The Talking Machine Forum — For All Antique Phonographs & Recordings

So I'm in the process of simple servicing on a 5a reproducer for my HMV 163, and I need help with how to keep the isolator ring in the correct orientation, and how I should inject the silicone. I'm using GE silicone 2+

My understanding is that the ring should not be touching the back of the reproducer, so how should I keep it elevated when I inject the silicone? What I would assume is the best idea is to place a layer of silicone in the back, and gently place the ring, letting it dry, and then filling in the rest. How should I keep the 2 set screws lined up.

Last question, what is the best method of placing the silicone? Should I inject it straight from the tube, or fill it in manually?

Hopefully someone who has done this can help me out
Thanks
Martin

Single-part, silicone adhesive isn't the best for this application. If you're in the US then I'd be happy to mail you some two part mold-making silicone. Using mold-making silicone, you would place a piece of card-stock or similar inside the reproducer to keep the silicone from getting past the flange and into the reproducer. Then find a piece of tubing or a socket the same outside diameter as the tonearm and place it into the reproducer. Then mix the two parts of the silicone, pour into the void and wait for it to cure. After it's cured then it will pull right out, if you're happy with it then glue it in place with a silicone adhesive.

Following Wyatt Markus' (MicaMonster) advice regarding this, you cut a short length of white gasket tubing (the same size used for Exhibition reproducers) enough to make a "donut" ring at the bottom of the isolator back. Use a bit of silicone on the bottom and sides before inserting the white tubing, and then put a bit more silicone on top of that. Then place the ring on top of this in its proper position and carefully fill in the space around the ring. Done properly the ring top rim will sit at or just above the isolator rim area. You can add the silicone directly from the tube; just go slowly and if necessary, using a toothpick, push the silicone down and around to avoid air pockets. Use a moistened fingertip to smooth the top around the ring and leave it at least overnight. The silicone will cure further over a period of days, so be careful when putting the reproducer on or taking it off, as you might pull the ring out when removing. Before doing any of this, check to see that the isolator ring fits on your tonearm snugly but without excess tightness.
This has worked very well for me for several reproducers.
Bob

Wyatt's process is probably the one to go with, but I'll throw this out in case anyone wants to try it. This was recommended to me by a repairman with whom I used to correspond, going back about 30 years:

Take some modeling (non hardening) clay, the kind you used in school, and which is available at most art or hobby shops.

Pat and roll the clay so the surface is absolutely flat.

Take a flat gasket, of the appropriate size & thickness, though made of a stiffer/harder material than you would use for the sound box, and press it into the flat surface of the clay to make an impression or mold.

Carefully remove the gasket.

Squirt some GE silicone calk into the impression, level the top, and let it completely set overnight.

Carefully remove the silicone gasket, and wash off any clay residue. Trim, if necessary, any imperfections with a sharp blade or small scissors.

Do this for both the front and rear gasket.

OrthoFan

Thinking about this, I'm wondering why silicone gaskets would be needed for an HMV 5a sound box, or any fitted with an Orthophonic diaphragm.

This gif image show the proper piston-like plunging action of an Orthophonic diaphragm:


The corrugated area is solely responsible for the diaphragm's flexing action; the outer flat rim, itself, does not move to any great extent.

Based on what I've read and heard, the gaskets used for the original Orthophonic sound box (relatively stiff paper backed rubber) and the HMV 5a (felt) were installed to create an air-tight seal while isolating the diaphragm from the casing, holding the outer rim of the diaphragm firmly in position.

It seems to me that silicone or any very soft gasket material would allow the diaphragm to flex from the outer rim instead of from the corrugated section. Wouldn't this adversely impact the piston-like movement of the diaphragm?

Just curious.

OrthoFan

The question being asked is not about the diaphragm gaskets, he is referring to the black rubber collar that is on the back of the soundbox, where it fits on to the arm.

Many thanks. I re-re-read the original post and saw where I fell down. I've also used silicone to re-mount the isolator ring, (which in my own lingo, I call a throat-gasket).

As to my posts, as Emily Litella*** would have said, "Oh, that's very different. Never mind."

OrthoFan
(*** https://www.youtube.com/watch?v=fZLeaSWY37I )

Thank you guys for the info. When doing this should I remove the back before doing this? I obviously dont want anything getting inside, but the isolator back is stuck. How should I safely go about removing it?

Thanks
Martin

This link should be helpful-it's MicaMonster's method for rebuilds.

http://www.lagparty.org/~wyatt/rebuild-orthophonic.html

The hardened isolator rubber usually needs to be chipped out, and that is covered in the above info.
After getting the brass isolator ring out you can use a blowtorch to quickly remove any rubber residue from it. Heat, and brush with a wire brush.
Hope this helps!
Bob

Yes, I read through that. I guess I worded it poorly, what i wasfering too was the back plate, I've removed all 4 screws and have tried to separate the 2 halves, but it wont budge. I understand that sometimes the gasket gets stuck to the diaphragm and can be damaged when removing the backplate, but doubt that will be a problem since the 5a uses felt ones.

Thanks
Martin