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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 4th Sep 2013, 11:16 pm   #1
ricard
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Default Replacing the infamous plastic bits on the Tandberg 12/1200X

As many who have attempted to repair a Tandberg 12 or 1200X will be aware, there are two identical plastic pieces which guide the record-playback-amp switches so that they a) latch in a positive manner in the playback and amplifier modes, b) are spring-loaded in the record position (and spring back to the playback position when released) and c) (1200X only) disengage the bias head lifting mechanism in record mode, so that the bias head comes into contact with the record/playback head (in playback mode the head is lifted away by a couple of millimeters).

As many will also be aware, these pieces tend to break, especially the first generation, where the spring providing the force is directly attached to a hole in the plastic piece. The second generation seems to fare better, with the spring being attached to a die-cast little lump of metal, which in turn pulls on the plastic piece; I think this arrangement spreads the force over a greater area of the plastic piece.

(There are also two variants in different colors: the original ones are grey, but some machines have replacement parts in black plastic; my guess is that the black plastic is not just different by color but also is stronger.)

However, even the variant where the spring isn't attached directly is prone to breakage, as the pictures below show.

As the piece once broken can't be mended in any way (glue would be way to weak, and the type of plastic wouldn't lend itself to gluing anyway), I was considering filing out a replacement part out of a sheet of nylon or similar. I then struck upon the idea of using 1mm piano wire which I had on hand, bending it into a shape that mimics the curvature of the original piece. Since it is spring loaded, the hole to the right does not actually have to be a hole, which means there the wire substitute doesn't need any loop to simulate the hole.

The result worked surprisingly well. In fact, after adjusting the wire, and also adjusting the arms for the record latch mechanism and bias head lift mechanism, it seems to work just as well as the original plastic piece.

I found there were two critical areas:

1. The angle at which the wire bends back around the lever to the right of it. Too acute an angle, and it becomes difficult to push the lever to the amp position. Too obfuse, and the lever tends to overshoot the playback position when moving from record to play (for some reason, this effect becomes worse if the machine is actually set in forward drive mode and then stopped; I suppose the additional vibration of the pinch roller arm moving back jars the mechanism more).

2. The loop at the very left. On the 1200X this presses against the arm which disables the lifting of the bias head in record mode. If the loop reaches too far forward and it is mounted on the ch L lever as in the picture, it will get bumped by the ch R plastic piece when its lever is moved between play and amp. If the loop doesn't reach far enough to the left it won't engage the head-lifting-disabler arm properly. (In fact in the first version I made I didn't think of adding the loop at all, so the bias head didn't move into its proper record position when only ch L was set to record mode).

All in all, a bit of trial and error is needed together with a thorough test of the positions of all the levers and arms in all combinations of modes. But the end result is definitely worth it.
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Last edited by ricard; 4th Sep 2013 at 11:21 pm.
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Old 5th Sep 2013, 9:45 am   #2
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Default Re: Replacing the infamous plastic bits on the Tandberg 12/1200X

Nice piece of work there!

I few years ago, and with the help of a friend who was very good at metalwork, we made replacements for the plastic pieces from a sheet of metal - can't remember what metal. Lots of filing and testting but it did work. Your "piano wire" solution is much more elegant however...
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Old 5th Sep 2013, 10:01 am   #3
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Default Re: Replacing the infamous plastic bits on the Tandberg 12/1200X

Plastic mechanical parts, the silent enemy! Excellent work, I like the lateral thinking!
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Old 5th Sep 2013, 4:24 pm   #4
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Default Re: Replacing the infamous plastic bits on the Tandberg 12/1200X

Indeed, an excellent bit of lateral thinking! Once, when it was necessary for a replacement part to conform to the external dimensions of the original part, I made a reinforced plastic replacement using wire as internal reinforcement for an Epoxy resin casting that was finished by filing to the required external shape. In that particular case the wire was arranged so that it would lie wholly within the finished item. It was some years ago, but as I recall I made up a mould in a piece of paraffin wax or Plasticene to cast the part roughly to size.
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Old 5th Sep 2013, 10:17 pm   #5
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Default Re: Replacing the infamous plastic bits on the Tandberg 12/1200X

Of course now there is an alternative for making replica mechanical plastic parts, 3D printing! The original part could be laser scanned then printed off on a 3D printer, possibly even using a superior material. Maplin actually stock 3D printers now! This is definitely the future as they will develop dramatically for the hobby market within a few years.
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Old 6th Sep 2013, 10:09 am   #6
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Default Re: Replacing the infamous plastic bits on the Tandberg 12/1200X

I was considering that, particularly since we have a 3D printer here at work. I'm a bit unsure about the material though, the original piece is under quite bit of strain (as is obvious from the amount that suffer breakage), it would seem that it would have to be some form of nylon or delrin type material in order to be flexible and tough enough. We used to use 3D printing at another company I worked for, long before it was known as that (this was in the late 1990s, we knew it as 'rapid prototyping' back then). However, that material was rather brittle and unstable, for one thing it tended to bend under stress. But I know next to nothing about the materials available for 3D printing these days so there may very well be some material that is appropriate.

A piece of sheet metal I would have thought would be much more stable. But of course difficult to mass produce using simple means.

BTW, regarding my original post, turns out I was slightly hasty: the loop to the right is actually a bit short leading the spring to be unduly stretched which in turn means that when returning to the playback position from the record position it's fine until the metal knob is mounted on the lever; the additional inertia then causes it to overshoot. Easily remedied by taking a slightly longer spring from my junk box. Also, the loop to the left actually does bump the lower plastic bit in some combination of modes, so I've had to adjust that again as well.

At any rate, any replacement will probably have to be adjusted depending on the individual machine encountered.
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Old 6th Sep 2013, 10:34 am   #7
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Default Re: Replacing the infamous plastic bits on the Tandberg 12/1200X

I remember those rapid prototype machines, the part emerged from a bath of resin. I think they were just intended to produce a part in solid for evaluation, not as a final component for use or production.
There are now even 3D printers that can produce metal parts.
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Old 6th Sep 2013, 12:19 pm   #8
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Default Re: Replacing the infamous plastic bits on the Tandberg 12/1200X

Still not without it's problems, though
http://www.bbc.co.uk/news/technology-23727229
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Old 6th Sep 2013, 12:51 pm   #9
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Default Re: Replacing the infamous plastic bits on the Tandberg 12/1200X

A friend who works for Renishaw in Gloucestershire came in to the pub a few weeks ago with a 1/12th scale model of a Sopwith Camel engine with moving pistons and cranks. It had been 3D printed IN ONE PIECE with no after-assembly and all enclosed moving parts worked smoothly.
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