The radio I managed to obtain was a Rootes-labeled Motorola 515T, which came out of a Series 2 Alpine. The brightwork looked reasonably good, and all the parts were there.
Wisely, I didn't believe the label that read that the radio was set up for positive ground, because someone had already made the negative ground modification. It was wired for sound, and when the switch was turned on, it sounded beautiful. I cleaned up the mechanisms and put some fresh grease on the moving parts.
Now the challenge was to identify where the amplifier input was. The easiest thing to do is to look for the leads going to the volume control. This particular model is delightfully simple and almost elegant. The power supply and amp section is a separate section at the back of the radio. The volume control leads go under the RF/IF section and simply solder to the amplifier board. Here's the challenge with that. The common of the amplifier is tied to +12v rather than the chassis of the radio. That means if I want to power my MP3 player off the car electrical system, I'll need a method of isolation since the audio common of the MP3 player sits at chassis potential.
To insert the signal to the amplifier before the volume control, I had to grind off the board trace between the CW lead of the volume control and the radio's preamp output circuit. The idea was to install a switch to go between AM audio and Auxiliary audio. To make this seamless the switch would be internal to the radio, and actuate at the far end of the tuning band. This proved very simple with the addition of a microswitch and a small bracket.
The orange wire on the far left of the board trace is where I ground off the board to reach the CW terminal of the volume control. This went to the NC terminal of the microswitch. The volume control CW terminal was wired to the black wire which went to the Common terminal of the microswitch. The final wire was a grey wire that I connected to the NO terminal of the microswitch and out the back of the radio, along with a wire attached to the CCW terminal of the volume control. When the microswitch closes at the far end of the tuning band, the signal input for the volume control is taken from the leads going out the back of the radio.
The ground potential problem of the audio signal was solved with an isolation transformer. I rummaged 'round in the junk box for a small transformer with about 500ohms impedance on the output winding, and a 200ohm impedance on the input leads. This arrangement worked fairly well and yielded acceptable sound quality, as long as I didn't try to connect up both stereo channels. I'll just modify the MP3 player decoder chip for Monoaural output.
The console enclosure was painted with a satin finish, and the components were re-assembled. It cleaned up nicely with just some mild hand soap and a toothbrush. All bright surfaces were then treated with a wax to protect them from corrosion.
The next challenge was to install the radio in the car, without any of the original brackets. I did have some heavy flat stock available to make brackets to suspend the radio from the underside of the dash. There are two rails that go from the dash back to the firewall. These do have many holes already available for attachment. The console was propped into position using shim wedges against the transmission hump to bring it up to the correct height, center it to the gear lever, and level it. Using a cardboard template, the brackets were sized in-position, then the flat stock could be drilled accurately and formed to fit.
The MP3 player sits under the dash and out of sight. It can be controlled by IR remote if needed. The Series 3 is now wired for sound.
