Back to Part 3
MMC: Micro Moving Cross
As mentioned above, when a wire is moved through a magnetic field, a current is generated in a wire that is proportional to the velocity of the movement. In order to increase the output, the wire can be wrapped into a coil, effectively lengthening the piece of wire moving through the field. Most phono cartridges make use of this behaviour by using the movement of the stylus to either:
- move tiny magnets that are placed near coils of wire (a Moving Magnet or MM design
- move tiny coils of wire that are placed near very strong magnets (a Moving Coil or MC design)
In either system, there is a relative physical movement that is used to generate the electrical signal from the cartridge. There are advantages and disadvantages associated with both of these systems, however, they’re well-discussed in other places, so I won’t talk about them here.
There is a third, less common design called a Moving Iron (or variable-reluctance(1)) system, which can be thought of as a variant of the Moving Magnet principle. In this design, the magnet and the coils remain stationary, and the stylus moves a small piece of iron instead. That iron is placed between the north and south poles of the magnet so that, when it moves, it modulates (or varies) the magnetic field. As the magnetic field modulates, it moves relative to the coils, and an electrical signal is generated. One of the first examples of this kind of pickup was the Western Electric 4A reproducer made in 1925.
In 1963, Erik Rørbaek Madsen of Bang & Olufsen filed a patent for a cartridge based on the Moving Iron principle. In it, a cross made of Mu-metal is mounted on the stylus. Each arm of the cross is aligned with the end of a small rod called a “pole piece” (because it was attached to the pole of a magnet on the opposite end). The cross is mounted diagonally, so the individual movements of the left and right channels on the groove cause the arms of the cross to move accordingly. For a left-channel signal, the bottom left and top right cross arms move in opposite directions – one forwards and one backwards. For a right-channel signal, the bottom right and top left arms move instead. The two coils that generate the current for each audio channel are wired in a push-pull relationship.
There are a number of advantages to this system over the MM and MC designs. Many of these are described in the original 1963 patent, as follows:
- “The channel separation is very good and induction of cross talk from one channel to the other is minimized because cross talk components are in phase in opposing coils.”
- “The moving mass which only comprises the armature and the stylus arm can be made very low which results in good frequency response.”
- “Hum pick-up is very low due to the balanced coil construction”
- “… the shielding effect of the magnetic housing … provides a completely closed magnetic circuit which in addition to shielding the coil from external fields prevents attraction to steel turntables.”
- Finally, (although this is not mentioned in the patent) the push-pull wiring of the coils “reduces harmonic distortion induced by the non-linearity of the magnetic field.”(2)
- reluctance is the magnetic equivalent of electrical resistance
- “Sound Recording Handbook”, ed. Glen Ballou
Lately, a large part of my day job has been involved with the Beogram 4000c project at Bang & Olufsen. This turned out to be pretty fun, because, as I’ve been telling people, I’m old enough that many of my textbooks have chapters about vinyl and phonographs, but I’m young enough that I didn’t have to read them, since vinyl was a dying technology in the 1990’s.
So, one I the things I’ve had to do lately is to go back and learn all the stuff I didn’t have to do 25 years ago. In the process, I’ve wound up gathering lots of information that might be of interest to someone else, so I figured I’d collect it here in a multi-part series on phonographs.
A warning: this will not be a tome on why vinyl is better than digital or why digital is better than vinyl. I’m not here to start any arguments or rail against anyone’s religious beliefs. If you don’t like some of the stuff I say here, put your complaints in your own website.
Also, if you’ve downloaded the Technical Sound Guide for the Beogram 4000c, then you’ll recognise a large portions of these postings as auto-plagiarism. Consider the TGS as a condensed version of this series.
A very short history
In 1856, Édouard-Léon Scott de Martinville invented a device based on the basic anatomy of the human ear. It consisted of a wooden funnel ending at a flexible membrane to emulate the ear canal and eardrum. Connected to the membrane was a pig bristle that moved with it, scratching a thin line into soot on a piece of paper wrapped around a rotating cylinder. He called this new invention a “phonautograph” or “self-writer of sound”.
