Recently my brother, who shares with me the interest in electronics and vintage even if he’s not a technician, asked me to provide him with a working tube radio to be used quite frequently at home, and therefore he was looking for a reliable, good performing device and, especially, that could call visitors’ attention thanks to some peculiarity.
After a short discussion, we oriented towards the German radio of the ’50s, which had all the required specifications, received FM band and were the most technologically advanced in the market (we remind that North-American market was already focused on the television, and American radio manufacturers were mainly producing inexpensive and standardized receivers). This is particularly true for the high end models, which were equipped with sophisticated audio circuits, with true tone controls, feedback circuit in low frequency, high quality loudspeakers such as the legendary Saba “Greencone”, “three-dimensional” audio systems, sets of multi-way loudspeakers , often with electrostatic tweeters and, starting from the end of the 50s, stereophonic sound.
The other side of the coin is that their designs lacked imagination, and the cases were all very similar to each other. Therefore, we had to find a product that could be distinguished thanks to some of its features.
Having already a Saba Freiburg Automatic 7, we decided to look for a similar model. Thanks to some online flea market, we bought ourselves a SABA Meersburg – Automatic 8 from a radio collector in Rhineland-Palatinate, also member of Radiomuseum and kindly showed his collection of old TVs, radios and cars. As typical for other SABA radios, model names for these radios are taken from South-West Germany regions (Saba was from the Black Forest area) and towns like Württemberg, Schwarzwald, Triberg, Villingen, Freiburg, Freudenstadt, Lindau, Wildbad, Meersburg, Konstanz, Breisgau, etc.
This radio features a technical device which, for its time, must have seemed science fiction: the automatic search tuning, exactly as in modern radio receivers. The latter ones work with a PLL, whereas here everything is analog and with tubes! The Meersburg 7 has even 10 tubes with a selenium rectifier: these characteristics, together with the good acoustic properties of the famous “Greencone” loudspeakers with AlNiCo magnets (aluminium, nickel, cobalt), make these radios more appreciated than the average German radios of the time.
Before describing the work necessary to bring our radio back to its full functionality, I feel obliged to list the technical details: from a radio-technical point of view, it is a super-heterodyne receiver, able to receive long waves, broadcast, short waves (the latter ones are not subdivided into multiple bands) and frequency modulation (88 – 100 MHz; unluckily it doesn’t reach 108 MHz since probably using the upper portion of the FM band were not allowed at the time).
Two separate pointers on the dial correspond to the two different methods of modulation; also the movements of the indicators are unconnected. Since there is a single tuning knob (which is connected to the automatic tuning motor), a system of clutches that switch between AM and FM, i.e. allowing the movement of one of the two pointers only, according to the wave band selected (FM button or one of the three AM bands).
There are three intermediate frequency stages for amplitude modulation and four for frequency modulation.
It is already evident that Saba could distinguish itself from other radio manufacturers: together with the alignment, with Saba IF transformers there is the possibility to adjust the degree of coupling of the coils, by meaning of coupling screws, allowing the adjustment the selectivity curve in order to obtain maximum selectivity from the receiver without compromising the band-pass curve, which would have cut higher audio frequencies. This is a technical feature which is extremely rare to be found in civil receivers, and confirms the complexity and accuracy of the design of these radios (which, on the other side, make them very difficult to service).
The radio has an internal dipole for FM reception, which is also used as internal antenna for shortwaves; its design is extremely accurate that there is a stub of a twin-lead aerial where the dipole (two aluminium sheets glued on the wooden case) is connected with the transmission line (a 300 ohm twin-lead antenna) in the upper part of the case. This was done in order to adapt the impedance of the aerial to the transmission line. To the untrained eye, it might seem that someone forgot a small piece of twin-lead antenna and, since it can be in a short circuit, I have seen that sadly removed by other repairers.
There is an adjustable internal ferrite antenna for broadcast and long waves. The AM ferrite rod can be rotated by a metal wire from the knob which is coaxial with the volume knob. A complex system of electrical contacts allows switching another coil when an external antenna is used, or to light a lamp behind Saba logo when the adjustable antenna is used instead.
Signal demodulation is the classic envelope detector for AM, and ratio detector for FM, using two solid-state diodes. There is also the automatic frequency control, but we will write about it later on, being part of the automatic tuning.
