W116 W116 W116 W116 W116

Becker Mexico Cassette

AUX input Upgrade

by Ken Tyler

A modification to fit a 3.5 mm stereo socket to the front face of a Becker Mexico Cassette to allow use of mp3 players and the like but still retaining AM/FM radio and cassette functions.

Becker Mexico Cassette

The socket is wired in before the volume control so the radio's volume and tone controls work on the external input. The input works well connected to the headphone output of my player, level is slightly down compared to radio at the same volume control position. The level out of a player depends on that particular player, its volume setting and the level of the "recorded" signal - no gauruntee your player will give the same results.

Read the following in conjunction with the accompanying pictures before starting. The job is not difficult but if you are not confident of your abilities with this sort of modification it might be an idea to find someone who is willing to carry out the modification for you. I would hate to hear of a damaged radio.

There are two separate components to the radio, the tuner/player part in the dash and the power amplifier, this is located under the glove box in my car.

To remove the radio, pull off the volume and tuning knobs, ease the tone and sensitivity controls off with a finger nail under them, the face and frame can now be unclipped and lifted off in one piece. Loosen the two screws, no need to remove them fully, then slide the retaining metal strips upwards and lift them out.

1. Top view of the radio.

Remove the glove box, slip the clip off the mult-pin connector and unplug it along with ground clip from the amplifier. Pull the radio out far enough to allow the aerial to be unplugged, the 12 volt supply (black wire with a bullet connector in my case) to be disconnected and finally remove the screw holding the ground connection. Tie a piece of string to the multi-core cable in the glove box and remove the radio completely, feeding the multi-core cable and string from the glove box as you go. The string is used to pull the cable back into the glove box.

Remove the top cover of the radio by depressing the locating tag at the back top edge of the cover and hinging it up, then pull it to the rear to free the front edge. Do the same with the bottom cover.

2. Two of the boards, the volume and tone control board, and the AM/FM detector radio/cassette selector board. This last board also has the mono input from a TMA, traffic management adaptor. No idea what the "proper" names for the two boards are but these names reflect their functions.

Unplug the TMA plug from the rear of the radio, remove the screw from the front right of the detect and select board, press the spring clip on the front left of the board outwards and hinge the board upwards.


The tracks on the back edge of the board could short to the case.

Audio cables
3. Hinged up detect and select board.

Picture 3 shows a closeup of the hinged up detect and select board. At the tip of the screwdriver are two screened audio cables, one yellow, one blue, these feed audio from the detect and select board down to the volume and tone board at the lower left, this connection is partly hidden by the spring clip in the picture. It is this pair of cables that are extended (replaced) to the added front socket.

Front routing
4. Pass through the gap between the cast aluminium front piece and the cassette slide.

I couldn't find screened audio cable that was small enough in diameter to pass through the gap between the cast aluminium front piece and the cassette slide as in Picture 4. The solution was to make up cables from a metre of Dick Smith's smallest twin screened figure 8 and two lengths of 2.5 mm heat shrink tubing, yellow and blue to match the old wiring. Four cables are needed, two of each colour about 200 mm long.

The figure 8 is split down the middle and cut into four single pieces a about 250 mm long. The outer insulation is stripped off about 15 to 20 mm at a time. Leave about 20 mm of the outer insulation in place to hold the screen together. A pair of adjustable wire strippers helps here. The insulation should be pulled off with a slight twist to keep the screen tight around the inner core. When a 200 mm length has been stripped, cut off 10 mm of the inner core at the free end and twist the fine screen wires tight to a point. Thread the stripped cable pointy end through the heat shrink tubing with a twist to keep the screen tight, otherwise it will bunch up and jam. When the screen emerges from the tubing shrink the tubing tight onto the screen with heat (hot air preferred) and cut off the untidy ends to ease threading them into the radio.

The whole process of cable making sounds fiddley but it works. I did a trial before committing to the method and was surprised how well it went. No doubt small diameter screened audio cable does exist somewhere.

A possible but untried alternative to making your own would be to remove a bit of the front alloy casting for more clearance but getting the casting off to do the filing looked too hard and I wasn't going to risk filings in the works by doing it in place.

The made up cables don't have to be yellow and blue of course, nor do they need to be small if the cassette player is not needed, the cable can pass through the deck.

Feed the four cables into the radio as in Picture 4. They pass above the bottom circuit board and between the alloy casting and cassette slide. The cables are NOT between the bottom cover and circuit board. With the radio upside down they can be pulled through when the come into view. I routed the top two cables to the detect and select board, the bottom pair to the volume and tone board.

New cables
5. New cables.

Unsolder the existing yellow, blue and screen wires from the three tags on the detect and select board. Strip the ends of a yellow and a blue pair, cut about half the fine screen wires off each (half as many is more than enough) and twist them together, solder the three new wires to the tags in the same positions as the old wires, see Picture 5. I used some black heat shrink to prevent shorts and to neaten up the ends. Identify this pair of wires in some way at the front of the radio. There is a letter "R" on the solder side of both boards to indicate the right channel, this is the yellow wire.

