HMV No:4 Soundbox question.

[poodling around]

I recently acquired a number of these sound-boxes for virtually no cost. All brass backed.

One of them has a problem. The needle bar has become detached from the mica diaphragm - in other words it is broken.

I thought that I could repair it with some bees wax.

Then I thought, "I have never seen inside these sound-boxes". So I removed the four back screws. How-ever I still couldn't prise open the back. So I thought maybe the back was just stuck.

But then I thought, if the front of the sound-box is made from pot-metal then maybe it has expanded ?

So, does any-one know if the front section of an HMV Number 4 sound-box (which has a brass back) is made from pot metal ?

Also, does any-one have experience opening up one of these sound-boxes ?

[epigramophone]

The metals used depend on the date of the soundbox.
According to HMG Page 258, pot metal was used first for the backplate in late 1928 and in 1930 for the front bezel as well.
If all your No.4's are brass backed, the front bezels should also be brass.

My experience of dismantling one ended in disaster. Heat, cold, penetrating fluids and colourful language had no effect. Trying to insert a razor blade between the two halves resulted in pieces breaking off. The two halves seemed to have welded themselves together, and the soundbox was a write-off.

It should be possible to re-attach the needle bar to the diaphragm without recourse to dismantling. Instead of wax I use a drop of PVA wood glue to seal the connection. It goes on white, enabling you to see what you are doing, dries translucent, remains flexible, and the next repairer can easily remove it by immersion in warm water.

[poodling around]

The metals used depend on the date of the soundbox.
According to HMG Page 258, pot metal was used first for the backplate in late 1928 and in 1930 for the front bezel as well.
If all your No.4's are brass backed, the front bezels should also be brass.

My experience of dismantling one ended in disaster. Heat, cold, penetrating fluids and colourful language had no effect. Trying to insert a razor blade between the two halves resulted in pieces breaking off. The two halves seemed to have welded themselves together, and the soundbox was a write-off.

It should be possible to re-attach the needle bar to the diaphragm without recourse to dismantling. Instead of wax I use a drop of PVA wood glue to seal the connection. It goes on white, enabling you to see what you are doing, dries translucent, remains flexible, and the next repairer can easily remove it by immersion in warm water.

Very interesting (and entertaining) ! I would have paid good money to see you trying to carry out that repair Thank you very much epigramophone.

It is a shame though because I was hoping to see inside the sound-box to understand how it was designed, replace the gaskets and try various different diaphragm materials.

I will do as you suggest in the next few days and repair it with PVA wood glue - the damage to the mica is slight.

I really like HMV 4 sound-boxes so that will be fine and I have plenty of other sound-boxes to mess around with such as Orchorsal and Meltrope.

Happy days !

[CarlosV]

Then I thought, "I have never seen inside these sound-boxes". So I removed the four back screws. How-ever I still couldn't prise open the back. So I thought maybe the back was just stuck.

The halves get stuck because the rubber gasket solidifies and keeps them glued. I managed to open one by dropping a mix of winter green and alcohol in the interface and letting it overnight. The mix is a rubber solvent, and when I tried the next day with a blade it opened up. But beware to try it only on brass ones, the pot metal ones are impossible to open without breaking them when they swell.

[poodling around]

Then I thought, "I have never seen inside these sound-boxes". So I removed the four back screws. How-ever I still couldn't prise open the back. So I thought maybe the back was just stuck.

The halves get stuck because the rubber gasket solidifies and keeps them glued. I managed to open one by dropping a mix of winter green and alcohol in the interface and letting it overnight. The mix is a rubber solvent, and when I tried the next day with a blade it opened up. But beware to try it only on brass ones, the pot metal ones are impossible to open without breaking them when they swell.

Very interesting. Thanks very much for the advice carlos.

It is a shame that these sound-boxes are so difficult to work with. I guess once opened even the even the brass one's would be difficult to put back together again.

Oh well, as I mentioned earlier, I have plenty of other sound-boxes to mess around with.

Happy days.

[CarlosV]

It is a shame that these sound-boxes are so difficult to work with. I guess once opened even the even the brass one's would be difficult to put back together again.

Once opened they are relatively easy to rework and close again. The small screws and the holding cylinder in the pivot bar require some patience but they are removable for cleaning and oiling. When I fix these soundboxes, I put normal gaskets instead of the red slotted ring that was used originally: I find that that the ones available today are too hard, and I get better results with a pair of tube gaskets.

[poodling around]

It is a shame that these sound-boxes are so difficult to work with. I guess once opened even the even the brass one's would be difficult to put back together again.

Once opened they are relatively easy to rework and close again. The small screws and the holding cylinder in the pivot bar require some patience but they are removable for cleaning and oiling. When I fix these soundboxes, I put normal gaskets instead of the red slotted ring that was used originally: I find that that the ones available today are too hard, and I get better results with a pair of tube gaskets.

Interesting.

Well, I tried soaking in hot water over-night - which did not work at all. I will spare you the horrific details ! (Just think Epigramophone).

Interesting to see inside and how it is constructed though.

It may be that the back was pot-metal as it seemed to only have a thin sliver of brass resting on some other material which looked almost 'volcanic' so that may have been the problem.

Anyway, I can still mess around with it.

Thanks as always.

[epigramophone]

A good guide to which back a No.4 soundbox has is the rubber isolator.
If it is flush fitting you have brass. If it protrudes you have pot metal.

[poodling around]

A good guide to which back a No.4 soundbox has is the rubber isolator.
If it is flush fitting you have brass. If it protrudes you have pot metal.

Thank you epigramophone.

Please see the three photographs 'below'.

I thought the one which was broken and I destroyed has a rubber isolator which indicated that it was brass.

However, once opened it seems to have a very thin 'coating' of brass and the rest of it appears to be pot-metal ?

I thought it would be solid brass ?

What are your thought please ?

[epigramophone]

That is indeed pot metal. Clearly the "guide" I read about is not infallible.