This device was conceived to record sounds in the air without any intention of playing them back, so it can be considered to be the precursor to the modern oscilloscope. (It should be said that some “recordings” made on a phonoautograph were finally played in 2008. See www.firstsounds.org for more information.) However, in the late 1870’s, Charles Cros realised that if the lines drawn by the phonoautograph were photo-engraved onto the surface of a metal cylinder, then it could be used to vibrate a needle placed in the resulting groove. Unfortunately, rather than actually build such a device, he only wrote about the idea in a document that was filed at the Académie des Sciences and sealed. Within 6 months of this, in 1877, Thomas Edison asked his assistant, John Kruesi, to build a device that could not only record sound (as an indentation in tin foil on a cylinder) but reproduce it, if only a few times before the groove became smoothed. (see “Reproduction of Sound in High-fidelity and Stereo Phonographs” (1962) by Edgar Villchur)
It was ten years later, in 1887, that the German-American inventor Emil Berliner was awarded a patent for a sound recording and reproducing system that was based on a groove in a rotating disc (rather than Edison’s cylinder); the original version of the system that we know of today as the “Long Playing” or “LP” Record.
Early phonographs or “gramophones” were purely mechanical devices. The disc (or cylinder) was rotated by a spring-driven clockwork mechanism and the needle or stylus rested in the passing groove. The vibrations of the needle were transmitted to a flexible membrane that was situated at the narrow end of a horn that amplified the resulting sound to audible levels.
Magnets and Coils
In 1820, more than 30 years before de Martinville’s invention, the Danish physicist and chemist, Hans Christian Ørsted announced the first link made between electricity and magnetism: he had discovered that a compass needle would change direction when placed near a wire that was carrying an electrical current. Nowadays, it is well-known that this link is bi-directional. When current is sent through a wire, a magnetic field is generated around it. However, it is also true that moving a wire through a magnetic field will generate current that is proportional to its velocity.
Forward to Part 2: Physics
So, it was obvious that the speed regulation wasn’t working properly at the end of Day 5. So, last night was spent digging for information on how centrifugal speed governors work and I came across this excellent explanation.
So, my theory was that the disc was seized on the axel and not moving correctly with the rotational speed. This means that everything came apart again, and the axel had to come out.
In theory, as the governor spins faster, the three weights get pulled out. This in turn should pull the disc in to rub against the friction pad. When it came out of the motor, the disc was immovable – it was stuck to the axel as I guessed. So, the three springs+weights were removed from the axel and, after a lot of WD-40 and a little repeated gentle “persuasion”, I got to here:
This is after I polished the rust off the axel with sandpaper, starting at 400 and working up to 3200 (lubricated with more WD-40). I was sanding along the length of the axel, since that’s the direction of movement of the disc.
Then it was “just” a matter of putting it all back together again… However, before I put it back in the case, I checked that the governor was working, which you can see below. Notice how the disc moves sideways to meet the friction pad, keeping things at a constant speed.
Then it was just a matter of putting everything back together again… And I have a working Telefunken Lido for those Sunday afternoon garden parties!
This was nearly the last day of the restoration (sort of…)
The beechwood that will hold up the top plate. This is 21.4 mm from the lip of the casing, as are all of the other pieces.
The opposite side of the same part of the case. I’m hoping that the yellow leather will darken over time…
All of the wood in the case. Notice the bent wood at the top… Turns out that this didn’t work… There wasn’t enough room between the inside of the case and the horn, so it had to go.
The alternative solution, using large washers directly on the inside of the case.
The new mainspring, ready to be greased and wound into the barrel.
The mainspring in the barrel, with teflon grease applied. Notice that it winds clockwise. Also note that the hook on the inner sleeve is grabbing the spring end. This is important, and a little difficult to manage…
Closing up the mainspring barrel, rotating about 45 degrees each time, and tightening only a little at a time.
Teflon grease on all the inner parts. This was a good idea – except for the axels of the speed regulator. The grease was a little too viscous, so it was replaced with WD-40.
Everything’s back together. All those photos I took at the beginning helped a lot during re-assembly.
The thick rubber compression washers are used to help level everything later.
Motor’s back in… time to level things up.
