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Need help restoring a Meltrope III soundbox
http://forum.talkingmachine.info/viewtopic.php?f=11&t=36232
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Author:  Orchorsol [ Thu Jul 05, 2018 12:53 pm ]
Post subject:  Re: Need help restoring a Meltrope III soundbox

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.

Author:  emgcr [ Thu Jul 05, 2018 1:17 pm ]
Post subject:  Re: Need help restoring a Meltrope III soundbox

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.

Author:  Inigo [ Mon Jul 09, 2018 2:22 am ]
Post subject:  Re: Need help restoring a Meltrope III soundbox

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.

Author:  Orchorsol [ Mon Jul 09, 2018 3:49 am ]
Post subject:  Re: Need help restoring a Meltrope III soundbox

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!

Author:  Inigo [ Tue Jul 10, 2018 2:35 pm ]
Post subject:  Re: Need help restoring a Meltrope III soundbox

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... ??? :?:

Author:  emgcr [ Wed Jul 11, 2018 3:31 am ]
Post subject:  Re: Need help restoring a Meltrope III soundbox

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.

Author:  Inigo [ Mon Jul 16, 2018 9:05 am ]
Post subject:  Re: Need help restoring a Meltrope III soundbox

Pm sent

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