10th-11th October 2009: As with every old car I have bought the radio didn't work. Owners seemed not to bother when the radio gave up the ghost and just left it there. Maybe it was because the radio is sometimes in an inaccessible place and is difficult to remove, maybe by the time the radio stopped working FM was all the rage and the owner didn't see much point in fixing an old AM-only radio. Without a crystal ball, one can only speculate.
Up until the late 1950s the valve-only car radios required a cumbersome vibrator circuit. A vibrator is a synchronised electromechanical device which converts the car's 12V DC supply to 12V AC. This is then fed into a step-up transformer which outputs about 275V AC. This is then rectified via a rectifier valve and this provides approximately 250V DC for use on the valves' anodes (plates). This is all a rather round-a-about way to get a high-tension DC supply for the valves but nevertheless an ingenious arrangement which fulfilled a necessity. Being electromechanical devices, vibrators were noisy, sometimes unreliable and eventually wore out and needed to be replaced. Once transistors started to become commonplace car radio manufacturers introduced hybrid valve and transistor circuits which obviated the need for a vibrator. This was possible basically because the only real high-power valve in a circuit was the audio amplifier. For this valve to provide the required gain it needed a plate voltage of about 250V. The other valves, such as the oscillator/mixer and IF amplifiers, did not need such high gain. So when transistors began to appear, car radio manufacturers replaced the output valve with an output transistor which could operate on low voltage, the remaining valves had their high-tension voltage reduced to 12V only and the vibrator was eliminated, along with the step-up transformer and rectifier valve. All-in-all the circuit was greatly simplified.
By the late 1950s most car radios used the hybrid valve and transistor arrangement. Many car manufacturers advertised
transistor power on their radios and Mercury was no exception. The sales manual states clearly that the 1958 Mercury was transistor-powered, somewhat of a tall story since there was technically only one transistor inside.
Removing the dead radio from my car necessitated lowering of the air conditioning register below the dash board. Once this was clear the nuts holding the radio to the dashboard were removed, the power, dial lamp, front and rear speaker wires were removed. Someone had done something dodgy with the rear speaker wires because they were just poked into the speaker socket. Interestingly when the radio came out I saw that it had a vibrator ! What was going on and why was the radio not as described in the sales manual. More on this at
Mercury Mysteries.
Disassembling the radio revealed lots of Texas red dust on bottom of the circuit board. The circuit board is mounted upside down in the radio so the components on the top side of the board were all relatively dust-free. First thing I did was to replace all the old electrolytic and paper capacitors as these are never pretty after 50 years. Either they are so dried out they do nothing or they are so leaky that they conduct and cause other components to self-destruct. Or occasionally they still work but I always find it's better just to replace the lot and be done with it. Access to the printed circuit board is difficult and so it had to be removed, making a note of all the wires that had to be unsoldered. There was one large compound can electrolytic which are nearly impossible to find replacements for so I disconnected it but left it in place because its three earth tabs actually joined different tracks on the circuit board ! I wired in replacement electrolytics in places where there was some space.
At this point I connected the radio to 12V but the radio had no life. I suspected the vibrator and so I removed it and prised the innards out of the can. I cleaned the contacts with a points file and then put the vibrator back in the radio. This time the radio came to life with an audible buzzing indicating that the vibrator was working. Some voltage checks showed that the step-up transformer and the rectifier were working fine. Then came some crackles from my test loudspeaker, always music to the ears of a radio serviceman ! I connected a long wire to the aerial socket and tuned around the radio dial and the stations came through with good tone.
I had noticed that the fade control had a charred section on the pot, presumably related to the dodgy wiring mentioned earlier. I connected my test speaker to the rear speaker socket and ajusted the fade control and it worked OK despite the damage. I lubricated the push-button mechanism with WD-40 and ensured it was all moving freely. While it was out I tested the dial lamp which worked. The dial lamp is on its own circuit and is switched by the second pole of the power switch. This means that the dial lamp only comes on if the headlights are on and the radio is switched on, a neat little example of 1950s decadence.
Finally because the vibrator was not working initially I was not sure how long it would last back in the radio. I therefore looked around for a replacement. Apparently electromechanical vibrators have not been manufactured for about 30 years but luckily to keep these old radios going someone manufactures solid state vibrators which are built into an aluminium can so that they look just like the originals. Being solid state switching devices they will run forever and never wear out. I have bought one and it works great, the only problem is the broken aerial on the car. That will have to wait for another day.
