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  • Writer's pictureDominik

Millimeters versus Millivolts

The previous blog articles related solely to the handling of classic electronic components in various studio devices. Tube or transistor amplifiers (if well restored) work in silence, they do not require much maintenance over a long period of time and therefore hardly change their technical and tonal characteristics. An overhaul is usually possible with simple means such as an oscilloscope and a multimeter. In this and the next episode, however, I would like to take a closer look at a device that has become very popular again and which deserves the term ‘machine’ for sure. Let’s talk about the classic studio tape recorder!

Calibrating a vintage tape machine

On to the Mechanics

Welcome to the next episode: Those who are not afraid of watchmaker's screwdrivers, belt tension scales or dial gauges are invited to join a journey into precision mechanics featuring flywheel, pinch rollers and other wondrous attractions.

Measuring and adjustment tools

For describing the maintenance of a studio tape recorder, I have chosen a machine that is a prime example of precision in the mechanical and electronic field, precisely because of its clear, simple construction, making it very service friendly as well. Furthermore, it was the practical standard in many radio stations and recording studios for a long time. Other machines (for example those made by Studer or Otari) have similar characteristics, but the procedure for restoration or overhauling is different in some parts. In general, however, they all follow the same principles of preserving analog signals on magnetic tape.

Telefunken M15A: Top Analog Technology for Magnetic Sound Recording

A true classic – which does not necessarily show at first sight. On the top, only a few, spartanly arranged controls are visible. Without a tape on, for example, the pick-ups for the winding plates appear inconspicuous and weak. The surface is dominated by the large roller and the steadying rollers, which have a large diameter and, when turned back and forth by hand, give a nice impression of what good bearings mean.

Bird's eye view onto the sober M15A layout

The heads hide under a stable cover and you can only see them when you bend far over the machine, almost standing on your head. The typical, very large and illuminated control buttons are located at the bottom left side. In the middle below the heads, there is a plain 4-digit electronic tape time display. It shows minutes and seconds very accurately, because even with repeated winding back and forth there’s just minimal slippage.

Under a lid at the bottom there is the main power switch, the speed selector (usually 19 and 38 cm/sec. are used in the studio) and the remote switch, which, when pressed, makes all operation on the machine itself impossible. The surface of the lid also holds the cutting and sticking fixture for the tape.

Control elements under the lid

There is a very important instrument on tape machines that I would like to point out: It is the counter for the operating hours, which makes it easier to keep to the maintenance intervals (5,000 or 10,000 hours). It's ‘nice’ when it has 4 digits and shows just a few hundred hours... so you never know how many times it has run over 9999 and how old the machine really is.

I recently had a machine with a 5-digit hour counter for restoration. It showed about 75,000 hours. A single tantalum capacitor was burnt in a playback channel, and everything else was working perfectly fine.

The Telefunken/AEG M15 or M15A types underwent numerous evolutions and extensions during their life cycle. Since they were already conceived as ‘modular kit’ at the design stage, additions could easily be implemented. The biggest versions were the 1" and 2" machines with up to 32 tracks on which Karajan recorded and cut and which ABBA and other legends of pop culture recorded their hits on. A rare piece in my collection is a real quad channel machine of the DGG (Deutsche Grammophon Gesellschaft).

Heads of a M15A quad machine with 4 tracks on a width of 1"

There were also units with pilot tone (which could run in sync with film) or time code functionality. Basically, it is irrelevant how many tracks a machine has: The mechanics and their adjustments are very similar for all formats.

First Maintenance Steps

Two ‘trades’ have to play hand in hand for the perfect operation of a tape machine: Mechanics and electronics. The one cannot do without the other. If you receive a machine and want to reactivate it, you should first clean it well, then examine the mechanics and disassemble it for further cleaning if necessary. Here, one should remember the order of disassembly to avoid unpleasant surprises during reassembly.

Don't worry, though – a mechanical clockwork is actually more complicated.

To clean the parts that come into contact with the tape, it makes sense to use pure alcohol in the form of spirit or ethanol. Other solvents would attack rubber or plastic parts. I have also used water, universal cleaner and a brush to make aluminum or lacquered surfaces shine again. During this treatment you must pay attention that no liquids find their way into the bearings, as it would dilute and wash out the all-important lubricants.

Capstan motor of an M15A

If rotating parts such as steadying rollers or the winding motors run unevenly, it is usually necessary to replace one or more bearings. These are still available today without any problems, since they are standardized. In older models, the pinch roller or even the capstan runs in so-called sintered bearings. The capstan motor does not have a collector and weighs only 800 grams. The whole machine weighs 55 kilograms.

Capstan motors have the advantage of running free of play and wear without much lubricant. After dismantling, they can be easily cleaned and then lubricated with sinter bearing grease. If such a bearing shows noticeable play due to corrosion, unfortunately only a complete replacement will help. You can use cotton swabs soaked in alcohol for cleaning the tape guides and the heads. More intractable dirt can be carefully removed with a wooden toothpick after soaking. Never use harder materials – the surfaces, especially those of the heads, are very delicate!

