching sound has lots of treble; all the soundbox treble is in there... But the Meltrope diaphragm sounds dead, and the feeling is that it is thicker than the others (I have no tools to measure the thickness). It also seems that the needle scratch sounds exaggerated, very loud, with the restored Meltrope. I mean at the mid-treble register, for the high-treble is indeed absent.

The air chamber in the Meltrope is smaller than that of the HMVs, and it has no phase plug nor any other of the HMV features, where the backplate is shaped following faithfully the valleys and ridges of the diaphragm… But the soundbox design is an equilibrium between many variables, and one good solution can be different from other.

All that said, my Meltrope III gives worse results than my restored HMV 5A/5B, properly adjusted. Either this... or I’ve been a lucky man to have a 5A/5B with very good adjustment and a marvelous sound... or I’ve been so many years tinkering with these small orthophonics that I’ve developed an unconscious mastery on adjusting them, which I cannot reach with the Meltrope. Or my only example of a Meltrope III is not a good one. Anything can be the reason.
Will continue my experiments and report later. I’m a stubborn man that never surrenders. Anyway, how funny is all this. And how satisfying when you finally get a good sound of something...!

Another future plan is to construct my own diaphragm. I’ve been joining suitable materials and tools to be able to make (or try to make) one by hand.


II - Comments by emgcr – Dec. 24th

I am particularly interested that you have omitted to replace the two small cones [the reinforcement domes]. I have several variants of this feature in Meltrope 111 boxes in my possession. The best condition ones have the twin cones a very close fit to the main diaphragm but some have just the outer cone with a deep “glue” connection and then riveted on the back. There seem to be different specifications and this may point to the fact that there was continuous development in period?

It is good that you are experimenting. I can tell you that my scruffiest looking and most bashed about Meltrope 111 is also the best sounding ! Yes, it is almost as good as my best EMG box !! Some of the others look good cosmetically but do not sound well at all. Eternally fascinating. I think a new soft rubber ring where the box fits to the tonearm is a definite advantage. Do you have such a thing ? Chunny has them in stock in Australia.
[I acquired recently some of those red rubber rings, and since then I’m only using them substituting the original rubber connections of HMVs (no 4 and 5A/B). I’ve found this gives better results. Also a silicone replacement for the original hardened rubber collar, poured between the brass collar and the soundbox backplate neck gives similarly good results.]

Anyway, I should be very interested in reading your future reports and please don’t hesitate to ask questions if you think I can help.


III - Later comments and answers by Inigo (black) and emgcr (red) – Dec. 24th & 26th

Would you mind if I edit the report and post it to the forum? I really enjoy with reports from colleagues and all the comments and discussions they generate. And this one can be interesting for others as well... Please do but I think it might be worth mentioning that I have never done what you have done ---ie removed the stylus bar from the diaphragm on a Meltrope 111. You are in new territory as far as I am concerned--- well done ! I’ve broken or spoiled many things with these experiments...

I have, of course, done the same thing many times with the EMG/Expert items and pretty much know what sounds best for them. The important thing there is to ensure a flat plane and an airtight seal where the tiny screw fixes the stylus bar to the diaphragm---a drop of beeswax. Beeswax always saves us.

By the way, do you know the thickness of the M 111 diaphragm ? EMGs were about 0.0035”. I should be interested in your measurement if you ever have time for such work. I don’t have a tool with such precision as needed to measure a diaphragm. I can only mention that I feel it a bit thicker than the HMV 5A/5B diaphragm, but it’s only an appreciation.

Reading again the Meltrope instructions leaflet I’ve noticed one thing I’d forgot: the adjustability of the tone by means of the pressure ring on the back rubber connector, and the ability to move the soundbox back and forth on the tonearm. Must try to change these settings. Yes, this can make a (small) difference in my experience. Of course, a hard rubber does create a problem. I’ve done several tests (non formal) and I cannot find much difference... but I have to repeat the tests in a more quiet environment and paying the maximum attention to see if any difference is audible.


IV – Other related comments

Your comment about the (small) internal diameter of the hole at the back of the soundbox casing was interesting. I shall check some of mine when I get a moment. You may see the photos of my Meltrope earlier in the post. I don’t remember if the hole can be clearly seen...
I’m planning to polish the edges of the back hole, creating a chamfer so the hole presents a sharp edge to the airstream. Maybe this does something to the sound...

