I have been recently contacted by a vintage car collector, which was attempting to restore a prestigious Jaguar E-Type Coupe, to ask if I could repair its car radio.

The car radio was a Blaupunkt Frankfurt, dated 1967/68, which the car was equipped with.
Both the car and the radio had American origin, but the car radio was purchased separately, since it came from the stripping of another vehicle.

The car radio is already a fully transistorized device, and it is a classical Blaupunkt receiver for standard broadcast, short waves and frequency modulation bands.
It has five push buttons for the mechanical selection of the pre-tuned stations: two AM, one shortwave and two FM stations.
As mentioned, the radio has an American origin, as shown by English acronyms of the wave bands and by the fact that the FM band was spread up to 108 MHz, whereas most European FM radios of that time were reaching only 100-101 MHz.
Correct identification of the model can be difficult, since the name Frankfurt name was shared with dozens of other models from different decades and with different technologies, which means that we have to rely on the chassis number usually printed on paper labels on the case of the radio. This is common for the production of this manufacturer: often a name of German city (München, Bremen, Stuttgart, Hamburg, Hannover, Köln, Ulm, Wolfsburg, Berlin, Essen, Mannheim, Hildesheim, Heidelberg, Emden, Dortmund, Karlsruhe, Solingen, Wiesbaden, Braunschweig, Koblenz, Marburg, Fulda, Flensburg, Minden, Bamberg, Coburg, Goslar, Ludwigshafen, Lübeck, Münster, Bonn, Ingolstadt, Kiel, Konstanz, Tempelhof, Göttingen, Mainz, Limburg, Nürnberg, etc.) or foreign (Vienna, Toronto, New Yorker, Montreal, Colmar, Lyon, Calvi, Lille, Le Mans, Windsor, Porto, Verona, San Remo, etc.), together with a numeric code.

Concerning the conditions of the radio, those were rather good aesthetically, but it could not be tested since the potentiometer of the volume control (which served as power switch, too) was broken.
The cause of the damage is easily guessed from the brute force evidence on the plastic knob still attached on the ‘stump’ of the shaft: since both the shaft and the bushing are made by aluminium alloy (or maybe even Zamak), they seized up because of oxidation. Taking off of the knob was therefore impossible, preventing the removal of the radio from the dashboard.

The ‘dismantler’ decided to use violence against the poor knob, breaking the pin. After further examining the conditions of the potentiometer, which was heavily deformed, I noticed that the damages continued even inside the radio, where some PCB pads were delaminated.

Unfortunately these Blaupunkt potentiometers (branded “Ruwido”) were practically custom-made for each of these radio models, and it is nearly impossible to find an exact replacement. Indeed, the potentiometer has a non-standard shape, and it fits not only the power switch, but also the balance control; and there are some intermediate pins along the resistance element, being an audio taper potentiometer. In conclusion, this was really a problem.

After some time, I went to one of the local electronics seasonal fairs, where I could find some Blaupunkt potentiometers very similar to this. Once back at home, I could verify that only one was mechanically similar, but still not compatible, not only because of the displacement of the pins which was different from the holes in the PCB, but also for the resistance values.

With patience of Job (note the teutonic maniacality and the enormous number of small pieces with which it was produced!), I dismantled both the old and the new potentiometers and transferred the resistive elements of the old one onto the new one. I also kept the switch of the new one. I finally adapted the mechanical parts with a metal file.

A friend with a lathe helped reproducing the black plastic knob.

The radio also had a faulty connection in the wave band selector, which was easily solved with some soldering.

Fixed the faulty connection, the radio was still perfectly in order and well aligned, so the restoration work was finally concluded.