The audio frequency section consists in a preamp penthode and a single ended EL84 output tube. Volume controls is done via audio taper potentiometer, there are two tone controls (bass and treble), and two different pre-set equalizations, “speech” and “music” (Sprache e Musik), which can be selected with two buttons on the front. Due to several sets of contacts being operated with these controls, these are operated via relays, not directly through the buttons.
Finally, 4 loudspeakers are used as output, two for intermediate and bass tones and two tweeters. A switch allows the selection of internal or external loudspeakers, or both, adjusting the impedance accordingly.
For servicing the radio, Saba installed a miniature 7-pole tube socket with all measurement points for alignment and repairing, very helpful considering the great complexity of the set.
The “muting” is working on the automatic volume control, which is operative every time any of the controls is operated, to avoid noises when buttons are pushed or whilst the set is searching for the next station.
Concerning the operation of the automatic tuning, as previously mentioned, it works thanks to an induction motor , powered by 220V alternating current. One of the two windings is connected to 220V mains via a big capacitor. In order to understand how the other windings is operated, determining the direction of rotation and therefore the automatic tuning, we may say that it uses the intermediate frequency (IF) signal: this signal is strong when the station is tuned and, when we move away from the station, it becomes weaker (the curve according to the signal decreases is determined by the selectivity curve of the system of IF filters or transformers).
IF signal is taken where is stronger, i.e. at the primary of the last IF transformer, and is modulated
with the line frequency at 50Hz by a tube. Afterwards, this IF signal, which has a frequency of 460kHz for AM and 10.7 MHz for FM, is detected.
We have in output a 50 Hz signal, whose amplitude and phase vary when tuning changes. This signal is amplified by a tube and sent to the second winding of the motor. When the station is tuned, the signals on the two windings are in phase and the motor stops. When we are out of tune, the two signals are out of phase and the motor drives the receiver tuning in the direction which reduces the reciprocal phase displacement, that is the one which brings to the correct tuning position.
You can easily guess that a minor dis-alignment or fault in the circuit can cause an imperfect operation of the whole system: for instance, automatic tuning can work in one direction much better than in the other. Since all the circuit is duplicated (AM and FM are separated), servicing is very laborious even following service manuals, better to give up without manuals. I would also recommend to solve all the problems of the circuit first (es: weak tubes and faulty capacitors) and eventually proceed with the alignment only afterwards and only if the problem is really major and evident. A mistake could be disastrous for the radio.
The search of a station is done by means of a small control lever, in the lower front part of the radio. When we move it gently left or right, it operates a relay that keeps the lever on the side and, at the same time, set the motor going until the next station is found. At this point, the lever is released and the motor stops. Further moving the lever, the motor is in direct drive and the rapid run is switched on, allowing to quickly move the pointer across the tuning dial. We have to recall that the motor is constantly driving the tuning: therefore, when the automatic tuning is on, the knob has some tuning jitter or vibration, always “following” the station in order to keep the it perfectly tuned. The motor warms, which is normal. If preferred (and recommended when using the radio for a longer period), the automatic tuning can be switched off with a push button, and the tuning becomes manual.
Speaking about the restoration, this radio was already functional, but the volume control was not working. Since volume control is physiological, find a generic spare part is impossible, so I was forced to disassemble and repair the potentiometer. Someone used brute force even when it reached the limit stop position and broke it, so I had carefully reconstructed using some two-part epoxy glue.
Another big problem was the contact oxidation: since the radio has several dozens of electrical contacts,
malfunctions were nearly everywhere. I cleaned the chassis through with a cotton cloth moistened with water only, with a great deal of patience I cleaned all the contacts with a small cloth soaked in a specific contact cleaner. One thing worth noting: the white paint behind the dial and on the chassis are extremely delicate, so cleaning must be done very gently, with clean water only. Unfortunately, even the potentiometer spray cleaner might remove the paint.
After this, I replaced the weak tubes and the most critical capacitors, those clearly faulty and those of the motor (note for who is repairing these radios: the big 0.4 one in aluminium is generally OK, whereas the big paper one is often leaky), leaving in any case the original parts at their original place (I believe that people doing complete “re-capping” of radios should buy brand new radios and change hobby).
Moreover, since the automatic tuning was working in one direction better than in the opposite one, and the knob was “dancing” to the rhythm of the music, I aligned the system of automatic tuning following the instruction of the service manual. I wouldn’t wish this experience on anyone, also because it is mandatory to align everything else first.