In my thoroughly restored 5A/B soundboxes I polished by hand (using fine steel wool) very carefully all the inner convolutions of the backplate, softening every part of the air chamber, even the phase plug and its three surrounding air passages, as I found them irregularly cast (with casting rests). I did what I could by hand to ease and soften all the air passage. I don’t know if it makes sense or not (it seems adequate to an engineer’s mind) but the sound I’ve got from my restored HMV soundboxes is simply astounding, with plenty of bass and a very good treble register. I’ve still not managed to get such a good sound from the Meltrope III nor from any other soundbox. My tricks are: clean thoroughly the diaphragm with alcohol, leaving no trace of dirt, dust, oxidation, etc., and seal all pores with vinyl glue (instead of wax); polish and clean all the air passages and chambers of the soundbox; mount the diaphragm and the soundbox so the gaskets exert only the minimum necessary pressure to avoid air leakages; perfect adjustment of the needlebar; a soft rubber collar.

I’ve done the same things to all my HMVs (no 4 and 5A/B) and the sound of all them is near perfect. I’ve never listened to an Expert-EMG soundbox, but these HMVs give a very good sound, open and clear, full of bass and crisp treble, either through the big horn of the HMV194 and on the smaller horns of the 127 tabletop, and the portables 101 and 102.

I’d like to try them in one of those true exponential horns (EMG) of yours... I think my HMV194 gives a good sound, yet I feel the horn gives only an approximation of the real thing.

I had plans since many years ago to build a big pseudo-exponential/tractrix horn using the corner of a room (walls and ceiling). Someday I’ll make a drawing and show it to you. In one single run (floor to ceiling) you’d get a 2.50 m long horn, but if you go back and forth along the vertical corner, you can make it twice longer, etc... And a reflector at the upper corner will convey the sound to the centre of the room, being the ceiling and walls the final sides of the giant horn… You only need to add a well designed ‘third side’ against the walls corner, forming a triangular cross-section expanding horn. A final triangular board against the ceiling corner will send the sound to the centre of the room. A nice project.

Another thing I’ve been thinking about is the manufacture quality of the soundboxes. I’ve seen on the web some images from other Meltropes and I’ve observed many differences in the shape and quality of the metal covers, for instance... the pits stamped on the corners near the needlebar for holding the external pressure balls, for instance, seem very different from one example to another. Interesting. I’ve seen covers with very well stamped pits, deep and rounded, almost spherical. My cover is not very well stamped, and it seems to exert too much pressure on the balls if I screw at full depth the two nearest screws. Yes, that is definitely not a good thing. My cover is very thick and hard to bend, so it was not at all easy to make it perfectly flat and with the cones raised so they exert less pressure. By careful re-bending and much test-and-trial I’ve managed to adapt the shape to my ideas.

It is certainly a good and happy design, for the many variants one can see of the same model surely sound all very well... And it seems cheap to produce. I think that may be the reason they changed from the M 11 ?

I will try to purchase another examples, just for fun. Good idea and I shall follow your work with great interest. I wish I had time for further investigation and experimentation myself but the demands of life never stop do they ?!


V – Illustrated epilogue

I’ve posted two videos in Youtube, for interest of colleagues, comparing the same record played in the HMV 194 using the original (restored) 5A/B soundbox and the Meltrope. See if you can notice the difference in sound...

Video using Meltrope III: https://youtu.be/Zx0mH8aND_E

Video using HMV 5A/B: https://youtu.be/Ikfj1Qo4L4o
Inigo


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 Post subject: Re: Need help restoring a Meltrope III soundbox
PostPosted: Tue Jan 08, 2019 10:57 am 
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Victor III
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I do applaud you for all this painstaking investigation and reporting - it's excellent to read it all.

I think the two reinforcing cones sandwiching the centre of the diaphragm must be what the designers referred to (very loosely) as the "spider". I also suspect they make a difference to the sound by forcing the diaphragm to work more like a piston, as opposed to deflecting from the centre point with more flexing. Unfortunately, given the variability of one example to the next, controlled testing would be almost impossible and we will probably never know!

I also go back to my previous comment - by design, the HMV soundbox is matched to the HMV horn system, whereas the Meltrope is a different entity and may perform better on other machines. Maybe we will never know what type of horn(s) the designers of the Meltrope intended it for, or rather, how they set about optimising the design to be as universal or tolerant as possible in terms of impedance matching. Presumably most of the intended sales were to owners wanting to upgrade from lower quality soundboxes, in which case there would not have been much need to cater for contemporary HMV designs.
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 Post subject: Re: Need help restoring a Meltrope III soundbox
PostPosted: Mon Jan 14, 2019 6:38 pm 
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Victor II
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Orchorsol, thanks for your applause. But although reporting is a heavy task, is the way all this knowledge (or unknowledge...) is shared with others, leading to comments and new ideas, etc. Unreported work is somewhat useless.