The platter is a little high on the left (relative to the top side of the plywood) so two screws get loosened and two get tightened. The rubber washers keep things from vibrating, and allow for this adjustment.
The finished gramphone!
With the tonearm clipped back for transport. The crank is clipped into the lid on the right side.
Crank in place, ready to wind up the spring.
The tonearm out, but in its resting position.
Playing a record for the first time in a long time!!!!
Seems that I need to work on the speed regulator… But it works – which it hasn’t done, probably for many years…
Some more bits and pieces of work this time, mostly leather work.
Yesterday was spent colour-testing the dye with some scraps first. The bottom piece is vegetable tanned leather without dye. The middle piece is with one light coat of dye. The top piece is thoroughly soaked with the same dye. The white balance in this photo is a bit weird – but the top one is the winner. It’s a yellow alcohol-based dye.
Then the remaining pieces were cut and dyed, the hardware is in, and the insert for the handle is formed. (At this stage, nothing was glued, since the leather was still wet.) The binder clips are there to shape the leather around the small piece of 2 mm thick leather inside the handle that creates the shape. The irregularity in the colour is due to the fact that the dye hasn’t finished drying yet.
All the stitching is done, and the handle is burnished. The handle and tabs are 2 mm leather, and the straps are 1.3 mm thick, give or take. I’ll punch the hole in the strap when it’s all assembled so as to ensure that it’s in the right place.
The above photo shows grease-proof paper glued to the inside of the bottom casing. This will protect the interior from any grease or oil that drops off the drivetrain. This is a pretty safe assumption, looking at the black grease stains that are there already.
The paper is cellulose-coated baking paper, and it’s glued in with water-based bookbinders glue. Once it’s dry (tomorrow), the white color will become transparent. Then I’ll put in another piece that wraps around the sidewall, since the player will often be set on its end.
In addition to this, the blocks of wood are ready to be inserted – almost all of them cut out of 10 mm thick beech. Instead of the canvas, I plan on using a 5 mm thick strip of beech, but this will have to be steam-bent to follow the curve of the top. We’ll see how well that works out – never tried steam-bending wood before… these will all be held in place with M3 Chicago Bolts with the non-slotted nut on the outside of the case. This will look almost exactly like the original rivets, but it will mean that everything will be much easier to disassemble in the future – just in case…
The new mainspring arrived in the mail today from Lindholts; it looks like it might need a couple of small modifications to work, but it’s a much better fit than the one I had on hand. So the next big days will be spent re-assembling the drive train and inserting the wood parts.
One small setback today. I found the right-shaped screws (to replace the random ones that were holding it together) at Birger A. Handel in Slagelse. The right shape – but the wrong colour. They’re brass, and the originals are all either nickel- or chrome-plated. A found a nickel-plating company near here in Herning, but they emailed me today to tell me that they’re not interested in plating 30 tiny screws for me. Not much profit in that I guess… Oh well. Hopefully, some day, I’ll find replacement screws. Until then, my Lido will be a lovely chrome / brass burst of colour!
Today was spent doing a bunch of small jobs while I wait for some replacement parts to arrive.
For starters, I found two needles under the hinge on the bottom half of the case. They’ll come in handy later!
The interior of the case, showing the four wood blocks to which the top screws on. Funnily, these are made of three different types of wood: pine, birch, and beech. I suspect that this was not strategic – but just a question of using whatever was on-hand. The lining is a waxy paper – the black stains are grease that has dripped down from the drive mechanism.
The four pieces of wood are two different heights from the top edge of the casing, although I suspect that this would have been custom-fitted. They’re attached to the outer casing using either tacks (the largest piece on the right) or split rivets that were bent over to lock the wood in place. The wood parts are not glued onto the case. All of the rivet and tack heads of these are badly rusted – so they’re coming out…
One of the split rivets after the wood has been removed. The wood did not survive the removal process. The hole on the far left is the opening for the crank.
The wood in the process of removal. I’ll just make new blocks to replace these…
The handle and strap are a different colour than the case, and are in quite bad shape. In addition, the hardware is badly rusted. This will all get replaced with new parts.
