Mercury 8MM991 FM Add-On

I designed and built an FM board to provide an intermediate upgrade option for customers...something offering a little more than just AM but less than the full blown conversion. Some customers simply did not need or want stereo or higher output and more speaker outputs provided by the conversion board and they wanted to keep their radio original. This is one feature that is attractive to some people. The process is also relatively easy to reverse.

My first working add-on was performed on a 1955 Buick Sonomatic. The radio was first electronically restored and then the FM board and associated hardware installed. Once I got it working the way I wanted, I decided to offer it as an option. The circuit was developed around the Phillips TDA 7000 mono FM chip and uses the radio's original circuitry for AM and amplification. As of now I can install the FM add-on only in radios with preset push buttons as I use these for band switching.

I decided to document the add-on process for customer FYI. The patient: a Mercury 8MM991 in decent shape that I picked up at a recent swap meet for 5 bucks. Here's a picture before any work was performed:

The first challenge is the installation of micro switches to allow the use of preset buttons to switch between the AM and FM bands. There is no generic fit for these switches...each radio presents its' own challenges. I prefer to use the outer presets as it is easier to remember. However for this radio, I had to install the AM micro switch so that the second from the left preset activated it. Here's a picture showing how the micro switches were installed.

The next challenge is installing the slider potentiometer. This potentiometer is used for tuning and is attached to the tuner treadle bar. Like the micro switches, there is no generic fit. Here, I drilled a small hole in the front of the case and then through this hole drilled a small hole in the treadle bar. I then fabricated a link between the treadle bar and the potentiometer. The potentiometer must be chosen so that it doesn't limit the travel of the tuner!

The last challenge is finding a location for the FM board. It's just a matter of finding open space to install it. Surprisingly, there wasn't much room in this Merc but just enough...beside the tuner unit. See the above picture showing the installation of the FM board.