Apart from that, I'm sorry for having a tough defect: I forget to take photos while in the process of repairing, because it absorbs my attention too much. I try to amend this using the photos that I've taken after the repair work, so you can see the differences.

Pages 1 & 2 of this topic contain photos of the Meltrope after repair session no.1, when I simply replaced the gaskets. They show the original diaphragm aspect (copper painted) and the straight cover legs with the screws at an intermediate depth, to avoid too much pressure on the fulcrum. There were also the original large lower balls under the fulcrum plate.

There are no photos after session no.2, but the youtube video mentioned at the end of the long report in page 3 (see above video using the Meltrope) shows the Meltrope with the cover legs altered (Z-bent) in order to moderate the pressure on the upper balls with the cover screw fully screwed into the front ring. They also show the lower balls replaced by smaller ones, so the fulcrum plate raises only a little over the front ring. A screenshot is included herein:

Attachment:
File comment: See the small balls under the fulcrum and the Z-bent cover legs. The fulcrum plate is lowered due to the smaller balls.
meltrope III after session 2.png
meltrope III after session 2.png [ 262.42 KiB | Viewed 474 times ]


Photos in this later post show the current status of the soundbox after repair session no.3, where you can see some changes already done in session no.2, as the cleaned diaphragm and the new gaskets made from emgcr tubing. Other session no.2 changes, as the Z-bending of the cover legs and the smaller lower balls, have been reverted in session 3. See details below.

REPAIR / AMMENDING SESSION no. 3

I've redone the soundbox once more for several reasons. I've disassembled the Meltrope, even detaching again the diaphragm from the stylus bar, and I've cleaned and straightened the diaphragm once more. I've reassembled it, this time replacing the two small reinforcing cones at either side of the diaphragm (spider?) which I omitted in session no.2. I did several mistakes in session 2 which had to be reverted. I'm not a clever guy…

A) The inverted diaphragm mistake
After session 2, I noticed that I had mounted the diaphragm in reverse, with the stylus bar arriving to the convex side of the central dome (the peak of the mountain). Seeing my own photos after session 1, I noticed that originally the stylus bar should arrive to the diaphragm by the concave side of the dome (the cup). So I decided to detach again the stylus bar and replace the diaphragm in its original position. I also cleaned and straightened the diaphragm again, and this time I reattached the stylus bar on the right side of the central dome (concave) and also replaced the two small cones (spiders) at either side. I fixed all with super glue (fast cyanoacrylate) which has worked well. I got all fixed in a few minutes, and the surplus glue drops also help filling the gaps, leaving the connections perfectly airtight. No further need of adding wax.

Attachment:
File comment: See the cleaned diaphragm and the stylus bar coming to the concave side of the central dome. Notice the spider cone added.
IMG_20190107_184703.jpg
IMG_20190107_184703.jpg [ 2.89 MiB | Viewed 474 times ]


B) The lower ball bearings mistake
I also replaced the original larger balls under the fulcrum plate, for the smaller substitutes I had installed in session 2 provoked a buzz with heavy recordings. The balls must be as large as they were at the beginning, and then there is no rattle. I've observed that the smaller balls fit too loosely into the cups under the fulcrum plate and in the front ring, and this must be the cause of buzzing.

Attachment:
File comment: See the original large balls under the fulcrum. Notice how high the fulcrum plate is regarding the front ring. This was the original configuration.
IMG_20190107_184813.jpg
IMG_20190107_184813.jpg [ 3.31 MiB | Viewed 474 times ]


C) The cover plate legs Z-bending mistake
I've also undone the changes in the cover plate (Z-bending of legs pressing on the upper tiny balls) for the pressure was then more relaxed on the balls (good) with the cover screws fully screwed in, but this way I lost the means for regulating the pressure (= soundbox tuning). So I've reverted the cover to its original straight shape. Then the pressure on the balls can be regulated by the two nearest cover screws installed at intermediate depth, and not fully screwed in. In order to improve this, while helping to fix the screws when they’re loosely screwed, I've added three small rubber washers to them, under the cover.

