fixing RCA MI-11850-C cutting head

[Steve E.]

Hello all,

I am posting this on behalf of Serg, who writes in Russian, and hopefully he can respond directly to your responses:


Serg writes:

I have an RCA MI-11850-C cutting head. The old
viscoloid damper in it has fallen into disrepair. How can it be replaced?

I've tried neoprene, silicone, rubber. None of this has produced good
results. The head has a strong resonance at a frequency of 900 Hz.

[markrob]

Hi Steve,

That seems odd to me. Most moving iron heads have much higher main resonances. For example, the Presto 1C/D is up in the 4Khz range IIRC. I don't know the RCA heads well, but I wonder if it has lost its stiffness due to an incorrect mechanical adjustment or maybe metal fatigue?? How is the armature centered on this head?

Mark

[Serg]

The armature is centered correctly. All settings correspond to the factory settings

The fact is that the native damper was quite rigid and viscous, so the resonant frequency was higher.

My homemade dampers are softer, so the resonant frequency has decreased significantly.

I have not yet been able to find a material for a damper that would be identical in its properties to a viscoloid

[Serg]

Two sound recordings are presented here. The first recording was made with the original damper. The second recording was made with a homemade damper.

A gramophone is not the best device for playback, but even on it you can hear a significant difference in the sound of recordings

https://www.youtube.com/watch?v=h_zsAQheEo0

[Steve E.]

This is maybe off-topic, but what is the cutting medium here? Are you cutting or embossing? Is that lacquer or a plastic?

[markrob]

Thanks for posting that. It was interesting. It sounded to me like the second recording started off very similar to the first and then the highs dropped out slowly. Maybe the damping heated up and changed its properties??

Mark

[emorritt]

My vote is for the original; sounded great, even on the acoustic phonograph. Second recording sounded 'thready and tinny' to my ears.

[Steve E.]

Given that he said "fallen into disrepair," I'm assuming that retaining the original damper isn't an option anymore.

[Serg]

I make records by cutting on plastic. I also mastered the technology of production of stampers and pressing gramophone records from shellac mass. I'll post a video with the results here sometime

It is almost impossible to find materials for recording factory production on the territory of the former USSR.

It's a great stroke of luck that I was able to find this RCA cutting head.
I also have a Telefunken R12b head and German Decelith recordable discs. A lot of them were exported to the USSR after World War II.

The problem with replacing the damper is bothering me.. I can't find a suitable damper material. All recordings are obtained with a "tinny" sound and a low level of high frequencies

[Serg]

A little off topic, but here's a video of playing a homemade shellac record


https://www.youtube.com/watch?v=XIUvlT1-ZzQ

[Steve E.]

I'm not sure you can test this: Have you eliminated the possibility that your plastic material is wearing out your cutting stylus?

[Serg]

This plastic cuts well. In any case, the wear of the cutting stylus would only lead to increased noise on the recording.

I conducted a test with a light flare (in our literature it is called the "Meyer Glare"). He recorded a pure tone with frequencies from 30 to 10000 hz. At a frequency of about 900 hz, there is a very large peak in the light glare, which indicates the presence of resonance at this frequency, which was not previously with a viscoloid damper

P.S. There may be inaccuracies in the translation, as some technical terms in English and in Russian are slightly different

[Steve E.]

You are communicating just fine in English, and it sounds like you are approaching this methodically.

[Steve E.]

I definitely want to know how you are making homemade shellac records, and I'm not alone here!!

[sameal]

If viscoloid is the term for the material and not a general term for plastics, then the viscoloid company was gobbled up by dupont, who in turn was gobbled up by dow corning. If anybody has the information on the blend and a possible substitute it's dow. They *might* be able to locate a sutible substitute in there inventory.

Otherwise you'd have to find a material to blend into your homemade dampers to make the plastic more rigid, like they do in the extrusion process.

The dampers in my RCA heads are tacky kind of, and not like neoprene. Kind of a beeswax like look and feel but a little more rigid

[Serg]

The technology of production of shellac plates turned out to be quite simple. I took my knowledge from both foreign and Soviet books on electroplating and record production.
I have access to Arshinov's archive. This man in the 30s from the Soviet record company Gramplasttrest went on business trips to recording studios in the USA and brought a lot of materials from there. Such as descriptions of technological processes, diagrams, photographs and drawings. At that time, it was supposed to organize a joint work between RCA and Gramplastrest.

So the technology itself is quite simple.
1. First, the sound recording is made on a plastic film
2. The plastic is chemically coated with a thin layer of silver
3. Copper with a thickness of about 1-1.5 mm is built up on the silver layer in the sulfuric acid electrolyte

Then the plastic is separated from the metal negative. It is this negative that is used for further small-scale pressing of records.

I get plastic for pressing by recycling old 78 rpm gramophone records (fortunately in the USSR such records were produced until the mid-70s).

The processing of shellac plastic is carried out using a technology developed in the mid-30s by a group of Leningrad inventors Zaikin, Abramovich and Tovstoles. It is no secret that at that time, not only in the Soviet Union, but also in the world, there were problems with the supply of shellac. Therefore, the inventors proposed to introduce synthetic rubber mixed with rosin into plastic to save shellac

I have been studying this topic for the last 5 years. I made my first recording machine at the age of 16, now I'm 21 and I already know how to make stamps and press records..

[Serg]

If viscoloid is the term for the material and not a general term for plastics,

I do not know what is usually understood by the term "viscoloid" in the rest of the world. This is what we call a translucent rubber-like material from yellow to brown colors. Our collectors really appreciate it precisely for its damping properties. But in Russia, the composition of this rubber still remains a big mystery. We have not produced anything like this

[emorritt]

Sorbothane would probably be a good substitute for Prestoflex, which was a viscoloid-like material. You can get sheets and strips of sorbothane online. It's been a while since I've had my Presto cutting heads open, but if I recall correctly, the damper was about 1/8 to 3/16 inch thick, anchored at the top of the head in a bar clamp and held around the armature in a circular clamp with four tiny screws. Sorbothane prevents vibration between about 10 and 30,000 cycles, which should damp any resonance across the frequeency range of most mono recording heads by Presto, RCA or other manufacturers.

[Serg]

I managed to find sorbotane from old stocks. The sheet has lain unpacked since the 90s.

It turned out to be insufficiently hard and viscous. Sorbotan could not suppress all resonances.

It may be necessary to change the setting of the cutting head for the sorbotane damper

[sameal]

In a stroke of (bad) luck, my RCA head has come down with the same issue. I'm going to be trying out some ideas me and soulbear talked about for forming a new damper. In the meantime I got my damper out in mostly one piece. Heres a photo