Controlling a Carver TX-11a Tuner from the 1980s, Part 2

05. August 2018 Blog 0
Controlling a Carver TX-11a Tuner from the 1980s, Part 2

In the previous installment of this exciting series, I looked at the documentation for the integrated circuits that control the Carver tuner, in particular the TI TC9147BP. Now it’s time to see if it actually works as expected. The answer is yes, if after some puzzling over unexpected readings.

I first looked at the connection from the chip that sends out the new frequency when it is changed. This is done through a serial line with two clocks. Once I had figured out that they send out the least significant bit first, it was all clear: ignore the first bit, then 11 bits of frequency information (in 0.1MHz steps in this case), and then five bits for the band. The capture below shows a value of 28 for the frequency, which translates into 87.5+28/10=90.3MHz. The frequency band is 00100 for FM North America.

After the long clock pulse, ignore the first bit, then there are 11 bits for the frequency and then five bits for the frequency band.

With that out of the way, I connected my logic analyzer to the pins for the station buttons. Here, the situation is rather confusing. Below, I’m capturing three lines: M1 and M2 are directly connected to the memory buttons and would also connect to memory locations 9 and 10. There is a pair of input lines called MC1 and MC2 that switch between the lower 8 and upper eight memory locations. Each button connects to one Mx input and one of the two MCx inputs.

I initially only captured MC1 and what I saw made no sense. How could M1 be active for memory locations 1 and 2? And then M2 was active for locations 3 and 4. But then it dawned on me that Carver may have just used the buttons differently: button 1 is location 1, button 2 is location 9, button 3 is location 2, button 4 is location 10, etc. Capturing both MC lines shows that that’s how things do indeed work:

Pressing buttons 1 through 5, capturing the two lowest button lines M1 and M2 (top) and the button bank control lines MC1 and MC2 (bottom).

So with this out of the way, I’m fairly clear on how to control the tuner. I don’t know yet if I need 5V inputs to trigger the buttons or if 3.3V might do it (the TC9147BP shows about 4.7V at its VDD pin and supposedly wants 3.5V for a High logic level). But reading in the frequency is pretty straightforward, and simulating button pushes shouldn’t be too difficult, either.

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