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 Post subject: HMV 5b Reproducer / Soundbox distortion and lack of bass
PostPosted: Sat Sep 29, 2018 9:41 am 
Victor Jr
Joined: Sun Jul 23, 2017 4:46 am
Posts: 4
By way of introduction I am Richard in the UK and this is my first posting. It is in response to soundgen asking the question on 17th April 2017, What makes a good reproducer work well?

I have 4 HMV 5a reproducers (soundboxes over here in the UK), and 2 HMV 5b

I noticed some time ago that if they are held to the ear and the needle bar brushed with a finger they each exhibit different natural frequencies from their respective diaphragms showing that each diaphragm has a slightly different temper or inherent stiffness.

The actual exhibited pitches, are not so relevant as the comparison between them.

The range extends across about 6 tones - or 6 notes on a piano keyboard say, with the best sounding one (a 5a) having the lowest natural frequency, and therefore the greatest compliance. This sound test has long been used in workshops for testing the rate of annealing on various metals.

The others exhibit progressive thinning of tone and lack of bass with higher natural frequency.

In answer therefore to the question, I would say that the best soundboxes are the ones that have diaphragms with the greatest amount of compliance.

However, this is not the end of my story as I found out later that this simple test was not measuring the compliance of just the diaphragm.

I took one of the 5b soundboxes that had much less bass response, and also produced some distortion and thought that I would dismantle and examine it.

It has been said many times before, but the back of these are pot metal and it expands with age making it difficult to remove the front/body. I carefully removed the four screws at the back with a perfectly fitting screw driver. They are brass and can shear easily so moderate force only is preferable. If they are tight I have read that soaking in WD40 may help, but luckily I didn't need to do this. However having removed them, I found that the back would not come off - at all. I realised that if I tried to lever it off with a small screw driver the pressure point exerted on the pot metal back would just break a piece from it. I therefore got a one sided razor blade, the type with a folded edge over one side, and by holding the soundbox in rubber vice jaws, very gently tapped the blade into the joint a bit at the time working around the perimeter several times. This gradually separated the two halves and spread the load against a much larger surface area of the back during the process. The back would still not come off so I repeated the process with a flat knife that was slightly thicker, and so on until the two halves did separate.
Before re fitting I carefully filed the diameter of the rebate in the back so that it would just 'kiss' fit the body on re assembly.

I then removed the diaphragm from the body by removing the pivots from the fulcrum bar.

Things I found:
The spider that the needle bar is soldered to, is joined to the main diaphragm with tabs which pass through holes in the main diaphragm. These are sealed with a very small amount of beeswax. When holding the diaphragm flat against the bench with one hand and wriggling the needle bar slightly with the other I could hear a crinkling sound. It was difficult to see at first but with a watch maker's lens I could see that a few of these tab joints were moving slightly. I re sealed them with hard wax which stopped this.

I couldn't see anything else wrong and wanted to re assemble the entire soundbox to see if the distortion had been eliminated. I noticed that the pressing around the edge of the diaphragm is different on the 5b compared with the 5a. My instincts were that the pressings on the 5a would make the diaphragm more compliant as they are more concentric with the round shape whereas on the 5b they are triangles pointing from centre to edge, so I considered that this may be why the 5a was giving more bass response. It was only a thought at the time.

On reassembly it was a real struggle adjusting the pivots and I found it impossible to tell whether they were slightly too tight when the soundbox was re assembled. Even though when fully assembled I could flex the diaphragm when pressing the needle bar, I could not tell whether the pivots were resisting movement of the needle bar or the compliance of the diaphragm alone.

The next day I had an idea. I disassembled the soundbox again but left the pivots in place. By holding the body I could move the diaphragm freely, and this showed that the pivots were indeed binding very slightly. I unscrewed one of the pivot screws about a sixth of a turn and immediately the diaphragm dropped ⅛" within the body and was absolutely free to move under its own weight. After much fiddling and adjustment I concluded that the angle of the end of the piece of the chrome body into which the pivot goes was not dead square, and so over tightening the pivot screw was just enough to squeeze the pivot offline and make the fulcrum bar bind on the side of it (just before the end that contacts the ball.) This situation alone had been enough to seriously reduce the available compliance of the diaphragm, make the needle far less likely to easily track the grooves and consequently cause much increased record wear.

I finished my final adjusting of the pivots so that the diaphragm would move under its own weight, by leaving the pivot screw not fully tightened up and then re assembled the soundbox. As an aside, doing it in this order I was also able to check that the edges of the diaphragm definitely did not come into contact with the body, but were centred between the two felt gaskets.

The result is, I now have a 5b soundbox with no distortion and the most glorious mellow tone across the entire frequency range with as much rich bass as my best 5a.

Testing was all carried out on an HMV163 which is the smallest model with a re-entrant horn.

After quite a few records I can note that the pivot screw has not come loose. Also the 5b has a more even response than the 5a and does not show an inclination to be peaky at some mid bass frequencies in the way that the 5a can. HMV must have known what they were doing back in the day when they introduced the 5b.

It would be interesting to know what these soundboxes sounded like when new, and whether soundboxes were sold with poor adjustments.

 Post subject: Re: HMV 5b Reproducer / Soundbox distortion and lack of bass
PostPosted: Tue Oct 02, 2018 8:45 am 
Victor II
Joined: Thu Apr 27, 2017 5:52 am
Posts: 220
Location: Redruth, Cornwall, U.K.
Thank you, Richard! A very valuable post and well worth preserving for future reference. I too have used the sound made by brushing the tip of the needle or stylus as a rough guide to the behaviour of the box, but since overhauling an 'Exhibition' some forty years ago I have never ventured any further.

Oliver Mundy.

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