The handle and strap are attached with the same split rivets, but bent around a canvas material that’s glued to the inside of the case, as can be seen in the photo above. The canvas is the dark square and rectangle. This canvas can’t be seen normally, because it’s covered by the wax paper. I peeled this off, because I’ll be replacing it with something a little more sturdy.
The rust on the exterior metal parts was cleaned up with a small wire brush on my Dremel tool. The before-and-after can be seen above. This took some time so as to not slip and carve into the case covering.
I used Simichrome to polish the metal tonarm. This appears to be plated brass. I’m not going to take apart the reproducer (the black part that holds the needle and contains the diaphragm).
If you look carefully, you’ll see a small set screw sticking out of the half of the tonearm on the right. That’s used to stop the front portion of the tonearm from making a full rotation when it’s swivelled back and forth onto the record, by hitting the portion of the threaded pipe that can be seen below.
So, if you’re dismantling the tonearm, remember to back off the set screw before separating the two parts.
I also started the leather work to make a new handle and strap. The replacement hardware and dye were ordered from laederiet.dk, near Aarhus (which is where I buy all my leather supplies).
Apart from all of that, I washed the exterior of the case with dish soap and a soft cloth – not too wet because there are some places where the covering is worn through and I don’t want water getting in there.
I also sprayed the interior fabric (maybe taffeta?) with an enzyme spray and rubbed it gently to remove some of the stains. Too much rubbing frays the fabric, so I had to be gentle…
Time to get inside and find out what’s wrong…
The cap comes off the mainspring barrel by tapping it with a hammer while holding onto the barrel itself. The inside shaft was already able to move up and down, so it was obvious it was no longer attached to the mainspring itself. The block of wood is used to prevent the hammer from damaging the cap edge. You hit the wood instead of the metal.
With the cap off, it’s easy to see that the mainspring is unfortunately broken. So, there are two options: Try to drill a new hole at the end of the remaining spring. This will mean heating it up to soften the steel a little… OR Try to replace it with a new spring.
A shot of the axel and the broken end of the spring. There’s a hole at the end of that spring that is caught by the hook that you can see on the axle. That hook is actually part of a sleeve that slides off the axel itself, as can be seen below.
The gap in the sleeve sits on either side of a pin that sticks out from the axel. This prevents it from rotating.
The rest of the mainspring is out of the barrel. This has to be done carefully to prevent it jumping out and either breaking something or punching a hole in me. One way to do this is to hold onto it on both sides of the barrel with two hands, and lifting one side of the spring out. This will push its way out until it gets stopped by your other hand, then you just alternate hands to let it out 180 degrees at a time. The only thing to be careful of at the end is to avoid bending the spring, since it will be caught on the pin on the inside of the barrel.
The bottom plate and the centrifugal speed regulator. The axel with the toothed gear just lifts out.
A close-up of the speed regulator and the clutch wheel, still covered in grease. Note that the pins on the end of that axel sit in brass bearings that are just holes drilled into pins. However, the holes are not centred. So, if you back off the set screws on the “front” of the vertical post, you can rotate the brass pins to change the height of the axel. Only the set screws are threaded.
The underside of the top plate. The spring that can be seen there is used to prevent the screw from rotating counter-clockwise. (Clockwise rotation loosens the spring. Counter-clockwise tightens it.) The portion that sticks out on the right is the part that the handle screws into from the outside. So, you can screw it on, tighten the spring, but then, when you reverse the rotation of the handle, it just unscrews because the rotation is stoppped by that spring grabbing the axel.
Another view of the same part. Notice the small cotten pad that sticks out of the arm connected to the tall rod in the back. That’s the part that pushes against the clutch wheel to slow things down.
The top of that same part. There’s still plenty of old grease in the worm gear… I didn’t take anything apart more than this. All of the degreasing was done in the state the you see in the photos above.
Degreasing started by just scraping off the goop with small wooden picks that I made from scraps I had lying around. The next step was to spray on WD-40 degreaser and start wiping things down with paper towels and a stiff plastic brush. That procedure was repeated until things were looking clean, but not necessarily shiny.
The photo above shows most of the bits and pieces degreased and cleaned up.
