Blaupunkt classic car radio step by step upgrade with bluetooth

If you drive a vintage car from the 70s, you should also choose a correspondingly old car radio, a modern car radio in a vintage car is an absolute no-go! Cars around 1970 were often equipped with blaupunkt car radios from that time, these fit visually well into the overall look of the vehicle. With a few modifications to the car radio, it is possible to adapt it to today’s technical requirements without changing its external appearance. Using the example of a blaupunkt frankfurt rallye stereo from 1970, some changes are described here step by step. This model is typical for this time and is representative for a whole model series with similar technical construction. Here, the DIN socket for the AF input/output as well as the mechanical key set for the different wavebands are common to all these devices. the following measures are therefore not limited to the frankfurt model, but can also be applied to corresponding other models. The typical structure of such models is described below Fig. 1 shown.

picture 1 basic structure of the blaupunkt car radio models described here

measures without interfering with the car radio

amplifier power

The aforementioned car radios have a quite good VHF reception in stereo, if they are technically flawless and have an appropriate antenna. However, the relatively modest output power leaves much to be desired. Here an external amplifier with z.B. 4 x 25 W remedy as it was also available from blaupunkt in the past, but is still offered by other manufacturers as an accessory today. The auxiliary amplifier is installed in a hidden place under the dashboard or in the trunk and therefore does not disturb the optics. The amplifier is controlled via the speaker outputs of the car radio. 4 speakers are connected on the output side. For these loudspeakers, there are usually installation positions provided in the cars of the 70s. If these are not available, the front left and right footwells and the area of the rear parcel shelf can be used for inconspicuous installation. Fig. 2 shows the basic connection of such an additional amplifier, it can be controlled either via the loudspeaker outputs or also the AF output of the car radio.

Fig. 2 principle of connecting an additional amplifier

AF feed

the NF-DIN socket of blaupunkt car radios is perfectly suited for feeding in external sources, today mostly MP3 players or directly from the cell phone. In the normal case, the inputs and outputs of the socket are bridged via a plug. Here is then an adapter cable with a 5-pin DIN plug connected, at the other end is a 3.5 mm jack with switch to connect to the MP3 player or cell phone. corresponding adapter cables are available in stores, but can also be made by yourself. The circuit in picture 3 provides a template for this.

picture 3 adapter 3.5 mm jack to DIN plug connection

  • When switching between radio operation and external power supply, the plug has to be changed constantly. This is annoying and also an external switch as a replacement for the switch socket does not provide a satisfactory solution.
  • The connection between the stereo decoder and amplifier is not cleanly separated within the radio, the left channel still comes through very weakly. The fault is in the radio’s circuitry and can only be eliminated by reaching into the device. The emergency solution is to set the tuner to a transmitter-free range (z.B. via the LW button which is not needed), but then noise is usually still present!

As an alternative, a much more modern solution via a bluetooth connection is available. Not only is the unsightly audio wiring to the cell phone no longer necessary, but the bluetooth connection can also be used as a hands-free system at the same time. for this in the trade (ebay, amazon) appropriate devices are offered, from far eastern production often clearly under 20, – €. For my purposes I have chosen the device in picture 4 with the following features:

picture 4 bluetooth receiver for music transmission and call transfer

the receiver must be paired with the cell phone once via the bluetooth connection, later the cell phone is automatically recognized and the connection is established. The pairing is indicated by a blue LED, to accept calls via the hands-free mode, the button shown above must be pressed. A microphone is built into the receiver, the playback takes place via the loudspeakers of the car radio. The device is supplied with 5 V via a micro-USB plug, as is the case with most cell phones, but the built-in battery also allows several hours of operation without external power supply. There are two buttons on the side of the machine that can be used to advance or rewind a track during music playback. This button/function has been omitted in the further considerations, because the operation can also be done on the player (cell phone).

the device can be connected to the car radio and operated using the jack adapter shown or the enclosed jack cable with the help of the jack/DIN adapter described above. However, without a fixed and easily accessible position for the receiver, as well as a clean cable routing, the operability of the device is severely limited.

For better handling, the receiver, together with a switch and the power supply for the cell phone, was installed in a small housing and attached to the cell phone holder. Thus the plant could be operated and has also a test enterprise over 3.000 km survived. Surprisingly, there was no more interference from the radio receiver when the amplifier was connected to the bluetooth receiver by a wave switch.

show the cell phone mount described here picture 5 and Fig. 6.

Picture 5 general view of the cell phone holder

picture 6 bluetooth receiver with control button, toggle switch and LED switch for 12 V supply

Of course, the annoying switchover between radio operation and external audio feed was still necessary. Also the operation at two places, once at the radio and additionally at the cell phone holder was not optimal and it was further searched for a more elegant solution. This could only lie in an implantation of the bluetooth receiver into the radio, so extensive interventions and modifications to the car radio itself were required.

Measures involving intervention in the car radio

VHF station buttons

If you already open the car radio to make major changes, you should first adjust the radio to the new conditions by a simple handle. Devices of the type described here usually still have all wavebands, such as longwave, mediumwave, shortwave and of course VHF. Since today a reception over LW, MW and KW is hardly given and thus in most cases no longer makes sense is the waste of three station keys for these wave ranges of course questionable and should be changed.

Switching between the individual wave ranges is done via a single slide switch in the radio, which is mechanically moved into the corresponding position via a linkage of the station buttons. If this rod is carefully bent upwards out of the slide switch with a pair of needle-nose pliers, the switch is deactivated. All you have to do is fix the switch in the FM position and you have a radio with 5 FM buttons that can be preset to the desired stations in the simplest way possible. I have already made this change on several radios, as it makes the old radios much more suitable for today’s daily use. The change is of course reversible, d.H. The original condition can be restored at any time by bending back the actuating rod.