M15 4-track 1/2". From the left: Eraser, record and playback heads

Powering On

Now you can dare to switch on the machine. On a M15A, the speed indicator may light up if the corresponding lamp has not burned out. This is often the case with the 38 indicator, since most machines were operated at this speed more or less all of the time. Maybe the control lamp for speed 19 will light up after pressing the middle switch under the flap. If you now press the tape sensing lever on the right side inwards, the capstan motor should start. The drive of the pinch roller can actually be irritating if you take a closer look: A very small motor with a diameter of only 50 mm including housing drives a huge flywheel via a wide synthetic rubber belt held under constant tension by a pulley.

This drive is also responsible for the characteristic operating noise of the machine: A more or less loud hiss, depending on the speed, can be heard when the machine is operated without a housing while the pinch roller is running. Here we have another possibility of doing some maintenance: A little bit of talcum applied and massaged into the rubber belt helps to reduce the noise level and to keep the rubber smooth.

Talcum is available at car accessory shops or from your partner’s make-up box: Face powder is the magic ingredient (the perfume fragrance will fade over time ;-)

Logics without Microprocessors

After 8-10 seconds of start-up, the stop button should light up (if the lamp is still ok – see above). This is the sign that the target speed has been reached. While holding the sensing lever, we can now press the start button. With a clacking sound, very familiar to older sound engineers, the pressure magnet pulls the pressure roller against the pinch roller and the right-hand winding plate runs counterclockwise at quite high speed. The stop button goes out, the start button lights up. Now the machine is in playback mode. If the record button is pressed at the same time as the start button, it lights up as well and the machine starts to record.

The next test concerns the rewind function: After pressing the rewind button on the far left, the speed of the winding motors can be brought to maximum using the shunting lever located between the record and play button. At the same time, this test can be used to check the winding motor bearings for suspicious noises. If the sensing lever is released, the machine goes into stop mode and the capstan motor also switches off. The speed and counter displays alone indicate that the machine is still switched on.

This little schedule is used to demonstrate that the machine contains control electronics for the drive in addition to its analog circuitry for recording, playback and erasing.

It receives commands from pressed buttons on the one hand, but also from several small magnetic and micro mechanic switches on the sensing levers and winding plates. All commands (buttons) and feedback (switches) converge on one board, which is called LG12 logic board. This PCB is located in a small rack at the rear of the machine. An electrical warning sign on the lid indicates that there are dangerous voltages present on these boards (the supply voltages for the winding motors).

LG12 assembly plan with soldering bridges
This board is swarming with NOR, NAND, and OR elements in the form of ICs. When reading the schematics, you are automatically reminded of switching algebra.

Here you can also ‘program’ some drive functions by using the soldering bridges. If a machine behaves strangely in terms of its tape transport not working properly or motors starting without the ability to stop them again and so on, it is possible that a command (push button) doesn't work properly, or a micro switch is gummed up. Rarely it is a fault in the electronics of this board. If this should still be the case, you have to go very deep into the circuitry and rely on the details found in the original manuals. However, from experience I can say that these components are very reliable.

Put on a Tape

Once these functions have been tested, a (preferably used and old) tape can be put on. Now you can evaluate more exactly if the logic works under in-use conditions (playback, shunting, rewinding) and above all: if the mechanics are functioning properly. It can be seen with the naked eye whether the tape is jammed, perhaps because a winding plate sits too high or too low, whether the adjustment of the tape guides between the heads has shifted, or whether a sensing lever is bent.

Tape tracking on the playback head

When fast winding, the wrapping should be smooth and tight enough. Removing the reel, which usually holds an open wrapping floating freely over an aluminum plate (sometimes called pancake because of the reddish color of the magnetic tape), should not result in chaotic tape salad. Adjusting the winding torque can be done by simply adjusting the corresponding knurled screws.

M15A braking system

The heads serve as the height scale for tape guidance during recording and playback. These are mounted with a tolerance of +/- 5µm. All mechanical (height) adjustments are based on this measure. With some experience and a good eye, the adjustments can be made so that the tape passes in front of the heads without any warping. To check how far the heads dip into the passing tape, you can color the shiny head surface with a grease pen.

After letting some tape pass by, you can examine the level of the rubbed-off grease. The head should dip exactly into the middle of the tape. Another hint at borrowing your partner’s tools… this time the lipstick… make sure to discard the test tape with the grease traces afterwards.


Now the next steps can be made: Connect your tape machine, press play and make sure to not miss the next episode, as we’ll deal with some electrical adjustments then. We will calibrate the heads and levels with quite simple means, adjust the electronics to a preferred tape type and so on… stay tuned!

This article is presented with kind permission of its original publisher, the amazing Studio Magazin, enriching Pro Audio since 1978! The author, Uli Apel, is an incredibly versatile and experienced engineer as well as one of the most qualified experts in vintage broadcasting and audio technology around these days.



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