Building a Magnetic Tape Recorder from Scratch, 1949-1952

Pre-amp schematic

6) Erasing the Magnetic Tape: Easy, use a magnet WRONG! Little did we know at the time, total magnetic saturation of the tape made the detection of future recordings for the much smaller recorded signals impossible.

7) The Final Test: We finally shouted into the carbon microphone as the 8mm film rushed across the slit of the recording head. We then connected the recording head to the input of our amplifier. After rewinding the 8mm film our prerecorded “magnetic tape” passed once more across the now playback head. - We heard nothing! We repeated it several times with the highest amplifier gain setting and still --- wait, did you hear that? No. – Again, nothing.

We realized then the 8mm film was too thick and indeed the whole thing was nothing but a pipe dream. I had lost all hope. Meanwhile I worked on other electronic projects that had caught my interest.

Had it not been for my friend Gerhard who encouraged me not to give up on the Magnetic Tape Recorder, the project would have stopped right then and there. Of course, we knew that without the tape the professionals were using, our project would have no chance for success.

The BASF ( Badische Annelin und Soda Fabrick) in Bavaria was the only company that produced this tape. But to purchase such a tape was another story! Our first letter to BASF: Received no answer. Our second letter was answered: “ BASF only sells to radio stations and film studios”. In our third letter we explained what we boys were trying to accomplish. Someone at BASF must have had a heart because one month later, to our astonishment, we received a 1000 meter (3000ft) tape for the price of the 20.- DM.

It took me a while to come up with a better tape transport for the much larger tape reels. Two old turntable discs supported the tape but the right motors were hard to find. With my 18^th birthday coming up by the end of the year I wished for money to buy a synchronous drive motor and another smaller (fan) motor to the wind our precious tape. I had also repaired radios, vacuum cleaners, doorbells, etc. that brought in some more needed money.

With a rather crude transport system mounted on a plywood board, the new magnetic tape and the home-wound recording/playback head we actually produced the first positive results. Nothing to get too excited about, but a low volume garbled voice was audible and sure sounded better than nothing at all.

Finally, we identified the recording head as the weak link in the system. Months later we discovered the location of the AEG supply store in Hamburg that dealt with spare parts for AEG and Telefunken magnetic tape recorders. The idea was to purchase a used recording head. After a long time the AEG clerk returned to the desk and asked for the recorder model number. We told him we had not assigned one yet. When he found out that we were going to build a tape recorder from scratch he advised us to save our money because it had been unsuccessfully tried before. Twenty minutes later we left the AEG supply store with a 30.- DM used playback head. One could tell it had been used because the playback head had worn down some of the Mu- metal across the slit. A distinct difference between our head and the AEG head was the width of the slit. Our slit was about 1 mm wide; the AEG slit was less than 0.1 mm (meaning better frequency response). The AEG Mu-metal ring consisted of two halves, each side wire wound. The two halves were mounted between two cast-iron rings; a very smart design. From now on we were able to show some rewarding progress, slowly, step by step. The new head functioned as recording and playback head and for the first time we heard our voices more clearly, but still not loud enough. We solved our problem by building a preamplifier with two additional tubes. However, excessive 50Hz hum put a damper on our progress. (The German Power stations generate 220 Volt AC, 50Hz)

The partial solution to the problem was found by relocating our power transformer as far away from the playback head as possible and then rotating the transformer slowly for a minimum of 50Hz hum. By now we had used about half of our magnetic tape without erasing the pre-recorded section. Erasing the tape with a magnet was a no-no!

Tube amplifier schematic

Since Christmas 1950 was near, I was hoping for money to buy the missing recording and erasing heads. Santa was good that year. So, we were not only able to buy two heads but also a better synchronous motor to allow us to move the magnetic tape in a more constant speed (At that time we had not heard of piston drives.) Meanwhile we found some literature (Radio Magazine) describing how magnetic tapes are erased with a 32 kHz signal. We found a schematic for a simple oscillator and calculated the capacitance and inductance required. We had to wind our own coil for the 32 kHz oscillator. Having found a coil of unknown inductance, we needed to determine the constant (K) of the ferrite core. We accomplished that by placing a 500 microfarad capacitor in series with the coil and placed the LC network between our radio receiver antenna input and the antenna. We selected a radio station with a known frequency and then started to unwind the coil until the station was no longer heard. We then disconnected the coil and counted the remaining windings of the RF coil. Now we knew the number of coil windings. We also knew the frequency and the capacitance. With that data we were able to determine the constant K of the ferrite core, enough information to calculate the windings for the 35 kHz oscillator. In order to make sure we were providing enough RF power we selected the high-power LS50, a tube used in military radio equipment. We also read that a small amount of the 32 kHz RF signal would be beneficial for the demagnetization of the playback head.

The final results were stunning. The quality of our voices had improved and the signal was also more amplified than before. However, the music recordings left something to be desired because the tape speed was not constant. The two guides that positioned the tape over the three heads were stationary and not turning. But, we later found some rollers that improved the tape’s constant speed.

In spite of all progress made over the last few months we still could not totally get rid of the 50 cycle hum during playback recordings. I am not sure who came up with the idea, but by placing a coil with about equal impedance of that of the playback head, wired between the playback head and the preamp, we were able to reduce the hum. We positioned the coil several centimeters below the playback head and turned the coil around until the 50 Hz signal was almost completely canceled and only a slight hiss was audible. We then found a spot where we could mount the coil, generating the same results.

Approximately 1 ½ years had gone by since we started this project. The reasons were component delays, other more interesting projects, and extreme hot and cold weather conditions experienced throughout the year under the attic roof, without insulation for my Radio Shack. Even a thick warm Jacket and Grandma’s heating pad on my back would not allow me to stay up there more than 20 minutes during the winter months.

At the age of 18, I started my three year apprentice’s period in a radio shop and schooling in my hometown (Kellinghusen) where I had access to a lathe, allowing me to improve the rollers of the tape feed and other drives. Finally, I mounted all components on a large aluminum sheet. A friend of mine did not like the wooden frame I had built and took it upon himself to surprise me with a more professional version.

Radio Shack in the attic