The other pair are connected to the volume and tone board in the same way with one exception, the existing screens connect to a black wire that then goes to an earth point on the board which is inaccessible without removing the board, which is difficult to do. Cut the earth wire from the existing screens keeping as much length as possible, slip a short piece of heat shrink tubing on to the pair of cables and the black earth wire and solder the screens to the the wire, then shrink the tubing over the joint. This is just visible in Picture 5 (not too much heat into the radio here).

The four cables need to have sufficient length at the front to allow the socket to be soldered, feed a generous amount of each of the cables into radio such that the cables sit in large radius, mark where they exit the alloy casting, pull them back out and cut them at the marks - not losing track of which pair goes where.

The socket is a 3.5 mm stereo change over socket from Jaycar, black plastic with 5 pins on one side and transparent plastic on the other. Don't know if other types are around or if they would fit.

Socket left view
6. Socket - left view.
Socket right view
7. Socket - right view.
Socket top view
8. Socket - top view.

Pictures 6, 7 and 8 show three views of the socket with the center conductors connected. The screens are thinned out to about a third the original number of wires and connected to the ground pin on the socket (not shown - no picture).

Keep in mind when soldering the socket there is not much room to the right or rear of the socket and that where the cables enter the radio is about in line with the right edge of the socket when mounted. Picture 12 shows the actual connections to be made.

Socket wiring diagram
12. Socket wiring diagram.

I did the connections to the boards first then to the socket, it was just the way it worked out but it would probably be easier to do the socket then the boards.

At this stage a test should be done. Screw and clip down the detect and select board and plug in the TMA plug. There is no problem operating the radio with the covers removed but sit the radio on something insulating, a newspaper for instance to prevent any possibility of shorting something on the bottom. Connect up the radio in the car, including the amplifier. First see if the radio works, including the auto tuning function. If you have problems first check the TMA plug is in place. If the radio is still not functioning properly check your wiring for shorts and opens.

If all is well, plug in your player with an appropriate cable and see if that works, if it doesn't check you wiring. If you have the ins and outs on the socket transposed the radio will work but not the external input.

Face drilled
9. Face drilled.

A hole for the socket needs to be made in the pinstripe face as in Picture 9. The hole position is critical, if too low the socket will hit the alloy casting, too far to the right will interfere with the cassette flap, too high or too much to the left and a plug in the socket will hit the volume control knob. Fortunately I had a new face to replace my cracked one if I got it wrong. As it turns out if the hole for the socket is drilled in exactly the same place as the hole for the aerial trimmer is on the other side it will be in the correct position.

The hole can be drilled from the rear with the face in the frame but it might be safer to remove the face from the frame. If you chose to remove the face, start by easing the face out at the bottom and then where the projections are near the cassette button holes are and then pull it downwards slightly to release it at the top, taking note of the cassette flap spring and projection.

Make an impression on a piece of paper of the edge around the cassette area and bottom edge of the face, mark the aerial trimmer hole and transfer the hole center to the left side of the face. Do this with much care and accuracy.

To drill the hole use a sharp bit with light pressure, start with a pilot hole and enlarge in small steps until it is the required size. I ended up at 6 mm but check with your socket choice. Blunt drill bits melt there way through plastic with friction and make a mess, sharp bits can grab if too much pressure is applied. A drill press is ideal but failing that a hand cranked drill might be the go.

Reassemble the face and frame if previously separated and trial fit the socket to the frame and the frame to radio, positioning the socket wiring as needed, pulling excess cable back into the radio. Check the cassette flap is free and springs back. When satisfactory unscrew the socket's nut before removing the frame, the socket and cabling should then be in the correct position for the final assembly when the radio is back in the car.

Put the top and bottom covers back on the radio, the flanges of the covers sit outside the rest of the case. The bottom cover has the aerial trimmer symbol on it.

Reinstalling the radio is the reverse of removing. Connect the earth first, use the string to guide the multi-core cable through to the glove box and plug it in to the amplifier along with the earth wire. Position the cable so it is clear of the heater box mechanism visible in the glove box. Finally, connect the +12 volts and the aerial. Slide the radio into the hole, if it doesn't slip all the way in, reposition the wiring and try again. Install the retaining strips top end first, small end down and tighten the screws just enough to hold the radio. Use a tooth pic through the hole in face to guide the socket into the face when clipping the front in place. Put the nut on the socket then use something that won't scratch the dial face to tighten it up. Check the cassette flap doesn't catch on the wiring.

Put the knobs back on lining up the flats and gaps on the shafts with the matching parts in the knobs.

Socket mounted
10. Socket mounted.
Closeup socket
11. Closeup socket.

Pictures 10 and 11 show the completed modification, note the cracked dial face, now replaced. I didn't notice at photo time but the cassette flap is stuck half way, this is due the socket being slightly rotated clockwise causing the wiring to foul the flap, it is close in there but there is just enough space.

Good luck.