One last close-up of the cotton pad that is used for the speed regulator.
Back to the spring… I decided to not try to heat the steel, bend it to a smaller radius, and drill a new hole. Instead, I remembered that I might have some lying around. About a year or two ago, I bought a collection of tools and leftover parts from a guy who had planned to try watch and clock repair as a retirement hobby. He had bought the collection from a retired watchmaker.
In that collection, there were some old mainsprings for mantle clocks. Time to dig those out…
First thing is to measure the Telefunken’s mainspring. Turns out its roughly 23 mm wide, 0.5 mm thick, about 3.5 m long (this is just a rough estimate based on pulling it as straight as I could for as far as I could…) and the barrel interior is 78 mm in diameter. This means that I’m looking for a mainspring that’s 23 x 0.5 x 3500 x 78 – give or take…
A box of old clock mainsprings that I happened to have lying around…
I selected the spring that best matched, based on the width, and thickness and unpacked it. This is a delicate matter that involves holding the spring in a thickly gloved hand, cutting the wire, and then slowly releasing it under a towel. That way, if it does jump, you’ll only get hit in the face with a towel…
Sadly, the spring that I had on hand was too short. So, I’ve ordered one from lindholds.dk. The one that’s coming is also not as long as the original, but hopefully, it’ll do the trick.
Tomorrow: Greasing and reassambling as much of the drivetrain as I can, and starting to clean up the case.
I recently bought a well-used Telefunken Lido portable gramophone. It’s in reasonable shape, but it certainly needs quite a lot of repair and/or restoration. For starters, it doesn’t work – probably because the drive spring is either broken or disonnected inside the barrel.
The plan is to get as much fixed on it this weekend… however, that plan may change as the work progresses.
I’ve already made use of this page, this page, and this video to get ready for the project (including learning from the mistakes of others…) My documentation might be of similar use to others – in addition to providing some info on how gramophones worked…
The lido, as-is before I start…
The platter just lifts off.
The diagonal arm is the speed control that adjusts a clutch mechanism that can be seen in photos below. The needle and membrane are locked in the “travel” position, which sits them down into the mouth of the horn (the dark rectangular area at the “back”).
The first step was to unscrew the locking lid stay on the left side of the horn opening. The next step is to unscrew the lid hinges from the main case. Both the lid stay and the hinges are riveted to the lid, so they stay on.
The next step was to remove the three screws that hold the pipe + membrane + needle assembly onto the wooden top plate in the top right corner. After these have been removed, it all just lifts off.
Next is to remove the 5 small screws around the outer edge of the wood top plate. These hold the entire assembly into the bottom part of the case.
The next step is to disassemble the mechanism from the wooden top plate. In order to do this, the speed regulation arm has to be disconnected from the pin that connects it to the clutch underneath. This is done by loosening at least one of the two set screws that grab the pin.
The photo above shows the control arm after separating it from the pin that goes down into the mechanism.
Once this is done, there are four large screws the have to come out. Those are the four holes near the right-hand yellow “Fona” sticker.
In order to remote the drive mechanism, it has to be gently angled to slide it out without the spindle hitting the wood, and snaking it out around the horn.
The mechanism after removal.
The underside of the wooden plate, showing the entire horn. This is probably made of lead by the looks of things…
The two vertical rods are the main spindle (on the left) and the clutch control (on the left). Turning the clutch control pushes a soft pad against the vertical clutch wheel that can be seen on the same axel as the centrifugal speed regulator weights.
There are four 11 mm hex nuts holding the top plate of the mechanism to the four posts. First, the rubber washers needed to be removed using a knife to separate them from the top plate. Then the four nuts are loosened and the top plate can be lifted off. This will take the clutch rod and the main spindle with it.
The photo above shows the bottom plate with the speed regulator and the spring barrel.
The two last photos, above, show the underside of the top plate, holding the main spindle on the left, the clutch rod in the middle, and the screw entry for the winding handle.
That’s it so far. Tomorrow will probably be spent disassembling the spring barrel and seeing whether it’s fixable. Then de-greasing and cleanup of the drive mechanism, re-greasing and re-assembly.