The measure described here is illustrated by the photo in Image 7 made clear.

Image 7 actuating rod from the pushbutton set bent up to deactivate the shaft changeover switch

Installation of the bluetooth receiver

to integrate the receiver completely into the housing of the car radio the following measures are necessary:

  • Creating space for installation
  • Provide a button to operate the receiver
  • move the led for bluetooth operation to the front of the scale
  • Move condenser microphone to the front of the scale
  • Installing the complete unit mechanically and connecting it electrically

As listed above, the required work steps are described here in detail.

Creating space for installation

Although the dimensions of the receiver are very small, together with the necessary mechanical fixings and the electrical connectors, it requires a space that is not available without modifications. my solution is to cut the board in the area of the right knob for station selection and stereo/mono switching. the part of the board to be disconnected is indicated by the blue dashed line in figure 6. The toggle switch is no longer needed and is rebuilt to operate the bluetooth receiver, two other components L1 and G1 are removed and soldered back on underneath the board. The location of the components to be changed is shown on the board and the corresponding part of the circuit diagram in Fig. 8 shown.

picture 8 section of the board with new position of the components L1 and G1

provide pushbuttons for operating the receiver

Cutting off the marked section of the board not only freed up space, but also exposed the plastic arm of the mono/stereo switch. this lever arm is slightly shortened and ground to later serve as an actuator for a button located on the new circuit board to be installed. A spring on the axis of the switch, which fixed the mono or stereo position is removed.

move the condenser microphone to the front of the scale

as well as the LED, the microphone for the hands-free system must be relocated to the front of the radio. To do this, the condenser microphone on the bluetooth board is desoldered and fitted with the appropriate connecting wires (note polarity). Since the rightmost area of the scale is not used by the pointer, a hole can be drilled in the plexiglass and the microphone capsule can be glued in. The diameter of the microphone is very small and the change is practically not noticeable in the picture of the car radio.

Install the complete unit mechanically and connect it electrically

The board of the bluetooth receiver is removed from the original housing and attached to a newly made carrier board. In a first attempt, this board was also used to house the lF switch, which has already been mentioned several times. This should be mounted over the above mentioned rod of the key set, z.B. The LW button should be pressed. This was mechanically quite a challenge and required some tinkering (see Fig. 9).

carrier board 1st draft

Figure 9 first draft of the carrier board with NF switch

In the course of further trials, it then turned out that such a switch was no longer necessary. With simultaneous use of the shorting plug on the radio, the output of the bluetooth receiver had to be connected only to the input of the amplifier part in the radio. With external music feed or a call the radio reception was automatically muted. This of course made things much easier, as not only could the switch be dispensed with, but also the associated cumbersome operation. The layout of the new carrier board is in picture 10 shown. The board now has to perform the following functions and contains the corresponding components:

Layout of the carrier board

Fig. 10 layout of the carrier board

  • fastening of the entire unit via an aluminum bracket to the housing of the
    radio set (right outer wall)
  • housing of the bluetooth board and the accumulator
  • inclusion of the button for the operation of the unit
  • Mounting of the connection pins for the required plug-in connections
  • The voltage converter 12V to 5V, this is realized via a
    7805 fixed voltage regulator with electrolytic capacitor as charging voltage for the battery

First, the aluminum bracket is screwed to the carrier board. for this purpose as well as for the fixation to the radio housing 3 mm screws were used, the corresponding nuts were fixed with 2-component glue. This will make subsequent final assembly much easier. the bluetooth board is attached to the solder side of the carrier board with double-sided foam tape. Before that, the two connecting wires for the AF signal and the ground are to be soldered in the middle of the carrier circuit board. The further wiring is then shown from the photo in picture 11 visible.

picture 11 arrangement of the bluetooth board on the carrier board

the blue wires lead from the original pushbutton to the new pushbutton on the carrier board, in this case the original pushbutton can remain on the board. The blue LED is different, it has to be desoldered and the new supply wires (red and black) soldered on there. in the right half of the picture the bare wires show the connection of the condenser microphone, in the lower half of the picture the bare wires represent the 5 V power supply. The entire structure of the module is clearly shown by the diagram in picture 12 shown photo of the backside of the carrier board. Here you can see the voltage converter with the pins for power and AF signal on the right side, the button and the pins for LED and microphone on the lower left side and the 3.7 V battery in the middle.

back of the carrier board

picture 12 rear side of the carrier board with the required components

The following pictures 13 – 15 show the different views of the radio with the bluetooth receiver board installed.

carrier board in installed condition

Fig. 13 receiver board in installed condition

radio board with NF input

picture 14 view from below with wiring for AF input and 12 V supply Fig. 15 front view with condenser microphone at the bottom right of the scale


The conversion of an old car radio to modern technology presented above is a practical alternative. If you don’t just use your oldtimer for a coffee ride on a sunday afternoon, but take part in events and rallies all over europe, you need a navigation device for the journey and a tripmaster for the tests on the spot. Both tasks can be solved excellently using the appropriate apps with the smartphone, so the cell phone is inevitably on board as well. it makes sense to connect the cell phone to the car radio as described above to ensure a minimum of safety and comfort in the oldtimer.

Fig. 16 blaupunkt radio in the spider

Fig. 17 blaupunkt radio in the berlina

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Christina Cherry
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