The conversion of the Elpico unknown model car radio is complete, the quality of MP3 audio played back through the audio stages is pretty good actually.
Here is a better picture of the beast.
Laughable British technology,and how cheap can we make them engineering, note the beautiful alignment of the push buttons and the slot in the front panel that they come through. I wonder if the term elcrapo came from this stuff, we used to use it to refer to cheap and nasty electronics like elcheapo too.
The radio had several problems the aerial cable was fatigued and broken where it entered the case, the braid was frayed internally with strands hanging just a millimeter or so from and internal screen which is at +12Volts.
The Band Change switch was noisy as expected and responded to a clean up. This revealed the real cause of scratchiness and intermittently going dead. Tapping the radio or just touching the RF/Mixer AF117 Transistor brought it back to life. All the AF117's have been 'flashed' at some time I suspect, they all bear scratch marks on their cans. I replaced the offending one with an AF239, OK its not now as original but neither is adding an MP3 input.
The life of these radios with original transistors must be limited as even brand new replacement old stock transistor, of which I should have a few in my loft, will exhibit the tin whisker growth short circuits which cause these problems, if they don't they soon will. In fact the car radios which used AF117's started the problems very early in use, more so I think than the Transistor Portable Radios, temperature variations I wonder.
Here are the internal connections.
Using a switched 3.5mm Stereo Socket we can change from Radio reception to MP3 input by inserting the plug from the MP3 headphone output. The 120 Ohm resistors combine the stereo L and R channels for the mono radio, the 22uF capacitor shown was the original idea to provide isolation from any DC potentials, with the unknown factors of what will get plugged in I opted for extra precaution and used a 0.47uF non polarised capacitor in each lead.
The Jack Socket connections.
The Elpico radio has an internal switch which enables the radio to run on +ve or -ve Earth vehicles. The actual radio circuitry is positive earth so the screen of the lead must not be connected to the earthy end of the Volume Control. There are several places and PCB tracks which are connected to the radio case, connecting the screen there is fine for +ve or -ve Earth cars.
The Volume control connections.
The socket and components need to be mounted in a small preferably screened metal box, where do you get one of those theses days, make it.
The input socket for the MP3 Player.