Attachment:
File comment: See the small rubber ring added to the screw under the cover.
IMG_20190107_184829.jpg
IMG_20190107_184829.jpg [ 3.45 MiB | Viewed 474 times ]


Attachment:
File comment: The three screws with rubber rings.
IMG_20190107_184916.jpg
IMG_20190107_184916.jpg [ 2.93 MiB | Viewed 474 times ]


Attachment:
File comment: The cover adjusted with the rubber rings. These allow the cover pressure over the balls to be relaxed, while the screws can be tight at intermediate depth. This system allows adjusting the pressure exerted on the fulcrum balls at different values, and the soundbox can be tuned.
IMG_20190107_185109.jpg
IMG_20190107_185109.jpg [ 3.03 MiB | Viewed 474 times ]


D) The soundbox neck reshaping
I've also tried what I announced to emgr by email: I've filed out the inner entrance of air to the soundbox neck. I've carefully made a chamfer or soft curved profile at that side of the air outlet, so the real hole has a more acute edge to the air flow, the outer side of the hole. I've done this by hand, using a file. A heavy bid, because this is not reversible to the original state...!

Attachment:
File comment: See the chamfer filed out at the inner edge of the air hole.
altered hole.png
altered hole.png [ 4 KiB | Viewed 474 times ]


Some photos, although it is difficult to see when assembled...

Attachment:
File comment: See the brilliant inner part of the filed out hole at the inner side of the soundbox.
IMG_20190107_184714.jpg
IMG_20190107_184714.jpg [ 3.03 MiB | Viewed 472 times ]


Attachment:
File comment: Maybe in this other photo it is more evident...
IMG_20190107_184723.jpg
IMG_20190107_184723.jpg [ 2.8 MiB | Viewed 472 times ]


E) The final sound
After all this work and trial-and-error, the final status of the Meltrope III is more or less the same after no.1 overhaul, but with the right gaskets (thanks, emgcr), the diaphragm cleaned and straightened, and the tiny rubber washers for regulation of pressure.
I must say that there is a noticeable difference in sound using this regulation system. It seems that the pressure is affecting directly the mid-bass register, which is stronger with a lower pressure, while it is weaker with more pressure. The bass and treble seem to maintain the same power, so the overall sound character changes with the pressure. The best results seem to be with intermediate pressures; the sound results more balanced with less mid-bass, the sensation is much more equilibrated, and it seems that this was the direct cause of that hated ‘tunnel’ effect in sound I noticed; it has disappeared at last. It was an exaggerated content of mid-bass frequencies in the mixture. The treble seems to be the same, nevertheless, with bamboo needles. There is still not much treble, but the sound is nicer and overall better.

See the final results in this youtube video:
https://youtu.be/7ppWGwCs_QM


E) Epilogue --- other unsolved problems
In the original status, as now, this Meltrope III had a defect: the angle of the stylus bar & needle on the record, when seen from the front, was not 90º. The stylus bar was angled so the needle was sloped out of the diaphragm plane, towards the edge of the record. Next figure illustrates the problem:

Attachment:
File comment: Notice how the stylus bar and needle attack the record surface.
meltrope III angle error.png
meltrope III angle error.png [ 1.05 KiB | Viewed 474 times ]


One of the ideas behind repair session no.2 (smaller balls under the fulcrum) was to lower the fulcrum plate over the front ring, improving this angle. The accidental mistake of reversing the diaphragm helped that, for the stylus bar head was attached to the convex part, making the stylus bar still better. The two things together made the error almost disappear.

Reverting to the original configuration (large balls under the fulcrum plate and deeper stylus bar head into the diaphragm cup) has recovered that original bad angle. By the moment I’m solving this by installing the soundbox in the tonearm at an angle with the vertical plane so the needle falls perpendicular on the record. It doesn’t seem a good solution. What to do…? Still I’m tempted to mount the diaphragm again in reverse, and improve thus the angle. But the sound could change… although after session no. 2 the diaphragm was reversed and the sound was still good. The only noticeable problem was the buzzing (caused by the small balls under the fulcrum plate).

Still I have to take a good orchestral record and try it comparing the 5B, 5A/B and the Meltrope, watching the differences. Next youtube videos in my channel (Idolfo Qé) will illustrate this.

About the problem of impedance matching, studying the Meltrope it seems it could fit smaller horns than the HMV 194, but in the other hand we have the experience of other colleagues, using this soundbox in the huge EMG/Expert horns, and the results are good. These are larger than the HMV 194, so it should also work on this machine. Judge the results by yourselves watching the abovementioned video. Pity I didn't record the hated 'tunneled' sound of the soundbox, Still I could lower the pressure on the fulcrum plate and do a test, for it seems to recreate that 'tunneled' sound. Will try.

Thanks for your attention and support, colleagues.
Inigo


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